Bacteria Flashcards
Give 3 key points about the development of germ theory
- Idea of transmissible, infectious disease from C16; disease as process C19
- Lister brought Pasteur’s observations to surgery - introduced antisepsis
- Robert Koch isolated pure cultures, showed specific bacteria cause specific diseases
What are Koch’s postulates (4)
- bacterium is present in every case of the disease
- bacterium must be isolated from the disease and grown in pure culture
- specific disease must be reproduced from pure culture in healthy susceptible host
- bacterium must again be recovered
Name some new diseases that have emerged as the population has aged and our lifestyle/ environment changes (4)
food-borne infections, Legionnaires’ disease, Helicobacter ulcers, toxic shock syndrome
Diseases also re-emerge, increasingly protected by
resistance to antibiotics (TB, pneumonia, whooping cough, STIs, nosocomial infections)
Name 2 bacterial diseases that are acute and last days and 2 that can be chronic, lasting for years
Name a bacterial disease that involves a carrier
pneumonia, gastroenteritis
syphilis, TB
typhoid
How do bacterial modes of transmission compare with viral modes?
often similar but as bacteria are rarely obligate intracellular parasites and can multiply outside the host they can spread in additional ways.
Give 4 modes of horizontal bacterial transmission and give examples of each
sexual contact e.g. syphilis, gonorrhea
• via respiratory tract (air-borne droplet)
Upper: pharyngitis, scarlet fever, diphtheria
Lower: whooping cough, tuberculosis, pneumonia, plague
• contamination from own flora e.g. urinary tract infection (UTI) from GI tract
• contact with skin, eyes e.g. boils, impetigo, fasciitis, conjunctivitis, leprosy, anthrax
How can bacterial disease spread vertically (2)
give eg of each
transplacental e.g. syphilis
parturition e.g. gonorrhoea
Other than vertical and horiztonal, how else can bacterial disease spread
indirect contact:
via inanimate objects, food, water, animals, soil
Give examples of bacteria that can spread via a) inanimate objects, b) food, c) water, and d) soil
a)e.g. nosocomial infections (hospital acquired) e.g. urinary tract following catheterization, surgical wound, burn infections. Caused by ‘opportunistic’, often drug resistant, pathogens e.g. (MRSA), Pseudomonas aeruginosa
b) intoxication e.g. Staphylococcal food poisoning, botulism
infection e.g. Salmonellosis (chicken, eggs), E.coli 0157 (beef)
c) cholera, dysentery, typhoid (these are faecal-oral)
Legionnaires’ disease (air-con, respiratory)
d) via wound (spores) – tetanus, gas gangrene
Give examples of bacterial diseases being spread via animals
(Zoonoses).
Animals have disease and/or are reservoirs, can include food-borne diseases, also from livestock (e.g. brucellosis, leptospirosis, E.coli 0157), wild animals (lyme disease via ticks)
Define the following
endemic
epidemic
pandemic
• Endemic: disease that occurs regularly at low or moderate frequency, e.g. dental
caries.
• Epidemic: sudden appearance of disease, or increase above endemic level e.g.
diphtheria
• Pandemic: global epidemic e.g. cholera
What are the 3 steps taken when there is a disease outbreak
traced to source, the mode of transmission determined, and the infectious agent identified.
True or false
bacterial disease outbreaks can arise from a single origin
As bacteria can survive and replicate outside the host, point source outbreaks can arise from a single origin, even non-living e.g. food poisoning, nosocomial infections.
When was there a key outbreak of Legionnaire’s disease
How is it typically contracted
What causes it
What happened to the outbreak
1976
from air con units
the bacterium Legionella pneumophila
as the origin was identified it could be address and it was only a point source outbreak
How does a point source outbreak become continuous source outbreaks
if the source is not eradicated e.g. typhoid (carrier),
travellers’ diarrhoea, food-borne infections,
hospital-acquired Staphylococci.
Describe the continuous source outbreak in 2012 of MRSA
rapid DNA sequencing identified an ongoing outbreak of MRSA in the Special Care Baby Unit at the Rosie Maternity Hospital. Screening of healthcare workers found one carrying MRSA. DNA sequencing confirmed that this MRSA strain was the source of the outbreak. The worker was treated with antibiotics, bringing this continuous source outbreak to an end.
What are propagated outbreaks of bacterial disease
host-to-host transmission results in ever greater numbers of infections
e.g. whooping cough, tuberculosis, gonorrhoea, dysentery, cholera, typhoid
Give an example of a bacterial disease that had point source, continuous source and propagated outbreaks
Bubonic plague, caused by Yersinia pestis, spreads from rodents (point/continuous source) to humans, in which the disease becomes pneumonic (propagated person-to-person)
Are bacteria obligate parasites?
Bacteria make ATP and protein, so generally are not obligate parasites. Rare exceptions include Chlamydia
Give a key difference between bacterial and viral replication
Unlike viruses, bacteria maintain structural integrity during replication
How do bacteria usually exist
typically unicellular and free-living, bacteria also live in complex multicellular communities (biofilms) and/or in association with eukaryotes.
do bacteria have organelles?
cell membranes?
usually no
yes - a lipid bilayer that acts as an osmotic barrier, and is the site of signal reception, transport of nutrients, and respiration. In Gram-negative bacteria this is called the ‘cytoplasmic’ or ‘inner’ membrane.
Give 2 ways bacterial cells can be motile
e.g. rotating flagella, extending/retracting pili ‘grappling hooks’
How do bacterial cells divide?
binary fission
Do bacteria have sexual reproduction
no meiosis,
but horizontal genetic transfer (HGT)
Briefly describe a typical bacterial genome
haploid, usually single & circular
+ plasmids, bacteriophage
Give 2 facts about bacterial mRNA
often polycistronic (co-linear genes), unstable
How does bacterial cell regulation differ from mammalian?
bacterial: mostly transcription initiation
mammalian: often post transcriptional
what are the different types of pathogenic bacteria (think shape)
give examples of bacteria and diseases for each
cocci (e.g. Staphylococcus, Streptococcus, Enterococcus),
rods (e.g. Salmonella, E. coli)
or curved rods (e.g. ‘comma-shaped’ Vibrio cholerae),
and are sometimes spiral e.g. Treponema (syphilis), Helicobacter (gastric ulcers)
Which bacteria produce endospores (4 examples)
where are bacteria that do this usually found
Clostridium tetani (tetanus), C. perfringens (gas gangrene, food poisoning), C. botulinum (‘botulism’ food poisoning), Bacillus anthracis (anthrax).
usually in soil
How to tell Gram positive from Gram negative bacteria
iodine-crystal violet complex is washed out of Gram-negative cells, but not Gram-positive cells.
The Gram stain reflects substantial differences in cell envelopes. Gram-positive bacteria have a thicker peptidoglycan wall, and Gram-negative bacteria have an extra (outer) membrane.
True or false
all bacteria are either Gram positive or negative
false
Some medically important bacteria do not Gram stain
(i) Mycobacteria, including Mycobacterium tuberculosis which causes TB, are called
‘acid-fast’ as they are difficult to stain/destain because of their waxy lipid coat.
(ii) Chlamydia and Mycoplasma, which cause non-gonococcal urethritis, lack a substantial cell wall. They are difficult to culture and insensitive to many antibiotics.
What is the bacterial cell wall made of
describe this material
peptidoglycan
- a huge macromolecule of alternating sugars N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), cross-linked by short oligopeptides.
Oligopeptide crosslinks also form vertically to connect peptidoglycan sheets into a thick layer
How does the size of peptidoglycan differ between Gram positive and negative bacteria
Gram-positive peptidoglycan is 150-500 angstroms thick, the Gram-negative peptidoglycan layer is thinner.
Which bacteria have a periplasm
what is this
Gram negative
between inner and outer membranes
What does the periplasm contain
enzymes e.g. proteases, lipases, phosphatases, β-lactamases (break down lactam-ring containing antibiotics e.g. penicillin, cephalosporin) and components of transport (import, export) systems.
Where are porins found in Gram negative bacteria
outer membrane
Where is LPS found
The outer leaflet of the outer membrane contains lipopolysaccharide
(only in Gram negative)
What are the different parts of the LPS
O-antigen: highly variable, generating different antigens and therefore
serotypes. An important defence against host attack
Core polysacchride: constant sugars
Lipid A: he endotoxin released from dying bacteria, and is a major PAMP ‘alarm signal’ that triggers a wide-ranging
immune response.
Which type of bacteria tend to have polysacchride capsules
what is the point
respiratory pathogens like
Streptococcus pneumoniae and Haemophilus influenzae.
Capsule prevents drying and protects from host defences:
complement and macrophage.
Proteins central to interaction with the host are typically exported and/or assembled on the bacterial surface e.g. toxins, adhesins, flagella.
what mustt these things cross in Gram postive and negative bacteria
In Gram-positive and Gram-negative bacteria, proteins typically cross the cell (inner) membrane using an N-terminal secretion signal and the standard secretion (Sec) pathway.
In Gram-negative bacteria, additional export pathways cross the periplasm and outer membrane
What are the 4 steps of binary fission
genome duplication, segregation, cell septation and division
How fast do lab cultures of E. coli replicate
how does this compare to in vivo infection
Laboratory cultures can double every 20-30 minutes, but doubling can take hours in an infection
Are bacteria active in stationary phase?
yes
Radical changes in physiology and gene expression enable survival and reduce susceptibility to many antibiotics.
What are most bacteria’s relationship with oxygen
Most pathogenic bacteria are facultative anaerobes, e.g. Salmonella, Staphylococcus
What does facultative anaerobe mean
able to grow with or (usually less well) without oxygen
What do facultative anaerobes use for energy when growing aerobically
use glycolysis, the citric acid cycle and oxidative phosphorylation to generate ATP from glucose
Name some bacteria that are strict anaerobes
how do they gain energy
e.g. Clostridium, Bacteroides
from fermentation (using various terminal electron acceptors e.g. nitrate, sulphate or ferric iron. Oxygen is toxic to many
Where do all bacteria find carbon and energy
from organic matter (including us)
How are bacteria usually motile
flagella
How do flagella provide motility
The long flagellar filament is rotated like a propeller by a motor anchored in the cell (inner) membrane
What direction do bacteria move in
sense chemical signals and move towards them (nutrients) or away from them (toxic chemicals) by chemotaxis, in which chemoreceptors at the ‘nose’ of the cell sense and transmit information, via a phosphorelay, to the flagellar motor switch. The switch controls the direction in which the helical propeller rotates: counter clockwise (CCW) to swim, or clockwise (CW), causing the bacterium to ‘tumble’ and randomly change direction
True or false
bacteria usually have access to lots of resources
false
Pathogenic bacteria lead lives of feast and famine
How is a bacterium usually able to switch genes on/off depending on its environment
largely determined by changes in the coupled transcription of contiguous genes
(operons), which is tightly coupled to translation and rapid mRNA turnover.
Name 2 things bacteria have adapted to sense the environment effectively
large SA:volume ratio
signal transduction systems called histidine-aspartate phosphorelay (HAP) signalling pathways
Name some signals that might make a bacterium change its gene expression
temperature, pH, amino acids, osmolarity, ‘stress’.
What is global regulation
bacteria’s ability to co-regulate expression of networks of operons (regulons)
What is quorum sensing
how do bacteria do this
cell density sensing
whereby some pathogens switch on
their armoury of virulence genes when its population reaches a specific density.
by secreting a small signal molecule and sensing its concentration -
a form of communication!
How do bacteria change their genes other than immediate physiological changes
what does this lead to
evolution
genetic change is driven by mutations in large populations of rapidly replicating cells.
This results, for example, in variation in cell surface structures e.g. Salmonella and E. coli have hundreds of O (LPS) and K (capsular polysaccharide) antigen serotypes
What are bacterial DNA rearrangements
shuffling the genome
caused by random capture and insertion of DNA elements called insertion sequences (IS)
What are insertion sequences (IS)?
e a few hundred base pairs long and enter the genome using their own recombination system
part of DNA rearrangement in bacteria
What are transposons in bacteria
genomic elements (bigger than and often including IS elements) which have picked up useful (to the pathogen!) genes encoding e.g. antibiotic/heavy metal resistance, or virulence factors e.g. toxins.
How are transposons incorporated into the bacterial genome
what does this allow
s can be incorporated into the main genome or carried by plasmids
allow bacteria further flexibility and have enabled rapid spread of antibiotic resistance and virulence genes.
What are the 3 ways for bacteria to transfer DNA
transduction
transformation
conjugation
What is transduction (bacteria)
transfer of DNA via bacteriophages
What is transformation (bacteria)
uptake of DNA from dead/ lysed bacteria
What is bacterial conjugation
direct transfer of DNA requiring contact between 2 bacteria
Give 3 historical landmarks that are important for DNA transfer in bacteria
1928: discovery of transformation in Streptococcus pneumoniae (dead capsulated bacteria ‘transformed’ live non-capsulated bacteria and killed mice).
1944: DNA demonstrated to be the component responsible for transformation
1952: only DNA is introduced during transduction of bacterial cells by bacteriophage
What are the key events in bacterial pathogenesis? (3)
• colonization of epithelia/mucous membranes. This can involve stable adhesion on an epithelial surface and/or entry into host cells
• multiplication requires the ability to evade or survive the hosts defences.
This can lead to spread, locally or to distant sites
• transmission to a new host
These key events differ in sequence and mechanism in different bacteria.
What does the pathology of bacterial disease arise from
from host cell malfunction or death caused directly by toxins or bacterial invasion, and/or indirectly arising from the host response to the bacterial infection.
Give 3 facts about virulence genes
- are specialised. Not constitutively expressed, but switched on in response to host environmental signals. Often co-regulated by the same signals - global regulation.
- are often carried on extrachromosomal plasmids and bacteriophage, and/or grouped on pathogenicity islands (PAI) in the chromosome.
- encode proteins or other molecules that are exported to the bacterial cell surface, secreted into the environment, or delivered into host cells.
Where are bacterial genes for infection usually found
on pathogenicity islands (NOT randomly distributed in the genome)
How have pathogenicity islands evolved
by integration of transposons, plasmids and bacteriophage and incorporation of transformed DNA (pathogenicity islands typically have a G+C content that differs from the surrounding chromosomal DNA).
Name 2 Salmonella pathogenicity islands
What do they do
SPI-1 and SPI-2
determine, respectively, entry into non-phagocytic cells and survival in macrophages.
These increase dissemination of virulence traits and facilitate virulence gene co-regulation
For which bacteria is cell motility important in colonisation
e.g. enabling bacteria such as Vibrio cholerae and Helicobacter pylori to move through mucus.
Give 2 uses of flagella in bacterial colonisation
Many bacteria swim through liquids by means of rotary flagella.
Some bacteria also use flagella to ‘swarm’, moving rapidly across surfaces (e.g. urinary tract epithelium) as a coordinated population.
What do cell surface adhesins allow
enable bacteria to attach stably to host cells or extracellular matrix, via carbohydrate or glycolipid receptors
Where are bacterial adhesins usually found
give an example
assembled on the tips of long rigid pili (or fimbriae) projecting from the bacterial cell.
eg P-pilus adhesin of uropathogenic E.coli which binds a kidney receptor.
How are UTIs usually caused
by uropathogenic E. coli (UPEC) from the
host’s own colon flora by ascending infection.
Which types of people tend to get UTIs
women (shorter urethra)
catheter users
Which bacterium is associated with UTIs following catheter use?
Proteus (highly motile)
what is cystitis and what can it lead to
bladder infection
can ascend to kidney (pyelonephritis)
What can proteus infections of the urinary tract lead to
kidney stones (Proteus cleaves urea to ammonia by producing urease)
How does cystitis lead to kidney stones? (5)
Cell motility aids ascent of urinary tract
Common (type 1) pili bind bladder cells
Pap (P-pili) bind to receptors on the kidney
Damage by pore-forming toxin (haemolysin) and inflammation (LPS)
In Proteus infections, urease cleaves urea to ammonia causing kidney stones
Give an example of long term colonisation of the urinary tract
UPEC can cause recurrent UTIs
During acute infection, superficial epithelial cells undergo ‘exfoliation’, allowing UPEC to invade underlying epithelial cells where they persist as quiescent reservoirs, protected from antibiotic treatment. Quiescent UPEC can then resurge and enter the bladder lumen, causing further acute infection
Name 2 bacteria which adhere to host epithelial cells extremely tightly
enteropathogenic E.coli (EPEC) and enterohaemorrhagic E.coli (EHEC) causing intestinal disease
How do EPEC and EHEC adhere to epithelial cells
Interaction with the host cell cytoskeleton induces ‘pedestal’ formation by the host.
The bacterium uses specialized needles to deliver (‘inject’) proteins into the host cell, including a specific receptor (translocated intimin receptor, Tir).
Binding of the bacterial surface protein intimin to Tir subverts host cell signal transduction and promotes actin polymerisation
Which protein does EHEC inject into epithelial cells to cause actin polymerisation and pedestal formation
Tir (translocated intimin receptor)
this interacts with intimin on the bacterial surface to subvert host cell signaling
Give 3 examples of bacteria forming biofilms
- plaque-forming Streptococcus mutans and other bacteria on teeth (glucan formed from food sugar, acid leads to demineralisation i.e. caries)
- Staphylococci which colonise catheters e.g. in the urinary tract
- Pseudomonas aeruginosa which can colonise contact lenses, causing conjunctivitis
What is Pseudomonas aeruginosa
How does it cause damage
how are its genes encoding virulence factors and biofilm formation controlled
Gram negative opportunistic pathogen
Damage is direct (toxins etc) and indirect (inflammation).
co-regulated by quorum sensing.
Where do P aeruginosa biofilms form
in the lungs of individuals with cystic fibrosis
What is a critical component of chronic P. aeruginosa biofilm lung infections
the copious production of alginate polysaccharide ‘slime’. This protects bacteria in the biofilm from the host immune response and antimicrobials
What makes opportunistic bacteria like P aeruginosa particularly dangerous
often multidrug resistant due to multiple efflux pumps in the cell envelope
What are the 2 ways bacteria can force entry into non-phagocytic cells, e.g. intestinal epithelial cells rather than remaining on the surface
what do both mechanisms have in common
zipper mechanism
trigger mechanism
both induce changes in (‘remodel’) the host cell cytoskeleton by binding actin directly and/or subversion of host signal transduction.
What is the zipper mechanism used by bacteria to gain access to a non-phagocytic cell
Receptor mediated endocytosis e.g. by Listeria.
Here, bacterial ‘invasins’ mimic eukaryotic ligands e.g. fibronectin, a component of the host extracellular matrix that binds integrins. This causes cytoskeletal rearrangement to internalise the bacteria
What is the trigger mechanism used by bacteria to enter non-phagocytic cells
Subversion of host cells by bacterial effector proteins, some binding to actin, that are delivered directly into the target host cell through bacterial needles e.g. by Salmonella
These effector (invasion) proteins mimic eukaryotic cell proteins e.g. in actin binding and polymerisation (SipA and SipC) or signalling pathways (SptP). In Salmonella, genes encoding effectors (and needle) are on a pathogenicity island (SPI-1)
Name 2 pyogenic cocci
Staphylococcus and Streptococcus
How do Staphylococcus and Streptococcus spread through tissue
by using batteries of enzymes to spread through tissues, e.g. collagenase to break down connective tissue, coagulase to induce fibrin clot formation, staphylokinase to break down blood clots (opposite effect to coagulase), and DNAse to break down DNA in pus
How does Listeria spread in the body
enters cell in a vacuole via zipper mechanism
Inside the cell, it lyses the vacuole to enter the cytosol
Here it replicates and manipulates the cytoskeleton to become motile (actin comet tail) to spread into neighbouring cells in another vacuole
repeat
What are actin comet tails
a form of cytoskeletal rearrangement - actin polymerisation
induced by the pathogen, this time at one end (pole) of the bacterial cell granting it motility
What is a key difference between the behaviour of listeria and salmonella in the cell
listeria lyses its vacuole before beginning intracellular movement and replication
salmonella stays in its entry vacuole to replicate
What causes typhoid
which cells does it replicate in
salmonella typhi
macrophages - stays in entry vacuole so is not destroyed. this allows it to be spread locally and throughout the body by the host defense system.
Which diseases does chlamydia trachomatis cause
i) trachoma, the most prevalent form of infectious blindness in the developing world.
(ii) non-gonococcal urethritis, a common sexually transmitted infection, is often asymptomatic, many ‘silent’ carriers. However, can cause acute prostatitis (inflammation triggered by e.g. chlamydial LPS) and chronic infection can lead to pelvic inflammatory disease and infertility.
How big is the chlamydia genome
1.04 Mbp encoding ~900 genes (cf. Salmonella: 4.85 Mbp, ~4450 genes),
Why can the chlamydia genome be so small
has lost many metabolic enzymes and pathways and is adapted to obligate intracellularity, scavenging essential metabolites from the hostcell.
What are the 2 forms of the chlamydia bacterium in its lifecycle
infectious elementary body (EB), which has low metabolic activity and cannot replicate,
and
reticulate body (RB) that replicates inside host cells.
What are the 4 key stages of the chlamydia life cycle
- EB attaches to host epithelial cell and induces uptake in endosome.
- EB in the endosome differentiates to the metabolically active RB, which replicates. The RB-containing vacuole segregates from the normal endosome maturation pathway and becomes a protective, nutrient-rich vacuole or ‘inclusion
- The RBs then produce both EBs and RBs.
- The host cell lyses to release EBs that go on to infect more host cells
How is damage caused in chlamydia infections
by cell lysis and inflammation triggered by chlamydial LPS
How do chlamydia EBs induce uptake into host cells
Involves rearrangement of host actin cytoskeleton by chlamydial effector Tarp (translocated actin recruiting phosphoprotein) injected into host cell using virulence needle (similar to EPEC, Salmonella).
Name 5 of the physical and chemical barriers to bacterial survival and multiplication (not immune system)
skin mucous lysozymes gastric acid/ bile salts iron limitation resident microflora
How does the skin protect against bacterial survival and multiplication
- dry acidic, cool, high salt (limit growth)
* sloughing cells (remove bacteria)
How do mucous membranes protect against bacterial survival and multiplication (4)
- cilia (pulmonary airway)
- eyelid blinking, urine flow, GI tract motility
- mucin layer (physical barrier, traps bacteria)
- tight junctions (limit invasion between cells)
How do lysozymes protect against bacterial survival and multiplication
• in tears – splits bacterial peptidoglycan
Which 2 chemical barriers inhibit growth of bacteria in the body
gastric acid/ bile salts
Fe limitation via sequestration by lactoferrin and transferrin
How do resident microflora protect against bacterial infection
some sites densely occupied e.g. colon can have1011/ml.
Inhibits colonization by occupancy, toxic waste (pH) and chemicals (e.g. fatty acids, bacteriocins).
What are the early immune responses to bacterial infection (3)
• acute inflammation
• complement - alternative pathway activation by bacteria
(e.g. LPS)
• macrophages etc bind bacteria via lectins.
Kill in phagolysosome
Give 8 things that happen in the induced immune response to bacterial infection
- enhanced inflammation
- classical pathway of complement activation
- Ig production - CD4/MHCII/Th2
- IgG opsonization- Fc-mediated phagocytic killing
- Ig neutralization of toxins
- IgA at mucosa - blocks colonisation
- MHCII/Th1 activates macrophages;
- CTLs in some cases
Name bacteria that infect the following areas:
a) oral cavity
b) upper resp tract
c) skin
d) large intestine
e) urethra/ vagina
a) lactobacillus
b) Strep, corynebacterium, neisseria
c) staphylococcus, epidermidis
d) bacteriodes, E. coli
e) lactobacillus
Give 2 ways the body can make a hostile environment for pathogens
lack of essential iron
gastric acid
How do bacteria overcome the body’s attempt at making the environment hostile by reducing iron
Include examples
bind iron at high affinity (much higher than host) e.g. by secreting iron-binding siderophores and reimporting them loaded into the cell, e.g. green fluorescent pyoverdin secreted by the opportunistic pathogen Pseudomonas aeruginosa and equibactin produced by Streptococcus equi
How can bacteria overcome the hostile acidic gastric environment
Pathogens like Shigella (dysentery), and Helicobacter (ulcers) can resist
low pH by pumping H+ out of their cell and/or producing NH3 by urease action.
How can bacteria prevent macrophage uptake (4)
(i) Paralyse the macrophage by subverting its functions
(ii) Inhibit chemotaxis
(iii) Resist phagocytosis by shielding with antiphagocytic capsules
(iv) Kill macrophages with secreted cytolysins
How can bacteria paralyse the macrophage by subverting its functions
give examples
disrupting intracellular trafficking and signal transduction by injecting (needles again) anti-phagocytic proteins into the macrophage, e.g. Yersinia injects multiple effectors including YopT, a protease that targets small GTPases to disrupt the cytoskeleton, and YopP, an acetyl transferase that inhibits signaling, triggering apoptosis
How does Yersinia inhibit macrophage function (2)
injects multiple effectors including YopT, a protease that targets small GTPases to disrupt the cytoskeleton, and YopP, an acetyl transferase that inhibits signaling, triggering apoptosis
How can bacteria inhibit macrophage chemotaxis
Cleave complement C5a with C5a peptidase.
What are bacterial capsules commonly made of
which bacteria tend to have them
how can capsules help a bacterium avoid phagocytosis (3)
Capsules are commonly made of polysaccharide,
e.g. by bacteria causing pneumonia (e.g. Streptococcus pneumoniae)
- steric hindrance i.e. physical block
- non-immunogenic polysaccharide e.g. sialic acid
- lack affinity for complement factor B: C3b not formed
How can bacteria kill macrophages
with secreted cytolysins
What are cytolysins
pore-forming toxins that disrupt cell functions, cause apoptosis, and destroy cell
membrane integrity
Name 4 bacteria that secrete cytolysins
e.g. Gram-positive
Streptococcus pyogenes (streptolysins O and S),
pneumococcus (pneumolysin),
Gram negative E. coli,
B. pertussis (whooping cough).
How can bacteria survive inside a macrophage
interfere with phagosome maturation/ inhibit phagosome-lysosome fusion
resisting oxidative burst
How does salmonella avoid destruction inside a macrophage
name another bacterium that does the same thing
inhibit phagosome-lysosome fusion
mycobacterium
both also survive phagolysosome onslaught by resisting the oxidative burst.
Which bacterium causes Legionnaires disease
where does it multiply
how does it survive here
How does the organ get damaged
Legionella pneumophila
multiplies in alveolar macrophages.
prevents the oxidative burst by modifying the host phagosome with bacterial proteins that block lysosome fusion.
This stimulates cytokine release, and lung damage appears to reflect a vigorous inflammatory response.
How might Legionnaire’s disease survive air conditioning
L. pneumophila is a facultative intracellular parasite of amoebae.
How can a bacterium avoid complement
LPS O-antigen and capsules contribute to serum (complement) resistance by hindering access to the bacterial cell, preventing formation of membrane attack complex.
How can bacteria avoid the effects of host antibodies
evade Ab recognition
inactivate Ab
How can bacteria avoid Ab recognition (3)
(i) mimic host
(ii) shut off or switch expression of surface antigens by phase variation
(iii) More complex DNA recombination to generate antigenic variation
How do nbacteria mimic the host to avoid Ab recognition
e.g. by covalently attaching sialic acid to LPS e.g. Neisseria gonorrhoea (gonococcus), Neisseria meningitidis (meningococcus)
How do bacteria change their antigens to avoid Ab detection
e.g. Salmonella flagellin. Rearrangement (‘flipping’) of an invertible promoter region, flanked by recombination sequences recognized by site-specific DNA recombinase.
What does Neisseria provide an example of (in avoiding the immune system)
More complex DNA recombination to generate antigenic variation
in Neisseria, silent pilin genes, encoding antigenically distinct pilin proteins, recombine into a transcriptionally active expression locus.
What does Neisseria cause
Neisseria gonorrhoea causes gonorrhoea
What are the different paths a gonorrhoea infection could follow (5)
acute urethritis (Gram-negative diplococci in PMNs in discharge) may be followed by severe complications e.g. systemic spread - arthritis, endocarditis;
infertility following cervical infection;
in newborns,
conjunctivitis and blindness.
Compounded by antibiotic resistance and asymptomatic
carriers.
What are the 2 ways bacteria can inactivate Ab
(i) cleave secretory IgA
ii) bacteria bind Ig Fc component (prevents opsonization
How can bacteria cleave Ab molecules
Which bacteria do this? (4)
Antibody cleavage by specific proteases of mucosal pathogens like: Streptococcus pneumoniae, Haemophilus influenzae, Neisseria gonorrhoea, Neisseria meningitidis.
What is MRSA
Methicillin-resistant Staphylococcus aureus
Describe Streptococcus pyogenes
what diseases does it cause
Gram positive cocci, Lancefield group A based on surface carbohydrate
pharyngitis and other acute infections, e.g. tonsillitis, scarlet fever, puerperal fever, necrotizing fasciitis (‘flesh-eating bacteria’)
How does Strep pyogenes improve its colonisation of epithelium
produces numerous adhesins
Give 8 virulence factors that aid the spread, survival, and damage of Strep pyogenes
- polysaccharide capsule (Lancefield Group A)
- surface M protein (80 serotypes i.e. antigenic variation) - binds complement factor H
- C5a peptidase - inhibits chemotaxis
- streptolysins O and S, pore-forming toxins (β haemolysis - page 27 & practical class)
- pyogenic toxins - superantigens (toxic shock syndrome – see page 31)
- hyaluronidase - breaks down tissue
- streptokinase - lyses clots
- DNAse depolymerises DNA in pus (reduces abscess viscosity)
What complications can acute inflammatory Strep pyogenes lead to (2)
(a) Rheumatic fever – heart & joint granulomas plus fever, may lead to rheumatic heart
disease. Believed to involve autoimmunity, e.g. to M protein
(b) Glomerulonephritis - accumulation of Ab-Ag complexes that lodge in kidney glomeruli
to cause inflammation (type III hypersensitivity)
Name a pathogen of horses that is closely related to a human bacterial pathogen
What disease does it cause
Streptococcus equi
(Closely related to Streptococcus pyogenes)
strangles
What characterises streptococcus equi’s disease
strangles - characterised by a purulent nasal discharge and abscesses in the lymph nodes of the head and neck
What happens if strangles becomes systematic
(bastard strangles) with the bacteria spreading via the
lymphatic system to e.g. lungs, abdomen, brain. As with S. pyogenes infections, later
complications can include myocarditis.
What are the virulence factors of strep equi
S. equi (Lancefield Group C) has similar virulence factors to Streptococcus pyogenes (Lancefield Group A) and a specific iron-binding siderophore, equibactin.
How can bacteria directly damage host tissue
toxins:
- Cytolysins: target host cell membranes
- Enzymatic intracellular toxins: poison host cells by specific catalytic activity
What are exotoxins
protein toxins secreted by many pathogenic bacteria
How can you disable an exotoxin
Name a clinical use
can be denatured (‘toxoided’), e.g. by heat, sometimes to produce non-toxic immunogenic forms used for vaccination.
What are the 2 broad types of exotoxins
cytolysins
Enzymatic intracellular toxins
What are the 2 toxic mechanisms of cytolysins
give examples of each
(i) enzymatic degradation of membrane phospholipids by secreted phospholipases e.g.
Clostridium perfringens α-toxin
(ii) pore formation in membranes
e. g. Streptococcus pneumoniae pneumolysin, Streptococcus pyogenes streptolysin O, E. coli hemolysin, and Helicobacter VacA
What are the results of ctyolysin action
Lead to target cell lysis, but at lower concentrations disrupt (‘subvert’, ‘short-circuit’)
host cell signal transduction,
causing e.g. release of leukotrienes, histamine. And apoptosis
What is the advantage of bacterial cells releasing cytolysins (3)
disable immune cells, assist tissue damage and bacterial spread
What are A-B toxins
- Enzymatic intracellular toxins: poison host cells by specific catalytic activity
why are enzymatic intracellular toxins called A-B toxins
their function depends on receptor-binding (B) and intracellularly
active (A) components
Are A-B toxins monomeric?
Can be single polypeptides cleaved to active fragments
diphtheria, tetanus, botulinum toxins), or multimeric (anthrax, pertussis, cholera toxins
What are the specific actions of bacterial enzymatic toxins ? (7)
- ADP-ribosylating enzymes
- Adenylate cyclases
- Glycosdases
- Glucosylating toxins
- Genotoxins
- Deamidases
- Neurotoxins
Which bacterial toxins are ADP-ribosylators (3)
What do they regulate and what does this affect
Cholera toxin,
E. coli enterotoxin (labile toxin, LT),
pertussis toxin (whooping
cough, Bordetella pertussis)
ADP-ribosylate regulators of host adenylate cyclase.
This leads to disturbance of cAMP levels, signalling and ion balance.
What does diphtheria toxin do
ADP-ribosylates translation elongation factor 2, so blocks protein synthesis
What produces cholera toxin
Vibrio cholerae
When were the last outbreaks of cholera in the UK
C19 (but worldwide pan/epidemics still exist)
Where is cholera endemic
in large areas of the developing world, spread by fecal-oral route from contaminated water
Is cholera acute or chronic
what are the possible outcomes
acute infection.
If untreated ~50% mortality, but electrolyte replacement reduces this to 1%.
How does V. cholerae infiltrate the host
e colonizes the small intestine mucosa by a fimbrial adhesin
What causes the symptoms of cholera primarily
cholera toxin (CTX)
Describe the genes encoding cholera toxin
carried on a bacteriophage integrated into the bacterial chromosome and are co-regulated with adhesin and other genes by a HAP signal transduction system (global regulation).
What is the structure of CTX
explain the function of each subunit
AB5
B binds to host receptor GM1-ganglioside, uptake by receptor-mediated endocytosis and retrograde transport to ER.
Active A subunit translocates into the host cell cytosol.
What is the effect of CTX on the host cell
n short-circuits control of adenylyl cyclase (AC) activity by ADP-ribosylating stimulatory G (GTP-hydrolysing) protein (GS), fixing it in the “on" state. This results in uncontrolled high levels of cAMP.
Increased intracellular cAMP disturbs activity of Na+ and Cl- (CFTR) membrane pumps
What does the disturbed activity of CFTR in cholera infections lead to
Ion imbalance leads to water/electrolyte loss (up to 20 litres a day) into gut lumen.
Copious watery diarrhoea (‘rice water’), shock, collapse, sometimes cardiac failure.
What causes diphtheria
This disease was a scourge of children prior to what ?
Gram-positive rod Corynebacterium diphtheriae -
an extracellular toxigenic bacterium
he introduction of the highly effective ‘toxoid’ vaccination.
Is diphtheria still a threat
still major killer in the developing world and has re-emerged where vaccination is inadequate.
What is the close counterpart of Corynebacterium diphtheriae in animals caused by
which pathogen is related to this bacteria
caused by C. ulcerans.
C. pseudotuberculosis causes a range of respiratory diseases in sheep, goats, cows and horses
What do Pathogenic C. diphtheriae, C. ulcerans and C. pseudotuberculosis all produce
diphtheria toxin (DTX)
Describe the gene encoding diphtheria toxin (location and regulation)
carried on a bacteriophage integrated into the bacterial chromosome.
The DTX gene is controlled by a bacterial transcription factor DtxR, which represses gene expression when bound by iron (Fe), i.e. transcription is switched on in the host where the concentration of free iron is low.
How does C. diphtheria invade the host what then happens
what does this lead to
colonises nasopharyngeal epithelium
then secretes DTX toxin (an AB toxin)
ntense local inflammation and damage to mucosal cells, growth of bacteria in inflammatory exudate, and formation of a pseudomembrane – occludes airway
Toxin can lead to irregular heartbeat, coma, and death.
Give the 3 steps of DTX action
- single (A-B) polypeptide binds to receptor (heparin-binding epidermal growth factor, HB-EGF) via B domain.
The A-B polypeptide is ‘nicked’ by a host protease (furin) but A and B domains remain covalently connected
by a disulphide bridge. Toxin taken up by endocytosis. - acidification of the endosome (by the V-ATPase proton pump) triggers B-dependent translocation of A across vesicle membrane into cytosol.
- in the host cytosol, the disulphide bond is reduced, A is released and blocks protein synthesis by ADP-ribosylating translation elongation factor-2 (EF-2).
Name 2 exotoxins that are adenylate cyclases
what is the result
Bordetella pertussis (whooping cough) and Bacillus anthracis (anthrax)
Though toxin action is mechanistically different, the effects on host cells are similar to cholera toxin (ion imbalance etc)
Name 2 glycosidase exotoxins
what do they do
Shigella, which causes bacterial dysentery, produces shiga toxin, and EHEC produces shiga-like toxin.
Both are glycosidases that depurinate 28S rRNA to blocks translation (protein synthesis)
what causes antibiotic-associated diarrhoea
what causes damage
Clostridium difficile
releases toxins which glucosylate small GTPases involved in signal transduction – disrupt actin cytoskeleton and tight junctions
What do genotoxins do
what produces them
cleave DNA
Salmonella typhi (typhoid), E. coli and Campylobacter (gastroenteritis) produce Cytolethal Distending Toxins (CDTs)
Name 2 bacteria that produce deaminase exotoxins
what specifically do they produce and what does this do
Uropathogenic E. coli (UPEC) and meningitis-causing E. coli
Cytotoxic Necrotizing Factors, deamidases that targets host cell GTPases, disrupting signal transduction to reorganize the actin cytoskeleton
Name 2 bacterial neurotoxins
tetanus and botulinum toxins
When is TeNT made
during anaerobic growth of the pathogen in a wound
How does the structure of TeNT contribute to its function
B chain binds to a receptor on peripheral nerve membranes.
The A chain is internalized and undergoes retrograde transport to the CNS to cleave synaptobrevin
True or false
It is the TeNT B chain that performs the all important cleavage which blocks inhibitory NT release
false
The A-chain cleaves synaptobrevin, blocking release of inhibitory neurotransmitters
What does the TeNT A chain’s cleavage of synaptobrevin lead to
blocks release of inhibitory neurotransmitters. This results in uncontrollable muscle contraction, spastic paralysis (death from spasms of respiratory muscles).
Compare the action of BoNT to TeNT
same proteolytic action but BoNT acts at on peripheral nerves, preventing release of stimulatory neurotransmitters, resulting in
flaccid paralysis
How can you tell tetanus from botulism in animals
essentially identical in animals
How can release of toxins from bacterial cells aid colonisation (give examples)
How can it aid transmission
e.g. kill ciliated cells in whooping cough,
aid transmission e.g. diphtheria (cough droplets), cholera, Shigella (faecal-oral route).
Is release of bacterial toxins always clearly useful
Some less clear - may be incidental to the human host?
what are bacterial effectors
Like exotoxins, bacterial effectors, delivered into eukaryotic cells by ‘needles’, act on intracellular targets to subvert host function e.g. E. coli Salmonella,
and Yersinia
What happens generally in conventional antigen presentation
How does this differ when superantigens are involved
antigen is engulfed by APCs and degraded to peptides. These are bound by MHC Class II molecules and presented on APC surface, where it is recognized by TCR
Protective T
cells are generated.
superantigen (SAg) bridges weakly interacting MHC Class II and TCR by binding outside the normal antigen-binding pocket. Thus, SAg activates various useless T cells by promoting tighter binding and stronger signalling in T cells. Results in cytokine storm.
Name a disease caused by superantigens
when was it first discovered
Toxic shock syndrome (TSS)
originally recorded in 1978 in women. Cause was superantigen (toxic shock syndrome toxin, TSST) produced by Staph. aureus growing in highly absorbent tampons.
True or false
Strep pyogenes causes toxic shock syndrome
false
TSS caused by TSST from Staph aureus
Strep. pyogenes produces similar toxin and causes toxic shock-like syndrome (TSLS) -
What are the symptoms of toxic shock syndrome
symptoms of shock, multi-organ failure, similar to those caused by LPS endotoxin
Name a disease caused by superantigens (not TSS)
Staphyococcal toxins involved in food poisoning are also superantigens.
What is the advantage of superantigen toxins? .
May deflect immune response
what triggers acute inflammation in bacterial infection
what is the process
lipid A component of Gram-negative bacterial LPS
LPS released from dead bacteria is bound by LPS-binding protein and delivered to macrophage receptors, CD14 and Toll-like receptor 4 (TLR4). This triggers signalling pathways that activate cytokine genes.
Why is lipid A a good host alarm singal
unique to bacteria
what is endotoxin
lipid A
‘endotoxin’ is a historical term that is nowadays a bit confusing as not a pathogen-specific toxin
Why are the initial symptoms of manybacterial infections similar
lipid A is unique to bbacteria so the triggered innate immune response underlies some of the common uncomfortable symptoms of
bacterial infections like pyelonephritis, gastroenteritis and cystitis
What is endotoxin overload
eg when large numbers of bacteria die, can be dangerous e.g. in septicaemia, causing septic shock.
Other than Lipid A, what else can trigger an innate immune response to bacteria (2)
other bacterial components, including:
peptidoglycan and flagellin, the major component of bacterial flagella (PAMPs)
Give an example of acute inflammation in the wrong place
meningitis
What causes meningitis
Caused by several pathogens, some of which colonize the nasopharynx asymptomatically (carriers can be important in outbreaks).
Neisseria meningitidis can colonize several percent of the population, but meningococcal disease is uncommon.
What is bacteraemia
What does this lead to
Bacteria cross epithelium into blood
Endothelial cell damage occurs, and bacteria cross the blood/brain barrier. Inflammation in meninges caused primarily by LPS lipid A
Name 4 bacterial pathogens that can cause meningitis
give their Gram stain and shape
Neisseria meningitidis (G -ve coccus )
Haemophilus influenzae type B (Hib) (G -ve rod)
Streptococcus pneumoniae (G +ve coccus)
E. coli (neonatal meningitis) (G -ve rod)
Give an example of host mimicry in E coli and Neisseria
How can the body protect against these strains
E. coli K1 and some Neisseria meningitidis strains have sialic acid capsules (sialic acid is common in host tissues)
Polysaccharides are weakly immunogenic, but anti-capsular antibodies can protect against specific serotypes
How common are the different types of inflammation in bacterial inflammation
acute - common
chronic - rare
Give 2 examples of long term inflammation in bacterial infection
long-term inflammation caused by:
Chlamydia can lead to pelvic inflammatory disease and infertility,
by Helicobacter pylori leads to gastric ulcers and cancer
What is a particular danger of chronic inflammation in bacterial infection
which disease is this particularly common in
granulomatous inflammation
TB (can result from extended host response to persisting Mycobacterium tuberculosis)
What happens in an extended immune response to mycobacterium tuberculosis
Mycobacteria can be successfully cleared by CD4+ T cell-activated macrophages, but the pathogen commonly persists and the resulting infiltration of further large numbers of activated macrophages leads to a granulomatous lesion surrounding the pathogen, characterised by ongoing inflammation, tissue destruction and repair (fibrosis)
Give 6 examples of immunopathoglogy following bacterial infection
chlamydia H. pylori leprosy syphilis Streptococcal glomerulonephritis Lyme’s disease
What causes leprosy
how is this similar to TB
Mycobacterium leprae
involves granuloma formation
What causes syphilis
Treponema pallidum (a spirochaete)
What is Streptococcal glomerulonephritis
(type III hypersensitivity reaction) follows lodging of immune complexes in the kidney after bacterial infection
What causes Lyme’s disease
what does it involve
Borrelia burgdorferi
not only LPS-induced inflammation, but also type III hypersensitivity - deposition of immune complexes in the joints induces inflammation and possibly arthritis, in the vasculature and meninges leads to neurologic symptoms
Name 2 possible examples of bacterial antigens contributing to autoimmune disease
(Streptococcal M protein cross reactivity)
rheumatoid arthritis (chronic Gram-negative bacterial infections).
What bacterium causes pneumonic plague
Yersinia pestis
Where does the term ‘bubonic’ plague come from during a Yersinia pestis infection
Infecting bacteria spread through lymphatics, lymph nodes enlarge (buboes).
When does bubonic plague become pneumonic
when infection spreads from lymphatics and leads to hemorrhagic inflammation, spread to blood,
lung,
then leads to meningitis, epticaemia, multi-organ failure
Why was it called black death
necrotic lesions following spread of Yersinia throughout body
name some of the virulence factors involved in a Yersinia pestis infection
antiphagocytic capsule and protein toxins including ‘injected’ effector proteins called Yops that subvert macrophage function to prevent engulfment.
Additional inflammatory damage caused by LPS lipid A.
What is a common ‘immediate’ cause of death in UK hospitalized patients
Which bacterium most commonly causes it
pneumonia
Streptococcus pneumoniae
What is the progression of a strep. pneumoniae infection (3 key stages0
• Colonizes nasopharynx (adhesin). Resists removal by mucous and ciliated cells by pneumolysin (pore-forming toxin) and IgA protease.
(Host factors predispose to colonisation – alcoholism, viral respiratory infection).
• Migrates to lower respiratory tract. Avoids phagocytosis by alveolar macrophages:
pneumolysin and polysaccharide (carbohydrate) capsule.
Capsule prevents opsonization and aids survival in droplets during dissemination.
• Lung damage by pneumolysin and inflammation
What causes whooping cough
the bacterium Bordetella pertussis,
How is whooping cough usually seen
as an acute severe disease of young children
What is DPT
(diphtheria-pertussis-tetanus) vaccine
How effective has the DPT vaccine been
vaccine is effective, but outbreaks returned in early 1990s when vaccine take-up dropped.
pertussis still infects millions in developing world.
Give the 3 key stages of bordetella pertussis infection
• Aerosol inhalation. Colonisation of respiratory epithelium, aided by adhesins.
Bacteria multiply in mucosa.
- Damage to ciliated epithelial cells and mucosal cells by LPS lipid A (triggers acute inflammation) and exotoxins including pertussis toxin and an adenylate cyclase.
- Accumulation of mucus, inflammatory cells, dead epithelial cells and bacteria in the airway. Fever, bronchitis. This and toxin action on neurons promotes paroxysms of coughing. Life-threatening in infants with underlying disease, can lead to neurological sequelae.
Name a chronic bacterial granulomatous disease and its relative in cattle
TB
Caused by Mycobacterium tuberculosis and closely related M. bovis in cattle
Why did TB incidence increase in the 1980s
is TB a problem now? Who does it predominantly affect?
exacerbated by infection with HIV
drug resistant strains now epidemic. In UK, TB is predominantly a disease of the elderly, homeless, alcoholics, drug addicts, immunosuppressed
Which animals does TB from M bovis affect
Why should non-vets care
a wide range of mammals
Infections in cattle are of particular concern in the UK, as a public health threat (zoonosis) and cause of major economic loss
(restrictions on trade/movement, TB testing, mandatory slaughter and compensation).
How does TB spread from cattle to humans
primarily through non-pasteurized milk.
Before introduction of pasteurization in the UK, there were > 50K cases of TB caused M. bovis.
Is this still a problem?
Bovine TB was brought under control in most of the UK by widespread testing and slaughter of infected cattle. However, the disease remains endemic in the South-West of England and there may be a link to infected badgers acting as a reservoir.
Describe M. tuberculosis as a microbe (4)
a non-motile
obligate aerobe
that grows slowly, 4-8 weeks on complex laboratory medium
acid fast
Why is M. TB called acid fast
it is difficult to destain due to a waxy impermeable cell envelope (mycolic acid) that makes the pathogen resistant to drying and disinfectants, and protects it from immune attack.
How does M. TB spread and what happens when it enters the host
a spread by small droplets
ingested by alveolar macrophages
M. TB Bacteria spread by small droplets and are ingested by alveolar macrophages. How are they not destroyed by these macrophages?
by establishing a ‘safe’ uptake pathway, arresting
trafficking and inhibiting phagosome-lysosome fusion (i.e. phagosome maturation).
What do M. tuberculosis bacteria do once they have entered the alveolar macrophages
multiply in macrophages,
when macrophages lyse, the bacteria can cross the alveolar epithelium, enter the lymphatics, drain to regional lymph nodes and may enter the blood to disseminate around the body - New foci of infection
Which protein secreted by M. tuberculosis facilitates lysis of host cells and dissemination?
what does it do
ESAT6
thought to interact with macrophage membranes and interfere with signaling pathways that e.g. down-regulate production of ROS.
What happens if a M. TB infection persists
immunological shift as CD4+ TH2 cells are produced in addition to protective TH1 cells.
Usually though, M. tuberculosis-sensitized TH1 cells activate macrophages, predominantly via IFNγ.
Increased killing follows TNFα secretion, and production of nitric oxide.
Cell-mediated immunity is established
What is the basis of the tuberculin skin test
CD4 T cell activity following M. TB infection
What happens in the (type IV) granulomatous inflammation following the response to M. TB infection
aggregates of activated macrophages (which may differentiate to epithelioid cells or multi-nucleate giant cells), surrounded by fibroblasts and a few lymphocytes, ‘wall off’ the pathogen
he intense activation often results in concentrated release of lytic enzymes, and leads to a ‘cheese-like’ centre.
Which processes do granulomas show
ongoing tissue destruction (necrosis), repair (fibrosis, scarring) and inflammation
What does caseous necrosis of granulomas in TB lead to
aids further spread of bacterium (miliary TB)
What does the clinical outcome of a M. TB infection reflect
what are these factors determined by?
a complex balance between bacterial survival, the level of cell-mediated immune protection and the amount of tissue destruction caused by the immune response.
largely determined by the immune status of the host and the
virulence of the TB strain.
Why are symptoms of food poisoning seen only a few hours after eating
caused without bacterial colonisation of the host
Name 3 bacteria that can infect you after eating food
what do they grow in and how do they cause symptoms
clostridium botulinum produces botulinum toxin (BoNT) during anaerobic growth in food (e.g. home-canned)
Staphylococcus aureus grows in e.g. custard, processed meats at room temperature (contamination from human carrier).
Stable enterotoxins interact with gastric mucosa and are thought to act as superantigens
Clostridium perfringens
germinates from spores that survive in pre-heated foods (e.g. meat products, anaerobic). Ingested and produces toxins in the intestine. Toxins interact with mucosa, may be superantigens.
What happens to the BoNT from C. botulinum during food poisoning
absorbed from the stomach. BoNT is a protease that cleaves synaptobrevin, a neuronal SNARE.
BoNT acts at the NMJ, preventing exocytosis of the stimulatory neurotransmitter ACh,
causing flaccid paralysis
How do staph. aureus colonise the host in food poisoning
Bacteria are ingested but do not colonize.
How common is botulinum poisoning from home canned food
rare but life threatening
What causes food/ water bourne infections
how long does it take for symptoms to show
Caused by invasive bacteria e.g. Salmonella, Listeria
or
by bacteria that colonize the epithelial surface e.g. Vibrio cholerae
y after one or more days e.g. gastroenteritis, characterised by nausea,
diarrhoea and abdominal pain
What does clostridium difficile cause
what should you think of when you hear this bacteria
pseudomambraneous colitis
antibiotic resistance
Are GI bacterial diseases worrying?
s range in severity, in the developing world they are a major cause of morbidity and mortality, particularly in children.
What causes gastroenteritis in developed countries
when did cases pa in the uk peak
salmonella enterica (usually from chicken and eggs)
1990s: peaked at c.30,000 cases p.a. in UK.
What are the 4 key steps of salmonella’s invasion of the host
- Salmonella induce host cytoskeletal rearrangement (requires SPI-1-encoded needle and effectors e.g. the actin binding SipA, SipC), forces entry directly via the apical surface of epithelial cells of the distal ileum and proximal colon.
- Salmonella remain within a membrane-bound vacuole, ‘a replicative niche’, and multiply.
- Released from epithelial cells - by lysis? Induces inflammation (principally via LPS lipid A). Gastroenteritis.
- Taken up by macrophages and replicate. Survives intracellularly by switching on SPI-2 genes encoding effectors that inhibit maturation of the phagolysosome and confer resistance to defensins and oxidative burst (pumps, superoxide dismutase, catalase)
Which of salmonella’s genes encode effectors that inhibit maturation of the phagolysosome and confer resistance to defensins and oxidative burst?
what are the gene products? (3)
SPI-2 genes
pumps, superoxide dismutase, catalase
Why does the host get diarrhoea during a salmonella infection?
Where does the infection spread to?
Host response activates cAMP production
and fluid secretion
confined to GI tract
What causes typhoid fever?
What is this bacteria’s key animal reservoir
Salmonella typhi
no animal reservoir (human host adapted)
What is the process of the S typhi infection
what causes symptoms
how can it be spread
replicates in macrophages and is spread systemically via the blood
stream to liver and spleen.
Severe symptoms caused by typhoid toxin (CDT,) and response to LPS lipid A.
bacteria are shed in bile, returning to the intestine and environment - carriers are important (eg Typhoid Mary)
How does ETEC colonise the host
how does it cause symptoms
what does it commonly cause
how can it be spread
by fimbrial (pili) adhesins and produces a cholera-like enterotoxin LT (Labile Toxin).
Common cause of diarrhoea in children of developing countries and travellers’ diarrhoea.
Spread typically via contaminated water.
Important disease of animals, especially piglets – same toxin, different adhesins provide host tropism.
What do the following stand for
a) ETEC
b) EPEC
c) EHEC
a) enterotoxigenic
b)enteropathogenic
c) enterohemorrhagic
… E. Coli
How does EPEC colonise the host
initially attach by pilus adhesin.
Delivers effectors, including its own receptor Tir (which binds intimin on EPEC surface), into the target cell to facilitate tight adhesion by cytoskeletal rearrangement.
Generates ‘pedestals’. Inflammation
How does EHEC colonise the host
Attach by pili and pedestal like EPEC.
Inflammation accentuated by Shiga-like toxin (SLT)
What can Shiga like toxin and the inflammation caused by EHEC lead to
renal failure
What is the predominant serotype of E coli found in beef commonly
EHEC O157
Discuss other strains of E coli (ie not EPEC, ETEC, or EHEC)
some cause neonatal meningitis and urinary tract infection (UPEC)
Others are harmless/beneficial members of the gut flora
What accounts for the variability of E coli strains that exist
it is a consequence of genetic flexibility and accumulation of different virulence gene combinations - pathogenicity islands.
Name a food bourne bacteria that behaves in a way analogous to Salmonella
Campylobacter
How can you be infected with Listeria
what are possible complications
who is particularly at risk
food-borne (unpasteurized cheese, salads) Listeria can invade and become systemic.
can cross to the placenta, endangering the foetus, and the blood-brain barrier (meningitis).
Listeria is predominantly dangerous to the immunocompromised.
What causes bacterial dysentery
what characterises this disease
shigella
highly infectious disease
characterized by acute inflammation of the colon and low-volume diarrhea containing blood
mucus and PMN
Is the damage from Shigella directly or indirectly caused?
directly by e.g. shiga toxin and indirectly by the inflammatory response.
What are the principles of disease control (4)
to understand epidemiology,
eliminate or control the sources of infection,
interrupt transmission,
to protect the population.
The principles of disease control are to understand epidemiology, eliminate or control the sources of infection, interrupt transmission, and to protect the population. What does this mean in bacterial disease (3)
- Reducing exposure by disinfection, treating water and sewage, pasteurising food and cooking it properly, limiting insect contact and avoiding significant contact with infected people/animals (quarantine, isolation care).
- Reducing susceptibility of individuals and populations by vaccination.
- Chemotherapy with antibiotics and other antibacterial agents.
Name some of the key vaccines against bacterial infection (8 vaccines, 8 bacteria)
DPT (toxoid for diphtheria and tetanus, killed pertussis)
Polysaccharide from S. pneumoniae, H. influenzae (protects against pneumonia)
2 against meningitis:
Purified polysaccharide from Neisseria meningitides
Capsular polysaccharide of Haemophilus influenzae B coupled to tetanus toxoid (carrier)
2 against typhoid:
Live attenuated vaccine (Ty21A);
Vi capsular polysaccharide vaccine
Cholera: Killed whole cell or crude fraction of Vibrio cholerae
TB: BCG, an attenuated strain of Mycobacterium bovis
What are current strategies for bacterial vaccine development (2)
genetically-engineered attenuated variants (to generate antibody
and cell mediated responses)
subunit vaccines (e.g. toxin, adhesins, capsules) which raise antibody responses to block colonisation or toxin binding to receptors.
What can attenuated salmonella be used for
heterologous (‘cloned’) antigen delivery.
What does the selective toxicity of antibiotics reflect (3)
the differential sensitivity of a target (trimethoprim, aminoglycosides),
differential reliance on a target (sulphonamide),
or
best of all, the absence of a target (β-lactams, glycopeptides) in the host cell.
What is the shape and Gram stain of H pylori
spiral-shaped Gram-negative
Why was the discovery of H pylori’s gastric effects important for medication
Patients no longer faced an expensive, life-long regime of daily ulcer medication, but could be treated with a relatively cheap short course of antibiotics.
What percentage of ulcers are caused by bacteria
how common is this bacterium
Helicobacter pylori causes c.90% of gastric and duodenal ulcers
Up to half the human population is colonized at some time, but only a few percent experience illness, depending on host factors and bacterial virulence.
Where does H pylori colonise
mucin layer in gastric antrum
How are H pylori microbes well adapted to colonise the stomach
Extremely motile by means of flagella
produces urease to raise local pH, making gastric mucous less viscous
both aid swimming towards epithelium
What does urease do after being secreted from H pylori
splits urea into ammonia to raise local pH
How do H pylori cells bind to host cells
using adhesins
H. pylori causes intense mucosal inflammation
and ulceration. What are the major factors in this (2)
(i)
Helicobacter-induced IL-8 (attracts PMNs) production by epithelial cells,
(ii) destruction of epithelial cells by a pore-forming vacuolating cytotoxin (VacA) secreted by H. pylori
Describe VacA
hexmeric toxin produced by H pylori
inserts into the host
cell membrane to form anion-selective channels that are endocytosed.
VacA pores disturb ion balance of late endosomes and water flows in, swelling endosomes to form characteristic vacuoles
What accompanies the progression of a H pylori ulcer
more inflammation, including increasing recruitment of PMNs etc, and tissue destruction.
What might explain the correlation between H pylori and gastric cancer (2)
Possibly chronic inflammation exposes proliferating mucosal stem cells to dietary carcinogens and generates ROS.
It seems also that the bacterial effector CagA, delivered into gastric epithelial cells, interferes with signalling pathways, leading
to increased cell proliferation
Give 2 key conditions caused by antibiotic use
antibiotic-associated diarrhoea
pseudomembraneous colitis
What can C. difficile do following a course of antibiotics
which non human species is this particularly important in
colonises the gut as the antibiotics have eradicated normal gut flora
horses
describe c difficile
a Gram positive toxin producing anaerobic spore-forming bacterium
What does C difficile do to the gut that it has colonised
causes diarrhoea and inflammation of the colonic mucosa, largely through action of two secreted toxins (TcdA and TcdB)
Cell damage is also indirect – inflammation, pseudomembrane
What are TcdA and TcdB
What do they do
toxins secreted from C. difficile
glycosylate small GTPases in intracellular signalling pathways, resulting in subversion of the actin cytoskeleton and disruption of tight junctions – the epithelium becomes ‘leaky’, resulting in cell destruction
What is the primary treatment for C difficile infection
what are other solutions
Antibiotics (e.g. vancomycin) and rehydration therapy
refractory and recurrent infections are treated using faecal transplants.
what is a faecal transplant
where Donor faeces, screened for enteric pathogens, are introduced into the patient’s colon.
name a disease which is reemerging thanks to antibiotic resistance
TB, particularly in immunocompromised patients
Infections of which bacteria are increasing thanks to antibiotic resistance
increased nosocomial infections by, e.g. Staphs (e.g. MRSA) and Gram-negative rods, many of which carry resistance-plasmids.
How is E coli relevant to anti
E. coli causing septicaemia and meningitis are becoming increasingly resistant to the clinically important
fluoroquinolone Ciprofloxacin
Give broad ways antibiotic resistance can be acquired
through point mutations in chromosomal genes or acquisition of ‘new’
genes by conjugation,
transduction or transformation (horizontal gene transfer),
can be selected for by exposure to antibiotics.
What are β-lactamases an example of
Enzyme-mediated inactivation
Give 4 mechanisms of antibiotic resistance
- Enzyme-mediated inactivation
- Alteration of target so that antibiotic no longer binds effectively
- Metabolic by-pass e.g. alternative enzymes for folate synthesis
- Efflux pumps determine multidrug resistance (MDR)
How does penicillin work
how do β-lactamases combat this
Penicillin mimics the D-Ala-D-Ala peptidoglycan crosslink. It binds to the active site of transpeptidase, irreversibly inhibiting peptidoglycan crosslinking. Causes cell lysis
β-lactamases cleave penicillins.
what does chloramphenicol do
binds the 50S subunit
and inhibits peptidyl transferase activity
What do aminoglycosides do
give examples
s (e.g. getamicin, neomycin) alter
conformation of the 16S rRNA in the 30S subunit, where the ribosome ‘reads’ mRNA and recruits aminoacyl-tRNA. Inhibit tRNA selection, polypeptide elongation.
How can chloramphenicol be combated by bacteria
Some resistant bacteria produce acetyl transferases that modify (acetylate) chloramphenicol and aminoglycosides to prevent ribosome binding.
Give 3 examples of alteration of target so that antibiotic no longer binds effectively
e.g. Tetracycline
Fluoroquinolones (e.g. ciprofloxacin)
Vancomycin (a glycopeptide)
What does tetracycline do
how can a bacterial cell become resistant
binds 30S ribosome subunit and blocks binding of aminoacyl tRNA
resistant bacteria produce ribosomal protection proteins (RPPs) that have structural similarity to translation elongation factors. RPPs can dislodge tetracycline from the ribosome.
How do Fluoroquinolones work
how can bacteria become resistant
block synthesis of nucleic acids by inhibiting gyrase and topoisomerase, enzymes that control DNA topology during replication and transcription
R-plasmid encoded quinolone resistance protein, Qnr, binds topoisomerase to physically block binding of the antibiotic.
How does vancomycin work
how do bacteria become resistant
binds D-Ala-D-Ala and blocks access to the
transpeptidase, preventing crosslinking of peptidoglycan
Vancomycin resistant Gram-positive pathogens produce different peptidoglycan biosynthetic enzymes that synthesise D-Ala-D-Lac alternative peptide crosslinks
Some antibiotics inhibit folate synthesis, an essential step in the synthesis of nucleotides. Give 2 examples
how can resistance arise
sulfonamides compete with pABA for binding to the enzyme dihydropteroate synthase (DHPS)
trimethoprim inhibits dihydrofolate reductase (DHFR).
R plasmid-encoded DHPS and DHFR have a much lower binding affinity for the antibiotics
than the normal bacterial enzymes.
name 2 bacteria which have developed efflux pumps determine multidrug resistance (MDR)
e.g. in Gram negative Pseudomonas and E. coli
Describe efflux pumps in Gram Negative bacteria
tripartite
Drugs bind to inner membrane transporter, an ATPase or proton antiporter, and are ejected through a unique TolC exit duct spanning the periplasm and outer membrane
What are some new targets for antibiotics (4)
e.g. lipid A synthesis, bacterial cell division, or resistance pumps themselves
might also target pathogen-specific processes, e.g. adhesin assembly, though these would be narrower range than current antibiotics (more like antivirals)
Other than creating new antibiotics how can we combat antibiotic resistance? (3)
combinations of antibiotics (‘combination therapies’) and/or antibiotic potentiators (e.g. clavulanic acid) that inhibit resistance enzymes (β-lactamase) might also counteract resistance, possible phage therapy?
And more and better vaccines.
