PMI02-2007/8 Flashcards
What is pathogenicity?
Ability of a microbe to cause disease, discrete (yes/no)
What is virulence?
Degree of pathogenicity of an organism, continuous (spectrum)
What are virulent bacteria?
Bacteria that usually cause disease when they infect a host
What is a virulence factor/gene?
Bacterial component/gene only involved in pathogenesis
What is a housekeeping gene?
Gene involved in all aspects of a bacterium’s life
List Koch’s Postulates.
- Pathogen occurs in every case of the disease and distribution corresponds to that of lesions observed
- Pathogen does not occur in healthy subjects
- After isolated and repeated growth in pure culture, pathogen can induce disease in susceptible animals
Why do Koch’s Postulates not apply to HIV?
Cannot be grown in pure culture (virus)
Poor animal models
Why do Koch’s Postulates not apply to Helicobacter pylori?
Present in healthy people
Poor animal models
Why do Koch’s Postulates not apply to Streptococcus mutans?
Present in healthy people
Not the sole cause of dental caries
Why do Koch’s Postulates not apply to Chlamydia?
Bacterium cannot be grown in pure culture
Why do Koch’s Postulates not apply to Diphtheria?
Bacterial distribution does not match lesions (systemic disease but organism only found in throat)
Why do Koch’s Postulates not apply to Mycobacterium tuberculosis?
Found in healthy (asymptomatic) people (1/3 of pop)
List the Molecular Koch’s Postulates.
- Disease phenotype should be associated significantly more often with the pathogenic organism than with a non-pathogenic strain
- Specific inactivation of the gene(s) associated with the suspected virulence trait should lead to a measurable decrease in virulence
- Restoration of full pathogenicity should accompany replacement of the mutated gene with the wild-type original
What are virulence genes often encoded on and why is this beneficial for microbes?
Mobile genetic elements
Can be swapped between micro-organisms
Give an example of a mobile genetic element.
Plasmid
Transposon
(Bacteriophage)
What is the main benefit of virulence genes for a microbe?
Improves competitive fitness advantage in host
What are pathogenicity islands?
20-50 genes with low G/C ratio found in genome
Often flanked by mobile elements so can be swapped in their entirety
Encode toxin gene systems and other virulence factors
What virulence genes are often found in plasmids?
Adhesins
Antibiotic resistance
Toxins
Why do bacteria adhere to surfaces?
Prevent rapid clearance/protection
Gain a source of nutrients
What three factors are usually required for a bacterium to cause infection?
Transmission
Adherence
Invasiveness (sometimes)
How could a microbe be transmitted?
Inhalation
Ingestion
Inoculation (needles, skin contact, insects)
Fomite route
What is fomite transmission?
Breathing/coughing/sneezing out droplets onto a surface and someone else touching that surface
What cellular structures can be used by bacteria to adhere to a surface?
Flagellae
Fimbriae
What are found within flagellae and fimbriae which aid adhesion?
Specialised surface proteins involved in:
- direct attachment
- signalling to eukaryotic cell to trigger further adhesion or ingestion
What qualities of a microbe can adhesion affect?
Virulence
Tissue tropism (what it can infect)
Is adhesion always linked to virulence? Why?
No
Long-term commensals need to adhere to surfaces too
What does adhesion of a bacterial to a cell/surface cause?
Change in gene expression within bacteria
Induction of eukaryotic intracellular signalling = changes in gene expression and production of compounds (eg antimicrobial markers, adhesion proteins)
What is colonisation?
Presence of micro-organisms without accompanying disease
What is infection?
Presence of micro-organisms resulting in disease
Describe the changes from contamination to infection.
Initial contamination is just the beginning of colonisation, bacteria not always bound
Become resident micro-organisms on adhesion = colonisation; can stay here or progress further
Division and invasion into deeper tissue = critical colonisation
Gross changes in tissue and host = infection
How can bacteria obtain nutrients from a host?
Soluble waste products or host cell not taking up some nutrients
Released from host cells through damage
What are the two options for a pathogenic bacterium to survive in a host?
Evade immune system
Oppose immune function
What is superoxide dysmutase used for?
By microbes to prevent phagocytosis/damage by oxygen radicals
What is the benefit of being invasive for a microbe?
Penetrate mucosal layers and establish at systemic sites
What is septicaemia?
When a bacterial infection enters the bloodstream
What factors can aid the invasiveness of an organism?
Secretion of bacterial enzymes
Anti-phagocytic factors
Toxins that control host cell uptake mechanisms
Give an example of a bacterial enzyme that can aid the invasiveness of an organism.
Strep. pyogenes = collagenases
Staph. aureus = proteases
Haemophilus leucocidins = coagulases
Give some examples of anti-phagocytic factors that aid the invasiveness of an organism.
Capsule to prevent phagocytosis (Strep. pneumoniae)
M proteins by Group A Streptococci prevent antibodies binding
Fc binding proteins to prevent effector functions
Leukotoxins to kill macrophages
Give an example of an obligate intracellular organism.
Some bacteria:
- Chlamydia trachomatis
- Rickettsia prowazekii
- Mycobacterium leprae
All viruses
Give an example of a facultative intracellular bacteria.
Salmonella
Mycobacterium tuberculosis
Legionella
Listeria
What is a facultative intracellular bacteria?
Bacterium that can survive within or outside of a cell
Often have intracellular phases of transcytosis
What cell invasion mechanisms do intracellular bacteria have?
Phagocytosis and escape of phagosome
Inducing macropinocytosis or endocytosis by:
- interaction of invasins with host receptors to control actin
- injection of bacterial proteins into cell
- complex interactions between toxins and proteins
What organism causes typhus?
Rickettsia prowazekii
What does Mycobacterium leprae cause?
Leprosy
Describe the possible life of a dimorphic intracellular bacterium.
Starts as infectious and metabolically inert elementary body
Reorganises itself within phagosome to form a non-infectious and metabolically active reticulate body
Multiplies and leaves cell to repeat
Describe Rickettsiae bacteria.
Gram-negative bacilli
Arthropod-borne
Replicates in cytosol of host endothelial cells after escaping phagosome
Uses host’s actin to escape cell
What common features of obligate intracellular bacteria?
Long generation time as not facing as many environmental pressures
Small size and small genome (can use host mechanisms)
Require exogenous energy supply
May infect non-phagocytic cells
Protected from lysosomal degradation
Use own expression and replication mechanism so not completely dependent on host
No environmental reservoir (acquired from other hosts)
Cannot be grown by standard bacterial culture techniques so difficult to study
What are the advantages of intracellular invasion for a bacterium?
Immune evasion
Easily carried around body
Obtain nutrients from host
Smaller genome (500-1000 genes vs 1500-6000) - “reductive evolution”
What is reductive evolution?
Intracellular bacteria lose genes for metabolic pathways as they can rely on host mechanisms
Not wasting resources
Adaptation to a defined ecological niche
Describe pyogenic inflammation.
Acute fever/increased body temperature
Recognition of pathogen by innate immune system causes:
- release of large amounts of cytokines
- neutrophil and macrophage recruitment
- opsonisation and bacterial killing
Massive reactions may lead to shock
What organisms may cause pyogenic inflammation?
Streptococcus pyogenes
Streptococcus pneumoniae
Staphylococcus aureus
Name an organism that causes granulomatous inflammation.
Mycobacterium tuberculosis
What is a cytokine storm?
Overactivation of a healthy immune system leading to massive production of cytokines
Surge in pro-inflammatory cytokines followed by surge in anti-inflammatory cytokines
Usually multiple surges; leads to tissue damage, multiple organ failure, shock and death
What are the main cytokine participants in a cytokine storm?
TNF-α
IL-6
IL-1
In what conditions are cytokine storms mainly seen?
Sepsis and septic shock
Viral pneumonias
What group of microbes use molecular mimicry to induce a harmful antibody response?
Group A β-haemolytic streptococci
What are superantigens?
Antigens that inappropriately, excessively activate the immune response leading to shock
What is endotoxin?
Membrane structure/toxin associated with Gram-negative organisms, a type of PAMP
LPS
Heat-stable and poorly antigenic but triggers a strong innate immune response leading to shock and fever
What is an exotoxin?
Pore-forming or surface-acting or intracellular toxin
Describe the structure of endotoxin.
Lipid A core with 3 polysaccharide regions
Polysaccharide region split into inner and outer cores
O antigen, a repeating motif of polysaccharides, may protrude from the outer core
What is smooth LPS?
LPS/endotoxin with a long O antigen
Gives shiny colonies
What is semi-rough LPS?
LPS/endotoxin with a very short O antigen
Gives slightly matte colonies
What is rough LPS?
Damaged LPS/endotoxin with no O antigen
Sometimes increases toxicity
How does LPS trigger strong innate immune responses?
Lipid A (of LPS) complexes with LPS Binding Protein (LBP)
Macrophage surface complex (TLR4, CD14 and MD2) recognises LBP
Triggers signalling in macrophage = cytokine storm, iNOS activation (vasodilation) and inflammation
What is endotoxinaemia?
Presence of LPS/endotoxin in blood
What are the possible different actions that membrane-acting exotoxins can have?
Enzymatic (digestion of membrane constituents)
Pore formation
Detergent-like action
Perturb signalling
What is the enzymatic membrane-acting toxin?
Phospholipase used by C.perfringens, Listeria, C.albicans
What size are the pores formed through action of membrane-acting exotoxins and which enzymes may be involved?
Small or large pores = 7-30 subunits
E.coli = α-haemolysin
S.pyogenes = streptolysin O
S.pneumoniae = pneumolysin
Describe the pore formation action by membrane-acting toxins.
Toxins associate with a particular receptor in the membrane (may be protein or cholesterol)
Causes polymerisation of subunits (toxin + receptor)
Undergo conformational change after polymerisation to protrude through membrane
How can a membrane-acting toxin have a detergent-like action?
Hydrophobic protein toxin associates with hydrophobic tails of membrane = disruption
Which types of membrane-acting toxins perturb signalling?
Stable toxin
Specific proteases
Superantigens
What is a stable toxin?
Small, cysteine-rich peptide produced by Enterotoxigenic E.coli (ETEC)
Heat-stable
Binds to extracellular domain of guanylyl cyclase C as it is similar to the natural ligand guanylin
Leads to inappropriate activation of intracellular domain to produce cGMP from GTP
Leads to efflux of Cl- and prevents Na+ influx => osmotic diarrhoea as water is drawn out of cells
How can membrane-acting toxins act as superantigens?
Associate with MHC class II on APCs and cross-link it to a vβ region of a TCR of a non-specific T cell
Causes non-specific activation of T cells (polyclonal activation)
Give an example of a membrane-acting toxin that can act as a superantigen.
TSST-1 from S.aureus (leads to toxic shock syndrome)
What do intracellular toxins do?
Bind cell surface, cross membrane and have enzymatic activity in the cell (may be injected in directly)
What are the different parts of an AB toxin?
A fragment is active
B fragment is involved with binding
Give an example of a neurotoxin.
Botulinum toxin
Tetanospasmin
What do neurotoxins do?
Attack signalling between nerve cells or vesicle docking in synapse
Associated with zinc proteases
What does tetanospasmin do?
Prevents postsynaptic inhibitory molecules from binding so signal never turns off
What does botulinum toxin do?
Prevents release of excitatory transmitter so no signal is generated
Attacks syntaxin, synaptobrevin, SNAP-25
What are the symptoms of tetanus?
Restlessness
Headaches
Irritability
Muscle spasms, eg lockjaw
What are the symptoms of botulism?
Muscle weakness
Excess sweating
Dry mouth
Parkinson’s type symptoms
Describe the diphtheria toxin.
AB toxin - additional T region as part of B region involved in endosome insertion/toxin translocation
T region = series of α-helices with negatively charged loops between
Acidic conditions in endosomes = H+ associate with negative loops which allows the α-helices to closely associate with the membrane and escape endosome
Blocks EF-2 involved in peptide extension (translation) by adding ADP ribose (from NAD) => ADP-ribosylase enzyme
How is diphtheria treated/managed?
Vaccine
Antitoxin
Antibiotics
Rest
What are the symptoms of diphtheria?
Inflamed throat
Fever
Diphtheric membrane in throat
Give examples of ADP-ribosylating enzymes and what they do.
Diphtheria toxin and P.aeruginosa ETA = modifies EF-2
Cholera and pertussis toxins, E.coli labile toxin = modify signalling proteins
Name some non-toxin diseases and their causative agents.
Plague = Yersinia pestis
Typhoid = Salmonella typhoid
Shigellosis = Shigella flexneri
How do non-toxin diseases cause damage?
Use secretion systems to inject toxins directly into cells
What did the type III secretion system evolve from?
Flagellae
What did the type IV secretion system evolve from?
Pili
What did the type VI secretion system evolve from?
Bacteriophages
Describe the type III secretion system.
Resting state = coiled, compressed protein complex spanning the membrane bilayer
Contact with eukaryotic cell causes decompression and protrusion of system into cell to allow toxin secretion into it
Name a genus that uses type III secretion systems.
Yersinia
Describe how the type VI secretion system arose.
Viral DNA injected into bacteria
Bacteriophage incorporated into inner membrane and genome
Over time, these mutate and can be used by the target organism to inject their own toxins into a host
Give an example of an organism that uses type VI secretion systems.
Vibrio cholera
Why are toxins so potent?
Target key cellular components (membrane, protein synthesis, signalling)
Usually enzymatic action so can be used repeatedly
