Principles Of Pathogenesis - Bacterial Infection

Question 1

Ways in which disease can come about

Answer 1

1) some bacteria are entirely adapted to the pathogenic way of life in humans. Never form part of the normal flora but may cause subclinical infection e.g. M tuberculosis
2) some bacteria which are part of the normal flora acquire extra virulence factors making them pathogenic e.g. E. coli
3) some bacteria from the normal flora can cause disease if they gain access to deep tissues by trauma, surgery, lines, especially is associated with a foreign body e.g. S. Epidermdis
4) in immunocompromised patients many free living bacteria and components of the normal flora can cause disease, especially if introduced into deep tissues e.g. Acinetobacter

Question 2

How do we know that a given pathogen causes a specific disease?

Answer 2

Koch’s postulates

• the pathogen must be present in every case of the disease
• the pathogen must be isolated from the diseased host and grown in pure culture
• the specific disease must be reproduced when a pure culture of the pathogen is inoculated into a healthy susceptible host
• the pathogen must be recoverable from the experimentally infected host

Question 3

The iceburg concept of infectious disease

Answer 3

Asymptomatic infection –> under water
Less sever infection –> water surface
Classical clinical disease –> out of the water

Spectrum of virulence
Rabies 100% of infections are clinically apparent
Rubella 59% of clinical infection are clinically apparent
Poliomyelitis 0.1-1% are clinically apparent

Question 4

How do we know that a given pathogen causes a specific disease

Answer 4

Find a link between the potential pathogen isolated from or detected in clinical samples
The patients clinical conditions
Recognised syndromes e.g. Septicaemia p, endocarditis, osteomyelitis, meningitis, UTI, pneumonia, pharyngitis

Question 5

What is the evidence for a potential pathogen being clinically significant - particularly for bacteria

Answer 5

Isolated in abundance 
Isolated in pure culture 
Isolated on more than one occasion 
Isolated from deep tissues 
Evidence of local inflammation
Evidence of immune response to pathogen 
Fits with clinical picture

Question 6

Body surfaces have a normal bacterial flora

It can be a nuisance in that -

Answer 6

It can contaminate specimens
It can cause disease
When being treated with antimicrobials the normal bacteria can be removed which can cause a super infection, usually with resistant microbes

Question 7

Body surfaces have a normal bacterial flora - this is beneficial in that

Answer 7

It can protect against infection by preventing pathogens colonising epithelial surfaces - colonisation resistance

Question 8

Bacterial virulence - a simplistic view

Answer 8

Some bacterial proteins - exotoxins can elicit the features of a bacterial infection when injected as pure proteins e.g.
- tetanus toxin, botulinum toxin, anthrax toxin, diphtheria toxin
Vaccination with inactivated toxins - toxoids led to the decline in the incidence of many bacterial infections
Lead to the simplistic idea that all bacteria need to cause disease is a single toxin

Question 9

More sophisticated view of bacterial virulence

Answer 9

May different ways to define virulence factor

• needed to colonise and /or damage tissues: Molecular koch’s postulates: delete gene = loss of virulence, add gene back (on a plasmid) = restore virulence, biochemical evidence of damaging potential
• distinguishes pathogen from commensal - comparative genomics
• expressed or essential in vivo, but not in the lab?

Virulence process is
MULTIFACTORIAL
MULTIDIMENSIONAL - programme of events organised in time and space

Question 10

Steps in successful infection

Answer 10

Sex comes before disease - acquire the virulence genes
Sense environment - and Switch virulence genes on and off
Swim to the site of infection
Stick to the site of infection
Scavenge nutrients - especially iron
Survive stress
Stealth - avoid immune system
Strike back - damage host tissue
Subvert - host cell cytoskeletal and signalling pathways
Spread - through cells and organs
Scatter

Question 11

Bacterial sex - acquiring virulence genes

Answer 11

3 ways in which bacteria exchange DNA

• transformation: cells take up naked DNA
• transduction: phages carry DNA
• conjugation: cells mate through specialised appendages

Question 12

Bacterial sex - mobile genetics elements

Answer 12

• transposons - ST enterotoxin genes
• virulence plasmids - e.g. Toxins
• phage- encoded virulence - e.g. Botulinum toxin, diphtheria toxin, shiga- like toxin, staph toxin, TTSS substrates in salmonella

Question 13

Bacterial sex- pathogenicity islands

Answer 13

Defining features:
Carriage of (many) virulence genes
Presence in pathogenic versus non-pathogenic strains
Different G+C content from host chromosome
Occupy large chromosomal regions (10-100kb)
Compact distinct genetics units, often flanked by DRs, tRNAs, ISs
Presence of (cryptic) mobility genes
Unstable, prone to deletion

Often encode secretion systems:
LEE region in EPEC
Spi1, Spi2 in salmonella
Cag in H. Pylori

Can also encode adhesins, siderophores, toxins

Question 14

Sense environment

Answer 14

Sense environmental changes
Temp, nutrient availability, osmolarity, cell density
Simple cases - changes in iron linked directly to gene expression
Complex- sophisticated signal transduction cascades allow bacteria to regulate gene expression in response to environmental cues

Question 15

Switch virulence factors on and off

Answer 15

-changes in DNA sequence
Gene amplification
Genetic rearrangements - Hin flip flop control of flagellar phase variation
- transcriptional regulation
Activators and repressors (helix-turn-helix motif)
mRNA folding and stability
- translational regulation
- post translational regulation
Stability of protein, controlled cleavage
Covalent modification

Question 16

Swim

Answer 16

Many pathogenic bacteria are motile
Motility is crucial for virulence in some cases
Usually organelle of motility = flagellum
Variants - twitching motility, swarming

Question 17

Stick

Answer 17

To avoid the physical and immunological removal bacteria must adhere to
- cell surfaces and extracellular matrix, GI, redo tracts
Solid surfaces - teeth, heart valves, prosthetic material
Other bacteria

Use:
Direct interaction
Molecular bridging - fibronectin
Adherence often combined its manipulation of host cell signalling and cytoskeleton, invasion and intimate adherence

Common adherence mechanisms
- capsules and slime 
- biofilm formation 
Gram pos adhesins 
-MSCRAMMs
- fimbriae 
Gram neg adhesins (CHO and protein receptors)
- fimbriae, afimbrial ashesins, other membrane proteins, Type III-IV

Question 18

Scavenge nutrients e.g. Iron

Answer 18

Free iron low in the body fluids
-acute phase response causes further drop
-iron overload inc susceptibility to infection
Bacterial mechanisms for scavenging iron
-siderophores chelate iron and transport into bacteria
- iron can be scavenged direct from host iron - binding proteins e.g. Lactoferrin binding protein
- other pathogens remove the need for iron
Iron used to regulate aggressive virulence factors
- diphtheria toxin
-Shiga like toxin

Question 19

Survive stress

Answer 19

Stresses pathogens face:
Nutrient, acid stress in stomach, heat shock during fever, oxidative stress within phagocytes
Stress response proteins - chaperonins feature as immunodominant antigens
Detoxification proteins play a role in virulence e.g. Periplasmic Cu, Zn-superoxide dismutases
Infectious dose for enteric pathogens much lower in achlorhydria (no need to overcome acid stress)

Question 20

Stealth - avoid the immune system

Answer 20

IgA proteases - metalloproteases active against IgA 
Immunoglobulin-binding proteins 
E.g. Protein A of S aureus 
Resistant complement, opsonisation
- capsule
-LPS 
-surface proteins and OMPs 
Antigenic mimicry 
Antigenic or phase variation 
- involves surface structures such as proteins, LPS, capsules, variety of mechanisms: slip strand mispairing, cassettes 
Adopt cryptic niche
- inside phagocytes
- in biofilm

Question 21

Strike back - damage host tissues

Answer 21

Endotoxin - actions:
Pyrogenicity, leukopenia then leucocytosis
Hypotension- Gram neg shock, life threatening complication of septicaemia, most effects of endotoxins are mediated by TNF - attempts at therapy using anti-TNF
Exotoxins
- toxins acting on cell membrane
- toxins active inside cells
- superantigens
Membrane damaging exotoxins
Form pores in eukaryotic cell mems, producing oligomeric rings
Other toxins such as phospholipases, degrade components of membrane
Toxins active inside cells
Toxins often consist of translocation and binding B subunit that delivers the active A subunit into the host cell cytoplasm

Question 22

Spread

Answer 22

Spreading factors - degradative enzymes that allow bacteria to spread through tissues
Strep pyogenes secrete-
Hylauronidase- destroys connective tissue
DNass this viscous pus
Streptokinase - activates plasminogen –> plasmin
Plasmin destroys fibrin clots