Community acquired bacterial infections Flashcards
Define virulence factor.
Molecules produced by pathogens that contribute to the pathogenicity of the organism
List some common bacterial virulence factors and include their function.
Flagella – movement and attachment
Pili – adherence factors
Capsule – protects against phagocytosis
Endospores – metabolically dormant forms of bacteria – they are heat, cold, desiccation and chemical resistant
Biofilms – organised aggregates of bacteria embedded in a polysaccharide matrix => antibiotic resistant
Give examples of bacteria that possess/exhibit the following virulence factors:
a. Capsule
b. Endospores
c. Biofilms
a. Capsule = S. pneumoniae b. Endospores = Bacillus sp. = Clostridium sp. c. Biofilms = P. aeruginosa = S. epidermidis
What are exotoxins?
A toxin released by a living bacterial cell into its surrounding
What are the five different types of exotoxin and what do they each generally do? Give examples of bacteria that produce such toxins.
Neurotoxins (act on nerves or motor endplate)
- Botulinum toxin
- Tetanus
Enterotoxins (act on GI tract)
Infection diarrhoea – Vibrio cholerae, E. coli, Shigella
Food poisoning – Bacillus cereus, S. aureus
Pyrogenic toxins (stimulate release of cytokines)
- S. aureus
- S. pyogenes
Tissue invasive toxins
- S. aureus
- S. pyogenes
- C. perfringens
Miscellaneous exotoxin
What is an endotoxin?
This is the lipid A part of lipopolysaccharide that is found on the outer membrane of Gram-negative cells (it is shed in steady amounts from living bacteria)
NOTE: so ONLY Gram-negative cells can produce endotoxins
Why can treating patients with Gram-negative infection sometimes worsen their condition?
Antibiotics can cause lysis of the bacteria meaning that the endotoxins are released into the circulation in large quantities
This can trigger an immune response that leads to SEPTIC SHOCK
What is an outbreak?
A greater than normal or greater than expected number of individuals infected or diagnosed with a particular infection in a given time period, or a particular place, or both
How can an outbreak be identified?
Surveillance
Good and timely reporting systems are necessary
Describe the set of criteria to be met for an epidemic case to be confirmed
Possible epidemic case:
- Any person that has developed the symptoms AND has met a laboratory criteria (e.g. isolation of agent).
Probable epidemic case:
- Any person that has met the above criteria AND has been in epidemic country, consumed possibly contaminated food, been in close contact with a confirmed epidemic case.
Confirmed epidemic case:
- Any person meeting criteria for a possible case AND has had strain isolated.
Describe how PCR may be used to identify the outbreak strain
The isolates can be screened by multiplex PCR for characteristic features of the outbreak strain.
- This can be done on stool samples for example (e. coli).
What was the 2011 E. coli outbreak in Germany caused by?
Enteroaggregative shiga toxin producing E. coli
Describe the structure of shiga toxin.
There is an A subunit that is non-covalently associated with a pentamer of protein B
Describe the action of shiga toxin.
Subunit A is the enzymatically active domain
Subunit B is responsible for binding to the host cell membrane
Subunit A cleaves 28S ribosomal RNA in eukaryotic cells thus inhibiting protein synthesis
Bacterial ribosomes are also a substrate for subunit A so it can lead to decreased proliferation of susceptible bacteria (e.g. commensal microflora of the gut)
How was the shiga toxin gene transferred between bacteria?
Bacteriophage
