2. Community and Hospital 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 the following virulence factors:
a. Capsule
b. Endospores
c. Biofilms
a. Capsule - S. pneumoniae
b. Endospores - Bacillus sp. and Clostridium sp.
c. Biofilms - P. aeruginosa and S. epidermidis
What are exotoxins?
A toxin released by a living bacterial cell into its surrounding
- toxins that damage biological systems
What are the different types of exotoxin?
Give examples of bacteria that produce such toxins.
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- Neurotoxins
- – act on nerves or motor-end-plates.
- Tetanus or Botulinum toxins.
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- Enterotoxins
- – act on GIT:
- Infectious diarrhoea – Vibrio cholera, E. coli, Shigella dysenteriae, Campylobacter jejuni.
- Food poisoning – Bacillus cereus, Staph. aureus.
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- Pyrogenic exotoxins
- – stimulate release of cytokines:
- Staph. aureus or Strep. pyogenes.
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- Tissue invasive exotoxins
- – enzymes that allow bacteria to tunnel through tissue:
- Staph. aureus, Strep. pyogenes, Clostridium perfringens.
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- Misc. exotoxins
- – specific to certain bacteria, function not well understood:
- Bacillus anthracis, Corynebacterium diphtheriae.
List FIVE different bacterial virulence factors and an example of a bacterium for each
- - Capsule - S.pneumoniae
- - Endospores - clostridium, bacillus
- - Biofilms - pseudomonas aeruginosa, staphylococcus epidermis
- - Neurotoxins - tetanus, botulinum
- - Enterotoxins - infectious diarrhoea (V. cholera, E. coli, C. Jejuni, etc …), food poisoning (S. aureus, B. cereus)
- - Pyrogenic exotoxins - S. Aureus, S. pyogenes
- - Tissue invasive exotoxins - S. Aureus, S. pyogenes, C. perfringens
- - Endotoxins - any gram negative bacteria
What is an endotoxin?
This is the lipid A part of lipopolysaccharide that is found on the outer membrane of Gram-negative cells
- NOTE: so ONLY Gram-negative cells can produce endotoxins*
- NOT a protein – a lipid A moiety of LPS - shed in steady amounts from living bacteria.*
Why can treating patients with Gram-negative infection sometimes worsen their condition?
- Antibiotics can cause lysis of the bacteria meaning that the endotoxins/LPS are released into the circulation in large quantities
- This can trigger an immune response that leads to SEPTIC SHOCK
What is an outbreak and how can it be identified?
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
Identified using:
- Surveillance
- Good and timely reporting systems are necessary
- PCR
What was the 2011 E. coli outbreak in Germany caused by?
What were the symptoms of infection by this bacterium?
What was special about the bacterial strain that caused by outbreak?
Caused by: Enteroaggregative shiga toxin producing E. coli O104:H4 strain
Symptoms:
- Gastroenteritis
- Haemolytic uraemia syndrome (acute renal failure + haemolytic anaemia + thrombocytopenia)
Bacterial stain:
- The bacterial strain was an enteroaggregative E. coli strain (EAEC) that had acquired the ability to produce shiga toxin (through phagetransfer)
- Shiga toxin production is a feature of Enterohaemorrhagic E. coli (EHEC)
- This produced a new strain called Enteroaggregative haemorrhagic E. coli (EAHEC)
Describe the structure of shiga toxin.
Describe the action of shiga toxin.
Structure: There is an A subunit that is non-covalently associated with a pentamer of protein B
Action:
- 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
Shiga toxins are encoded on bacteriophages and contribute to horizontal gene transfer meaning they can be given to other bacteria types in phages.
What is the important virulence factor in EAEC?
EAEC can colonise the larger and small bowel → affects gut flora.
EAECs virulence factor – Aggregative Adherence Fimbriae (AAF):
- AFF required for adhesion to enterocytes and stimulates IL-8 response.
- AFF also allows a biofilm formation.
Give examples of respiratory tract infections - the bacteria they are caused by and their virulence factors
a. Legionnaire’s disease – Legionella pneumophilia (gram-ve):
- i. Location – lives in amoeba in – ponds, lakes, air conditioning.
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ii. Route of infection (RoI) – inhalation of aerosols.
- Grows in alveolar macrophages.
- iii. Virulence factor – type IV secretion systems – legionella replicates in legionella containing vacuoles (LCVs) inside cells.
b. Tuberculosis – Mycobacterium tuberculosis (gram+ve):
- i. Virulence factor – has an extra lipid layer & can enter a dormant state for reactivation.
- ii. 77% success of first treatment and 54% success of second treatment – has MDR.
What feature of Mycobacterium tuberculosis makes it more difficult to treat?
It has a mycolic acid outer membrane – this prevents normal antibiotics from getting into the cell
