9. GITI Flashcards
(28 cards)
How is a gastrointestinal infection a food-associated (borne) infection?
- ‘food’ acts as a vehicle for transmission
- food-handlers contaminate ‘food’
- then pathogen is consumed
- Eg.. Salmonella, Campylobacter
How does GI infection cause food poisoning?
- toxin present in food; gets consumed
- Eg.. Staphylococcus aureus
How does GI infection cause Antibiotic-associated diarrhoea?
- BS antibiotics disrupt normla gut flora
- over-growth of pathogenic microorganism
- Eg.. Clostridum difficile
What is Clostridium difficile?
- bacillus difficilis; normal component of faecal flora of 70% newborn babies and 2-3% healthy adults
What are the characteristics of Clostridium difficile?
- gram-positive rod
- strictly anaerobic
- spore forming microorganism
- toxin producing
- has >100 different genetic types
What are the risk factors for C.diff infection?
- spores; persistent, transmissible, resistant to antimicrobials
- hospitalisation; length of stay & age (65)
- broad spectrum antibiotic use
- hygiene/dirty environment
- close proximity of patients
- naso-gastric tubes
- contaminated equipment
- BP monitoring cuffs
What is the summary of the C.diff infection pathway?
- susceptible patient ingests spores; resist stomach acid
- favourable conditions for spore germination; pH6-6.8, bile salts, aa (aspartic acid, histidine)
- vegetative cells; flagellated; metabolically active; produce potent toxins
- vegetative cells produce spores in the colon; excreted in diarrhoea
- spores can survive in clinical setting
- infection cycle continues
How do C.diff spores germinate?
- germination initiation; germinants include, sugar, aa, nutrients
- loss of heat resistance and ions (K, H, Na) and Ca-DPA complex
- partial rehydration of core
- hydrolysis of cortex and degradation of SASPs
- Rehydration of core and restoration of metabolic activity
- sensitive to antimicrobials
What is a spore resistant to?
antimicrobials
What is a germinating cell sensitive to?
antimicrobials
How does C.diff adhere and multiply?
- uses S-layer proteins (adhesins)
- flagella
How does C.diff produce toxins?
- genes located on PaLoc
- Toxin A: enterotoxin
- Toxin B: cytotoxin
What are the function of toxins produced by C.diff?
- modification of Rho proteins
- Stimulate cytokine production
What is the MOA for C.diff toxins A and B?
- Toxins A and B enter gut lumen and attach to surface of intestinal epithelial cells.
- toxins cause opening of tight junctions between cells, compromising integrity of barrier.
- The toxins target Rho proteins, which maintain the cytoskeleton. Toxins inactivate Rho proteins leading to disruption.
- Actin cytoskeleton is disrupted causing ‘cell rounding’.
- Damage stimulates production of IL-8, attracting immune cells; immune response activation
What are the clinical manifestation of C.diff?
- asymptomatic
- explosive diarrhoea
- pseudomembranous colitis
- fulminant colitis
- death
What did the C.diff ribotype 027 cause in 2003?
2003: Canadian outbreak
7000 sufferers, 600 deaths
- higher levels of toxin A + B
- base pair depletion in toxin regulating gene
- resistant to ciprofloxacin/ moxifloxacin
What did the C.diff ribotype 027 cause in the UK 2004?
- Stoke Mandeville
12 deaths, 300 infections - Royal Devon and Exeter
13 deaths - Maidstone and Tunbridge Wells
90 deaths, 1170 infections
What is the PaLoc?
- positive regulator gene controlling expression
- encodes for protein porin associated with release of Toxin A/B from the cell
- downstream negative regulatory gene controlling expression of tcdA and tcdB
How can C.diff infection be spread?
- close patient contact
- filthy conditions:
shower cubicles
eating/drinking area
faecal stained patient commodes
What is the lab diagnosis of C.diff associated diarrhoea?
- clinical sample; faeces
- cat2 pathogen
- non-culture techniques; toxin detection (cell lines/ EIA) and GDH detection (EIA)
- culture; selective agar (CCFA)
- typing; ribotyping; outbreaks
How can a culture be used for lab diagnosis?
- selective CCFA
(cefoxitin, cycloserine, fructose)
identification of colonies
- ground-glass appearance
- odour of colonies
Why is Ribotyping used?
- genomic DNA encoding for 16S and 23S rRNA is a highly conserved region
- genetic info coding for tRNA will vary less within bacteria of same strain than diff strain
- it is highly discriminatory
What is the basic principle of Ribotyping?
- C.diff genomic DNA is digested with appropriate restriction endonucleases and separated by gel electrophoresis
- specific universal oligonucleotide probes target conserved regions
- it is visualised
- band pattern compared to a database of established C.diff ribotypes
What are antimicrobial treatment strategies for C.diff?
- discontinuation of precipitating antibiotic
- replacement fluid and electrolytes
- antibiotics for treating C.diff
oral vancomycin (250mg QDS; 10-14 days)
oral metronidazole (250mg QDS; 10-14 days)
