GI Microbiology

Question 1

Describe the distribution of the normal bacterial flora in the alimentary tract

Answer 1

Stomach: Too inhospitable for normal flora, so has a few Lactobacilli. Bacterial frequency increases further down the alimentary tract
Duodenum: Lactobacilli + streptococci
Ileum: E.coli
Large bowel contains obligate and facultative anaerobes
Rectum contains Enterococci; staphylococci; lactobacilli. Highly variable between people

Question 2

What are the causes of disruption of the gut microbiome?

Answer 2

Dysbiosis: change in the normal flora which may be harmful
Causes inc: IBD + Crohn’s disease
Irritable bowel syndrome – IBS
C.difficile
All antibiotics disturb the normal flora. If this happens, pathogen growth causes Antibiotic Associated Colitis

Question 3

What are the host defences and normal flora?

Answer 3

Structural: Seamless epithelium, tight junctions
Mechanical: Peristalsis, chewing, fluid movement
Biochemical: Gastric acid, bile, mucous
Immunological: Secretory IgA, intra-epithelial lymphocytes

Question 4

What are the benefits and functions of gut flora?

Answer 4

Colonisation resistance; blocks pathogens by:
Reducing free epithelial receptors and pH
Controls oxidative potential (anaerobic growth)

Has metabolites Vit K2, B12 organic acids to benefit the host
Useful in tolerance; antigenic stimulation, Tregs, IL17
Aids digestion

Question 5

What is C.diff overgrowth?

Answer 5

Toxin mediated disease
3 known toxins, causing:
Patchy necrosis w neutrophil infiltration
Epithelial ulcers
Pseudomembranes: leucocytes, fibrin, mucous, cell debris.
Severe diarrhoea

Question 6

How is c.diff overgrowth treated?

Answer 6

Treatable by;
stopping antibiotics    
Using other antibiotics
Antitoxins
Faecal microbiota transplant
Surgery - colectomy

Question 7

What is gastroenteritis?

Answer 7

An acute syndrome characterised by GI symptoms inc:
nausea, vomiting
Diarrhoea and abdominal discomfort
Thought to caused by an infection

Question 8

What damage results from GIT infection?

Answer 8

Pharmacological action of bacterial toxins
Local inflammation in response to superficial microbial invasion
The infection can spread to blood, lymph and other body sites – enteric fevers
Perforation/ulceration of mucosal epithelium

Question 9

How can e.coli be pathogenic?

Answer 9

If an ETEC, EPEC, EIEC or EHEC strain is introduced into the GI tract, diarrhoea or dysentery (bloody diarrhoea) will result.
Virulence factors – toxins, adhesins, invasins
Commensal e.coli are part of the normal flora and these do not produce toxins

Question 10

What are the different mechanisms for diarrhoea?

Answer 10

Bacterial toxins:
Exotoxins increase cAMP which increases fluid loss
Cytotoxins direct cell damage or inhibit protein synthesis

Adherence: Pathogens adhere to and damage the epithelium

Penetration and invasion: disruption of tissue architecture and function, inflammation