Introduction to the Microbiology of the Gut

Question 1

Describe the distribution and function of the normal flora in the GIT

Answer 1

• All surfaces of the gut have normal flora
o Resident and transient flora change over its lifetime – affected by diet, environment, stress, hormones, age

• In babies, there’s a transition between Facultative anaerobes to Bifidobacteria (assist digestion of lactose in milk) to Adult-like gut microbes

Question 2

What are the biological relationships of gut flora?

Answer 2

• Symbiosis – interaction between different species
o Commensal, Parasitism, Mutualism
o No one species of gut flora is commensal, but together they can be

• Stomach is too inhospitable for normal flora
• Distal to the stomach, there are varying % of bacteria in different environments
o Flora can change in disease – can affect digestion, absorption, energy balance, metabolism, endocrine functioning

Question 3

What are the benefits of gut flora?

Answer 3

• Colonisation resistance – blocks pathogens
• Production of Metabolites of benefit to the host
• Normal development of immunity – tolerance, antigenic stimulation
• Aids digestion – fermentation of sugars

Question 4

Describe the host defences present

Answer 4

Structural:
• Epithelium, tight junctions – rapid turnover

Mechanical:
• Peristalsis, chewing, fluid movement

Biochemical:
• Gastric acid, bile, mucous

Immunological:
• Secretory IgA, intra-epithelial lymphocytes

Question 5

What are Prebiotics?

What do they promote?

What is their clinical relevance?

Answer 5

• Nutrients that alter the gut ecosystem
• Fermented in the colon by Probiotic organisms – promotes growth of Probiotes
• Control of diarrhoea/constipation etc.

Question 6

What are the 3 types of GI infections?

Answer 6

1. DIARRHOEA - watery/liquid stools usually with an ↑Stool Weight and Frequency
   o Causes severe dehydration – excess fluid and electrolyte loss
   o Causes long-term morbidity, and reduced growth
2. DYSENTERY - inflammatory disorder of GIT, usually in large bowel
   o Passing blood and pus in diarrhoea
   o Pain, fever, abdominal cramps
3. GASTROENTERITIS - inflammation of GIT, usually in stomach and small bowel
   o Nausea, vomiting, diarrhoea

Question 7

What are the patterns of GI infections?

Answer 7

• Acute diarrhoea with blood and pus – Dysentery
• Chronic diarrhoea with/without malabsorption – Poor fat absorption = Steatorrhoea
• Infective Proctitis – ano-rectal STIs
• Dyspepsia – Peptic ulcers
• Dysphagia – Oesophageal Candidiasis (yeast infection)

Diseases can go from non-inflammatory, to an inflammatory response, to penetrating fevers

Question 8

Compare Food poisoning and Food associated infection

Answer 8

Food Poisoning:
• Poisoning of pre-formed toxins in food
• Short incubation

Food associated infection:
• Infection with live organisms
• Long incubation

Question 9

What are the mechanisms of disease in the Gut?

Answer 9

1. Pharmacological action of bacterial toxins local/distant to site of infection - e.g. Cholera
   o Enterotoxins, Exotoxins, Cytotoxins
2. Local inflammation in response to superficial microbial invasion - e.g. Dysentery
   o Damage to epithelium through adherence
3. Deep invasion to blood and lymphatics - e.g. Enteric fever, Hep A
4. Perforation and ulceration of mucosal epithelium