Microbiology of the GIT

Question 1

What is a gram-negative, non-spore forming, anaerobic bacteria that is found in the colon?

Answer 1

Bacteroides fragilis

Question 2

What is meant by the term ‘obligate anaerobes’?

Answer 2

These microorganisms are unable to survive in the presence of oxygen (at least at atmospheric
pressure). They survive and thrive in the absence of oxygen.

Question 3

Which parts of the GIT are predominantly anaerobic zones?

Answer 3

Parts of the mouth, small bowel and Colon

Question 4

Describe four beneficial roles that bacteria play in the human GIT

Answer 4

o Prevent colonisation of pathogens
o Stimulate development of Mucosal associated lymphoid tissue
o Stimulate production of natural antibodies
o Kill non indigenous bacteria
o Assist in the digestion of certain polysaccharides
o Play a role in the synthesising of certain vitamins (B12, Vit K)

Question 5

A man is involved in a fight and punches another person in the mouth. He has a small
laceration on his hand as a result. Why is this small wound potentially very serious?

Answer 5

The human mouth contains lots of bacteria (especially anaerobic) and even a small laceration
(especially if it involves a joint) can lead to a serious infection and possible joint destruction. It is
relatively common to see a hand laceration caused by this mechanism.

Question 6

A man with a prosthetic heart valve needs some wisdom teeth removed. Explain why he
might receive prophylactic antibiotics?

Answer 6

This question is really just trying to reinforce the fact that the mouth contains a high concentration
of bacteria. Dental procedures (in fact even brushing and flossing) cause a bacteraemia (presence of
bacteria in the blood) which can increase the chances of a prosthetic heart valve being colonised by
these bacteria. Prosthetic heart valves can be colonised more easily than normal valves leading to
infection of the endocardial surface of the heart (infective endocarditis). Giving prophylactic
antibiotics is thought to reduce the duration of the bacteraemia and hence the likelihood of the
heart valve being colonised.

Question 7

A twenty year old man goes to his GP with what he thinks is tonsillitis. The GP agrees that
it is. Should he be given antibiotics?

Answer 7

There is no right or wrong answer to this very common question, but it is worth noting that most
cases of Tonsillitis are caused by viruses. The decision to give antibiotics should not be a knee jerk
reaction (usually in response to a demanding patient) but based on the presence of Tonsillar
exudates or signs of secondary pharyngitis. Most people should just receive supportive treatments
to reduce their pain and fever and to maintain adequate hydration.

Question 8

A 16 year old girl is brought to her GP by her mother with a twenty four hour history of profuse
vomiting with watery diarrhoea. The GP thinks this episode might be caused by the Norovirus
(formally known as the Norwalk virus). How is this virus spread?

Answer 8

Faeco-oral route by person-person contact or via aerolisation of virus

Question 9

What is the normal incubation period of the norovirus?

Answer 9

Normally 1-2 days

Question 10

What other symptoms other than vomiting and diarrhoes can be present in a Norovirus infection?

Answer 10

Apart from profuse vomiting (non bloody, Non bilious) and watery diarrhoea (non bloody)she may
have suffered abdominal cramps, headache and a low grade fever.

Question 11

How would the GP assess whether this girl was dehydrated?

Answer 11

There are many ways. Common simple ones include the assessment of mucous membranes, skin
turgor and whether or not the person has sunken eyes. Signs of shock in severely dehydrated
patients will include tachycardia and tachypnoea. The girl might mention that she has not passed
much urine.

Question 12

The GP wishes to reassure both this girl and her mother. What duration of symptoms is
likely from Norovirus and what are the treatment options for this illness?

Answer 12

The symptoms usually last 1-3 days and there is no specific treatment just supportive measures
surrounding correct fluid and electrolyte intake.

Question 13

How would you define traveller’s diarrhoea?

Answer 13

Traveller’s diarrhoea is defined as the production of three or more loose watery stools during or
shortly after travelling abroad.

Question 14

Entero-toxigenic E Coli (ETEC) is a common cause of traveller’s diarrhoea. What type of
bacteria is this?

Answer 14

Facultative anaerobic gram-negative bacilli

Question 15

What type of diarrhoea is commonly associated with Entero-toxigenic E Coli (ETEC)?

Answer 15

Watery diarrhoea without blood or mucus

Question 16

Briefly describe the mechanism by which this Entero-toxigenic E Coli (ETEC) causes diarrhoea?

Answer 16

ETEC bacteria colonize the mucosal surface of the small intestine. The bacteria use fimbrial adhesins
(projections from the bacterial cell surface) to bind enterocyte cells in the small intestine. Adhesion
to the mucosal epithelial cells allows for transfer of enterotoxins produced by ETEC bacteria which
stimulate the release of liquid from the cells lining the intestinal walls. ETEC make two toxins, heatlabile
(LT) and heat-stable (ST), that cause intestinal epithelial cells to secrete excess fluid. Some
strains produce only one of the toxins while others produce both.

Question 17

What symptoms does Entero-toxigenic E Coli (ETEC) infection of the mucosal surface of the small colon produce?

Answer 17

ETEC infection results in the production of abundant watery diarrhea and abdominal cramping.
Other symptoms such as fever, vomiting, chills, headache, muscle aches, and bloating can also occur
but are less common. Illness usually lasts 3-4 days following exposure to the bacteria but can persist
for up to 3 weeks.

Question 18

How can Entero-toxigenic E Coli (ETEC) infection be treated?

Answer 18

Supportive measures including rehydration tend to be sufficient for recovery, and
hospitalization or antibiotics are usually not required

Question 19

Name two other common causes (other than ETEC) of traveller’s diarrhoea?

Answer 19

Shigella

Salmonella spp

Question 20

Name a normal vaginal commensal, which can convert glycogen to lactic acid creating an acidic environment which prevents other bacteria (like candida albicans) growing

Answer 20

Lactobacillus (gram-positive)

Question 21

Which colonic bacteria can survive on perineal skin?

Answer 21

E coli
Enterococcus faecalis
Lactobacillus

Question 22

Which microbes are the most common cause of UTIs?

Answer 22

1. E coli (primarily) (gram-negative bacilli)
2. Enterococcus faecalis (gram-positive cocci)
3. Then various gram-negative enteric bacilli (is something abnormal happening to get an infection of these?

Question 23

Which two MSU Gram Stain results do not indicate a UTI?

Answer 23

Gram-positive bacilli

Gram-negative coccus