Surgical microbiology

Question 1

Staphylococcus aureus?

Answer 1

• Facultative anaerobe
• Gram-positive coccus
• Haemolysis on blood agar plates
• Catalase positive
• 20% population are long term carriers
• Exo and entero toxin may result in toxic shock syndrome and gastroenteritis respectively
• Ideally treated with penicillin although many strains now resistant through beta-lactamase production. In the UK less than 5% of isolates are sensitive to penicillin.
• Resistance to methicillin (and other antibiotics) is mediated by the mec operon, essentially penicillin binding protein is altered and resistance to this class of antibiotics ensues
• Common cause of cutaneous infections and abscesses

Question 2

Streptococcus pyogenes?

Answer 2

• Gram-positive, forms chain like colonies, Lancefield Group A Streptococcus
• Produces beta haemolysis on blood agar plates
• Rarely part of normal skin microflora
• Catalase negative
• Releases a number of proteins/ virulence factors into host including hyaluronidase, streptokinase which allow rapid tissue destruction
• Releases superantigens such as pyogenic exotoxin A which results in scarlet fever
• Remains sensitive to penicillin, macrolides may be used as an alternative.

Question 3

Escherichia coli?

Answer 3

• Gram-negative rod
• Facultative anaerobe, non-sporing
• Wide range of subtypes and some are normal gut commensals
• Some subtypes such as 0157 may produce lethal toxins resulting in haemolytic-uraemic syndrome
• Enterotoxigenic E-Coli produces an enterotoxin (ST enterotoxin) that results in large volume fluid secretion into the gut lumen (Via cAMP activation)
• Enteropathogenic E-Coli binds to intestinal cells and cause structural damage, this coupled with a moderate (or in the case of enteroinvasive E-Coli significant) invasive component produces enteritis and large volume diarrhoea together with fever.
• They are resistant to many antibiotics used to treat gram positive infections and acquire resistance rapidly and are recognised as producing beta-lactamases

Question 4

Campylobacter jejuni?

Answer 4

• Curved, Gram-negative, non-sporulating bacteria
• One of the commonest causes of diarrhoea worldwide
• Produces enteritis which is often diffuse and blood may be passed
• Remains a differential for right iliac fossa pain with diarrhoea
• Self-limiting infection so antibiotics are not usually advised. However, the quinolones are often rapidly effective.

Question 5

Helicobacter pylori?

Answer 5

• Gram-negative, helix-shaped rod, microaerophilic
• Produces hydrogenase that can derive energy from hydrogen released by intestinal bacteria
• Flagellated and mobile
• Those carrying the cag A gene may cause ulcers
• It secretes urease that breaks down gastric urea> Carbon dioxide and ammonia> ammonium>bicarbonate (simplified!) The bicarbonate can neutralise the gastric acid.
• Usually, colonises the gastric antrum and irritates resulting in increased gastrin release and higher levels of gastric acid. These patients will develop duodenal ulcers. In those with more diffuse H-Pylori infection, gastric acid levels are lower and ulcers develop by local tissue damage from H-Pylori- these patients get gastric ulcers.
• Diagnosis may be made by serology (approx. 75% sensitive). Biopsy urease test during endoscopy probably the most sensitive.
• In patients who are colonised 10-20% risk of peptic ulcer, 1-2% risk gastric cancer, <1% risk MALT lymphoma