Surgical microbiology

 

Staphylococcus aureus

• Facultative anaerobe

• Gram positive coccus

• Haemolysis on blood agar plates

• Catalase positive

• 20% population are long term carriers

• Exo and enterotoxin may result in toxic shock syndrome

and gastroenteritis respectively (enterotoxin is pre-formed → rapid onset of symptoms.)

• 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, abscesses, surgical site infections. Common cause of lactational mastitis

• Most common cause of septic arthritis.

Streptococcus pyogenes

• 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.

Helicobacter pylori

• 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 → CO2 & NH3→ 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.

Actinomycosis spp

• Gram positive bacilli.

• Facultative anaerobes.

• May be difficult to culture. Direct visualisation of organisms and sulphur granules from lesions themselves is the easiest way to make a diagnosis.

• It remains a differential of conditions such as hydradenitis suppurativa, particularly if it is occurring in odd locations and with deeper abscesses than usual.