Skin and Soft tissue – Short answer questions- annotated

  1. Describe how Staphylococcus aureus is identified in the lab : a discussion of methods should include:
  • Gram stain appearance
  • Colony morphology on blood agar – may be haemolytic- colour etc
  • Coagulase detection – bound coagulase and tube coagulase – NB these are not equivalent procedures. Tube coagulase is the most reliable of the two methods
  • Other phenotypic characters – utilises mannitol , DNASe production, presence of protein A on surface (usually detected in a combined latex particle agglutination reaction that also includes bound coagulase), MALDITOF – commonest now in Australia – look it up!
  • Molecular – presence of nuclease gene by PCR is a common way

In general for important specimens – e.g. blood, the identification should be by at least two phenotypic methods – eg Mannitol positive and tube coagulase positive.

2.  What antibiotics are useful for treating S. aureus skin/soft tissue infections (boils, infected exzema or scabies)

  • For Methicillin -susceptible S. aureus (MSSA):  fluclox IV for infection with sepsis or positive blood culture. Oral diclox or flucloxacillin, cephalexin or amoxycillin+clavulanate.  Some strains are betalactamase negative and susceptible to penicillin.
  • For MRSA: vancomycin IV for severe infection with sepsis or positive blood isolate. Depending on tested susceptibility, cotrimoxazole, doxycycline or a macrolide – erythromycin or lincosamide- clindamycin.

For either: dependent on the local antibiogram, cotrimoxazile or doxycycline may be suitable.

Distinguish the approach to impetigo: important trial –

  • Bowen-A, Short-course oral co-trimoxazole versus intramuscular benzathine benzylpenicillin for impetigo in a highly endemic region: an open-label, randomised, controlled, non-inferiority trial . Lancet 2014.

3. What is the current pattern of susceptibility for these antibiotics at PMGH in 2016 ?

More soon.

4. Describe the pyogenic (direct infection consequences) and non-pyogenic complications of group A strep (Streptococcus pyogenes) infection

Important text book reading and understanding about pathogenesis in general!

5. Read through the Tropical Microbiology presentation from Prof. Baird (NT) – describe some of the Gram negative pathogens associated with skin and soft tissue disease in the tropics.  Australian-tropical-skin-bacteriology-rbaird.  Vibrio, Shewenella, Aeromonas, Melioidosis.

About mdjkf

Microbiologist and Infectious Diseases Physician
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