Medical Microbiology and Infection

at a Glance

Fourth EditionStephen Gillespie and Kathleen Bamford

Case Studies

Case 2

A patient presents at A&E 10 days after discharge from hospital following a coronary artery bypass graft. The patient is febrile and there is redness, swelling and a pussy discharge from the sternal wound that has not responded to oral flucloxacillin prescribed by his GP 4 days ago.

  • 1. What investigations would you carry out?

    The patient should be investigated for a surgical site infection. A swab or sample of the pus from the sternal wound should be sent for direct microscopy and culture. Blood cultures should be taken. Evidence of raised markers of infection (peripheral blood total and differential white cell count and C-reactive protein should be measured). He should have a chest X-ray and an MRI to assess for evidence of underlying sternal osteomyelitis.

  • 2. A Gram stain shows numerous pus cells and clumps of Gram-positive cocci. What does this tell you?

    The pus cells confirm the clinical evidence of a purulent discharge. The Gram-positive cocci in clumps suggest a staphylococcal

  • 3. What is the commonest aetiology cause of surgical site infection?

    Surgical site infections are most commonly caused by Staphylococcus aureus. Other organisms such as β-haemolytic streptococci can cause serious infections. Enterobacteriaceae and non-sporing anaerobes can contribute especially following operations that involve the gut and female genital tract.

  • 4. What makes you think this infection might be different?

    It is a concern that this infection has not responded and actually extended while on flucloxacillin, which should have helped if it was caused by Staphylococcus aureus that was sensitive to methicillin. This raises the possibility of methicillin-resistant Staphylococcus aureus (MRSA) or infection with Gram-negative bacilli.

  • 5. What risk factors might predispose to MRSA infection?

    The main risk factor for MRSA infection is previous carriage of this organism or contact with another who is a carrier. Carriage increases with previous exposure to a healthcare setting, residence in a care facility and previous antibiotic exposure

  • 6. How could this infection have been prevented?

    MRSA carriage can be prevented by good infection control practices using contact precautions. Infection can be prevented by preoperative screening, using topical eradication therapy and appropriate antibiotic prophylaxis perioperatively. In this case a glycopeptide such as vancomycin could have been used 30–60 minutes before the operation started.

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