Medical Microbiology and Infection

at a Glance

Fourth EditionStephen Gillespie and Kathleen Bamford

Case Studies

Case 1

A young, previously fit man presents with a one week history of a ’flu-like illness which has worsened over the past 24 hours. His temperature is 39.5 °C with a heart rate of 100 and increased respiratory rate. His chest X-ray shows diffuse mottled shadowing in both lung fields. A diagnosis of community-acquired pneumonia is made and he is started on co-amoxiclav and clarithromycin. After 24 hours his condition has not improved, his temperature is 39 °C and a repeat chest X-ray reveals multiple nodular shadows with the possibility of a fluid level.

  • 1. What investigations will you carry out?

    Blood and sputum cultures should be performed. Pneumonia is frequently associated with bacteraemia. The inflammatory response should be assessed by measuring the peripheral white cell count, C-reactive protein. The severity of sepsis should be assessed by measuring blood gases and serum lactate.

  • 2. What organisms are the most likely to cause community-acquired pneumonia in a young patient?

    A. Community-acquired pneumonia (CAP) is most frequently caused by Streptococcus pneumoniae, followed in this age group by Mycoplasma pneumoniae. Other organisms that have to be considered in an immunocompetent patient are Streptococcus pyogenes and Staphylococcus aureus (including MRSA) and Klebsiella pneumoniae.

  • 3. What element in the history makes one of these organisms more likely?

    The history is suggestive of a viral infection preceding a worsening of symptoms. This is suggestive of secondary infection with Staphylococcus aureus. Klebsiella pneumoniae could also cause this but would be more common in an older patient with pre-existing lung disease.

  • 4. What features are particularly alarming in this history – what might this be due to?

    It is of concern that there appears to be a worsening of the X-ray appearance and development of lung abscesses. This is consistent with Staphylococcus aureus. The history is suggestive of infection with a Panton–Valentine leucocidin (PVL) producing S. aureus strain. PVL producing S. aureus infections are associated with abscesses and severe necrotising pneumonia.

  • 5. What changes would you make to the antibiotic therapy?

    Failure to respond initially could be due to methicillin resistance, in which case vancomycin should be used. Use of an antistaphylococcal agent that inhibits protein synthesis would be advisable. Clindamycin is a good choice for staphylococcal pneumonia; however, ongoing therapy should be tailored to sensitivity results.

  • 6. What laboratory characteristics will help identify this organism?

    Staphylococcus aureus is a Gram-positive coccus. Characteristically it is differentiated from streptococci by the clump-like configuration of cells and production of catalase. It produces the enzyme coagulase, which differentiates it from the coagulase-negative staphylococci, which are common skin commensals and usually only pathogenic in the presence of foreign material such as intravascular catheters and prosthetic joints.

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