Medical Microbiology and Infection

at a Glance

Fourth EditionStephen Gillespie and Kathleen Bamford

Case Studies

Case 20

Mr S. O'D is a 32-year-old man from Ireland who was released early from prison under the terms of the 1997 Good Friday agreement. Some 10 years ago he was the victim of a punishment shooting and since then has suffered from a discharging sinus below his left knee. The clinicians managing the case have always noted that the discharge appeared blood-stained. A number of different organisms have been isolated in the past and he has been given a succession of different antibiotics with varying success.

  • 1. What is the underlying diagnosis in this case?

    The discharging sinus is likely to originate in the bone, so a working diagnosis would be chronic osteomyelitis.

  • 2. How would you investigate this radiologically?

    In chronic osteomyelitis changes in the bone are well established, and thus a plain X-ray should reveal signs of chronic disease. A bone scan or labelled white cell scan is likely to show that there is chronic inflammation at the site.

  • 3. What further simple investigations are necessary?

    Making an aetiological diagnosis is important here as it is essential to choose the correct antibiotic that penetrates into bone. Empirical therapy would be with flucloxacillin and fusidic acid, but if the patient does not have a staphylococcal or streptococcal infection the natural history of the chronic osteomyelitis will not be interrupted. A sample of pus should be taken for culture and susceptibility testing.

  • 4. What other investigations may become necessary should the initial test prove negative?

    In view of the importance of making an aetiological diagnosis, an operative sample could be considered. This may have the added advantage of removing infected involucrum.

  • More info: The initial sample of pus grows Gram-negative bacilli, the colonies of which appear bright red on the agar. The laboratory identifies these as Serratia marscescens.
  • 5. What is the significance of the isolate?

    Because this isolate was from a swab that could have been contaminated during sampling the organism isolated must be evaluated for clinical significance. S. marscescens is an environmental organism and, given the circumstances of the original injury, could be relevant here.

  • 6. What is the significance of the red colour on the plates?

    This is a microbiological curiosity — as these organisms are pigmented. This information does help to establish the significance of the isolate, as previous clinicians examining this discharge have found it to be "blood-stained". This tends to support the idea that the organism has been present before and strengthens the case for its aetiological role.

  • 7. How should the patient be managed?

    The patient should be commenced on intravenous antibiotics to which the Serratia is sensitive. The first choice would be ciprofloxacin as this will penetrate to the site of infection more effectively than β-lactam antibiotics. Long-term antibiotic therapy coupled with drainage and removal of any involucrum may effect a cure.

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