Isolation of two methicillin-resistant Staphylococcus aureus strains with atypical colony morphology

Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging problem in Belgian hospitals. Control of the spread of this organism in a hospital is very important, and correct and rapid identification of MRSA is therefore essential. Previously, we found that mannitol-negative strains, nonpigmented isolates and strains that do not agglutinate when taken from the primary culture plates (unpublished data). These characteristics lead to increased difficulties in the detection of these problem organisms. Now, we describe two MRSA strains with atypical colony morphology which could result in a delay in identification and possibly also in misidentification. One strain was cultured from a patient with osteomyelitis, and the other from an ICU patient with pneumonia. The strains were isolated within a one-year interval. In both patients MRSA strains with a usual colony morphology had been isolated from previous specimens, and both patients had been treated with vancomycin. Several specimens from both patients were cultured, and a different proportion of variant colonies in combination with normal colonies was found on each occasion.