Characterization of small colony variants of methicillin-resistant Staphylococcus aureus regrown in the presence of arbekacin

We examined the biologic and pathogenic characteristics of small colony variants of methicillin-resistant Staphylococcus aureus regrown in the presence of arbekacin at concentrations above the MIC. The cells possessed both coagulase and catalase activities, but not DNase activity. Ultrastructural observations revealed that the cell structure was polymorphic and fragile. The growth rate of these cells was slower, but they were more resistant to arbekacin than those of the parent strain. They were also more easily phagocytized by human polymorphonuclear leukocytes than the parent strain. The 50% lethal dose in mice of small colony variants was higher than that of the parent strain and they were rapidly cleared from the peripheral blood of challenged mice. These differences were more pronounced in normal mice than in leukopenic mice. These findings suggest that the pathogenicity of methicillin-resistant Staphylococcus aureus small colony variants regrown in the presence of arbekacin is low. However, this organism may produce a latent infection by reducing the efficacy of arbekacin in compromised hosts, or in infected foci with deficient neutrophil activity.