Increase in cell size and acid tolerance response in a stepwise-adapted methicillin resistant Staphylococcus aureus mutant

Abstract

A methicillin-susceptible strain of Staphylococcus aureus (MSSA) showed stepwise adaptation when grown in increasing concentrations of oxacillin, eventually reaching a maximum of 35 ?g/ml. The resultant oxacillin resistant mutant strain was stable and did not revert to susceptibility on frequent subculturing. The response of the cells to different concentrations of oxacillin was examined by scanning electron microscopy, which showed that the size of the bacterium increased with increasing concentrations of oxacillin. These changes in cell size were dependent on the concentration of oxacillin and occurred only after addition of non-lethal concentrations. In the presence of lethal concentrations (?35 ?g/ml) that completely inhibited cell growth, the cell sizes were smaller than those of wild-type cells and irregular in shape. This stepwise-adapted methicillin (oxacillin) resistant S. aureus (MRSA) mutant showed a greater acid tolerance response (ATR) to lactic and citric acids than the parent susceptible strain. These data indicates that methicillin resistance alters the morphology and ATR in stepwise-adapted MRSA mutant cells. © 2007 Springer Science+Business Media B.V.