Methicillin-resistant strains of Staphylococcus aureus: relation between expression of resistance and phagocytosis by polymorphonuclear leukocytes.

Virulent strains of staphylococci are known to resist phagocytic destruction better than avirulent strains. In this context, in vitro elimination by human polymorphonuclear leukocytes of eight methicillin-resistant strains of Staphylococcus aureus of unknown virulence was studied. After incubation for 1, 3, 5, or 24 hr in a modified phagocytic assay, the methicillin-resistant strains survived as well as other virulent but methicillin-sensitive strains of S. aureus. Highly resistant subpopulations were obtained from three parent strains, and a methicillin-sensitive revertant subpopulation from one resistant parent strain. All subpopulations were eliminated to the same extent as were the moderately resistant parent strains. When the phagocytic assay was performed in the presence of 25 microgram of methicillin/ml, only the methicillin-sensitive strains and the sensitive subpopulation derived from one resistant parent strain were eliminated after incubation for 24 hr. These in vitro data are further evidence against the use of methicillin in infections due to these organisms.