CHARACTERISTICS OF METHICILLIN-RESISTANT STAPHYLOCOCCI

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Abstract
Cultures of naturally occurring methicillin-resistant staphylococci obtained from a number of hospitals were examined for the nature and degree of resistance to methicillin and other antibiotics. All the cultures tested were similar in that they consisted of mixed populations in which the majority of cells were of normal sensitivity to methicillin with a minority showing methicillin resistance. The resistant members also differed from the rest of the population in that they grew more slowly even in the absence of methicillin. Pure cultures of the resistant minority were obtained readily but, on repeated transfer in the absence of methicillin, resistance was lost and the cultures reverted to mixed populations similar to the original naturally occurring strains. When methicillin-sensitive staphylococci were repeatedly subcultured in the presence of methicillin, a mixed population was obtained in which only a minority of cells were resistant to the antibiotic; in this respect, the cultures of methicillin-resistant staphylococci selected in vitro resembled the naturally occurring strains. The original cultures of methicillin-resistant staphylococci comprised populations of cells with uniform sensitivity or insensitivity to other antibiotics. The resistance of these staphylococci to methicillin was not due to increased ability to inactivate the drug but to intrinsic insensitivity to methicillin.