Unstable resistance to methicillin has been demonstrated in an isolate of . Loss of resistance occurs rather slowly but consistently on nutrient agar kept at bench temperature. The resistant organism exhibits an “optimal zone” of inhibition by intermediate concentrations of the drug, and this is antagonised by cephalosporins and other penicillins. These effects are markedly modified by the medium, inoculum size and temperature. Neither of the phenomena has been observed with the sensitive variant. Against penicillin there is a paradoxical effect in that with light inocula the methicillin-resistant organism is much more resistant to penicillin than its sensitive variant, whereas the reverse holds for heavy inocula.


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