Suppression of intrinsic resistance to methicillin and other penicillins in Staphylococcus aureus.

The pH of the medium in which staphylococcal susceptibility to penicillins was determined was found to make a profound difference (128- to 8,000-fold) in the expression of "intrinsic" resistance, whereas beta-lactamase-mediated resistance was only slightly affected by pH; methicillin-resistant staphylococci that are beta-lactamase-negative are models of pure intrinsic resistance, and the common beta-lactamase-producing organisms (methicillin-susceptible) are examples of pure beta-lactamase-mediated resistance. Methicillin-resistant staphylococci were unable to express their resistance at pH 5.2. However, growth of methicillin-resistant organisms in acid (pH 5.2) medium, followed by susceptibility testing at pH 7.4, showed no elimination of the genotype for intrinsic resistance, indicating that the pH effect was due to suppression, rather than to elimination of the gene determining the intrinsic resistance. These pH changes had little effect on the susceptibility of staphylococci that possessed neither intrinsic resistance nor beta-lactamase-mediated resistance. Thus, the suppression of "intrinsic" resistance was highly specific, and probably not the result of a change in ionization of the antibiotic, which would have been expected to affect all cells essentially equally. It is unlikely that foci of inflammation in man become sufficiently acid to suppress methicillin resistance of the staphylococci causing infection and inflammation.