Resistance of Staphylococcus aureus to penicillins and cephalosporins: reversal of intrinsic resistance with some chelating agents.

Methicillin resistance of 10 out of 11 strains of S. aureus could be selectively suppressed by a factor as great as >1024 in the presence of EDTA, though the susceptibility of methicillin susceptible (M S) strains of S. aureus was essentially unchanged. EDTA also sporadically, selectively reduced the number of phages that reacted with methicillin resistant (M R) strains of S. aureus, but had essentially no effect on the phage susceptibility of M S strains. The selective effect of EDTA on M R but not on M S strains could also be seen in relation to susceptibility to penicillin, cephaloridine, but not to most other antibiotics. Two other chelating agents, o phenanthroline and 8 hydroxyquinoline, were also tested: the former also had a selective detectable (but lesser) effect on the methicillin resistance of M R strains, but the latter was almost without effect. These results suggest that a metal (or metals) play a critical role in the 'intrinsic resistance' of S. aureus to penicillins and cephalosporins.