Methicillin-Resistant Staphylococci: Genetics of the Minority Population

Abstract

SUMMARYA methicillin-resistant strain of Staphylococcus aweus (no. 5982) contained cocci with d8ering intrinsic resistances to methicillin. Most of the population was slightly more resistant than a typical methicillin-sensitive staphylococcal isolate, but a few were mutants with ability to grow a t markedly increased concentrations of methicillin (500 ,ug./ml. or more). Penicillinase production was not essential for methicillin resistance since a penicillinase-negative variant of strain 5982 was able to produce penicillinase-negative strains with high degrees of methicillin resistance. Methicillin-sensitive strains also gave rise to mutants with increased intrinsic resistance to methicillin, but, in the mutants from methicillin-resistant strains, the increase in resistance was much greater. The mutants were characteristically slower growing than the wild type. Many were small colony variants which reverted to the parent type when passed in liquid media. With 5 yo NaC1-methicillin plates strain 5982 exhibited a phenotypic increase in resistance. On such plates a few L-type colonies were produced at high concentrations of methicillin.

Mechanisms of heteroresistance in methicillin-resistant Staphylococcus aureus

“…The highly resistant subclones are not genetically stable in all strains (24), as shown here. In some strains subclones picked at high concentration revert more readily.…”

Methicillin-resistant staphylococci: detection methods and treatment of infections

“…Most strains of methicillin-resistant staphylococci are heterogeneous in their phenotypic expression of the resistance despite genetic homogeneity (56,68,76,85). Typically, 1 cell in a population of 104 to 108 expresses resistance as defined by its ability to grow in the presence of high concentrations of beta-lactam antibiotic (e.g., 50 ,ug of methicillin per ml).…”

Resistance of Klebsiella and Enterobacter to the Penicillins

“…2 2 , however, charaateristically produced mwtants with diminished colonial size. As wilth small colony variants from hetere resistant (methicillin resistant) staphylococci, revertants producing normal-sized colonies were frequent (2). An interesting although unexplained property of mutants from strain no.…”

“…Dicloxacillin suppressed confluent growth on high concentrations of penicillinase-susceptible penicillins thereby revealing isolated colonies of mutants with large one-step increases in intrinsic resistance reminiscent of highly resistant mutants produced by heteroresistant (methicillin resistant) staphylococci (2,13). Large increases in penicillin resistance contrast with small multistep increases previously considered characteristic o f penicillin resistance (14).…”

“…Use of a dilute inoculum reduces the effect of penicillinase activity and permits evaluation of intrinsic resistance. Penicillinase activity is completely eliminated in penicillinase-negative variants (1,2). No one has reported penicillinase-negative variants of gram-negative bact eri a.…”