Phenotypic expression of oxacillin resistance in Staphylococcus epidermidis: Roles of mecA transcriptional regulation and resistant-subpopulation selection

The MICs for many oxacillin-resistant (OR) Staphylococcus epidermidis (ORSE ) strains are below the Staphyococcus aureas methicillin or oxacillin resis tance breakpoint. The difficulty detecting the OR phenotype in S. epidermid is may be due to extreme heterotypy in resistance expression and/or transcr iptional repression of mecA, the OR gene, by MecI. To determine the role of these factors in the phenotypic expression of ORSE, 17 geographically dive rse mecI(+) ORSE isolates representing 14 distinct pulsed-field gel electro phoresis pulse types (>3 band differences) were investigated. Thirteen of t he 14 types contained mecI and mecA promoter-operator sequences known to be associated with maximal mecA repression, and in all isolates, mecA transcr iption was repressed. All 17 were heterotypic in their resistance expressio n. Oxacillin MICs ranged from 1 to 128 mu g/ml and increased for 16 of 17 i solates after beta-lactam induction. Allelic replacement inactivation of me cI in three isolates similarly resulted in a four- to sevenfold increase in MIC. Zn the two of these three isolates producing beta-lactamase, mecA tra nscription was regulated by both mecI and beta-lactamase regulatory sequenc es. Heterotypic expression of resistance in these three isolates was unaffe cted by either beta-lactam induction or mecI inactivation. However, prolong ed incubation in concentrations of oxacillin just sufficient to produce a l ag in growth (0.5 to 1.0 mu g/ml) converted the population resistance expre ssion from heterotypic to homotypic. Homotypic conversion could also be dem onstrated in microtiter wells during MIC determinations in one isolate for which the MIC,vas high. We conclude that the phenotypic expression of S, ep idermidis OR in broth can be affected both by mecA transcriptional regulati on and by subpopulation resistance expression.