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

Many oxacillin-resistant Staphylococcus epidermidis (ORSE) have minimum inhibitory concentration (MIC) values below the Staphylococcus aureus methicillin/oxacillin resistance breakpoint. The difficulty detecting the OR phenotype in S. epidermidis may be due to extreme heterotypy in resistance expression and/or transcriptional repression of mecA, the oxacillin resistance gene, by MecI. To determine the role of these factors in the phenotypic expression of ORSE, 17 geographically diverse mecI+ ORSE isolates representing 14 distinct pulse field gel electrophoresis (PFGE) pulse-types (>3 band differences) were investigated. 13 of the 14 types contained mecI and mecA promoter-operator (P-O) sequences known to be associated with maximal mecA repression and in all isolates mecA transcription was repressed. All 17 were heterotypic in their resistance expression. Oxacillin MIC values ranged from 1--128 mug/ml and increased in 16 out of 17 isolates after beta-lactam induction. Allelic replacement inactivation of mecI in three isolates similarly resulted in a 4--7 fold increase in MIC. In the two of these three isolates producing beta-lactamase, mecA transcription was regulated by both mecI and beta-lactamase regulatory sequences. Heterotypic expression of resistance in these three isolates was unaffected by either beta-lactam induction or mecI inactivation. However, prolonged incubation in concentrations of oxacillin just sufficient to produce a lag in growth (0.5--1.0 mug/ml) converted the population resistance expression from heterotypic to homotypic. Homotypic conversion could also be demonstrated in microtiter wells during MIC determinations in one isolate with a high MIC. Thus, the phenotypic expression of S. epidermidis oxacillin resistance in broth can be affected both by mecA transcriptional regulation and by subpopulation resistance expression. mecR1-mecI allelic replacement mutagenesis of a fourth S. epidermidis isolate unexpectedly caused a decrease in oxacillin resistance both in broth and on agar. No direct relationship between mecI mutagenesis and the decreased oxacillin resistance phenotype could be documented. Lastly, various factors affecting the heterotypic to homotypic conversion in a heterotypic S. aureus isolate were examined. The results suggested that both the intact mecA gene and an increase in mecA transcription were required for the heterotypic to homotypic conversion. Furthermore, various non-beta-lactam agents that affected cell growth increased oxacillin resistance subpopulation expression, although not to the level of homotypy. We conclude that the conversion from homotypy to heterotypy in staphylococci may be, in part, a cellular response to environmental stress.