Eagle-type methicillin resistance: New phenotype of high methicillin resistance under mec regulator gene control

We report a novel phenotype of methicillin resistance, designated "Eagle-ty pe" resistance, which is characteristic in its resistance to high concentra tions of methicillin (64 to 512 mug/ml) and susceptibility to low concentra tions of methicillin (2 to 16 mug/ml). The type of resistance was expressed in mutant strains selected with high concentrations (e,g,, 128 to 512 mug/ ml) of methicillin from the pre-methicillin-resistant Staphylococcus aureus strain N315, whose mecA gene transcription is strongly repressed by the me cI gene-encoded repressor protein MecI. The Eagle-type mutant strains harbo red no mutation in the mecI gene or in the operator region of mecA gene to which MecI repressor is supposed to bind. In the representative Eagle-type strain h4, repression of mecA gene transcription and penicillin-binding pro tein 2' production were found to be released by exposing the cells to a hig h concentration (128 mug/ml) of methicillin but not to lower concentrations (1 and 8 mug/ml) of methicillin, The strain h4 expressed paradoxical susce ptibility (Eagle effect) to the cytokilling activity of methicillin, Experi mental deletion of mecI gene from the chromosome of h4 by mecI-specific gen e substitution converted its Eagle-type resistance to homogeneously high me thicillin resistance. We cloned two novel genes, designated hmrA and hmrB, from genomic library of h4, which conferred Eagle-type resistance to N315 w hen introduced into the cell in multiple copies, The genes were shown to co nfer homogeneous methicillin resistance to the heterogeneously methicinllin -resistant strain LR5 when they were introduced into on multicopy plasmids, This result strongly indicated that the genetic alteration responsible for the expression of the Eagle phenotype is identical, or equivalent in its e ffect, to the genetic alteration underlying heterogeneous-to-homogeneous co nversion of methicillin resistance in S, aureus.