[Detection of methicillin-resistant Staphylococcus aureus using PCR and non-radioactive DNA probes: III. Mutations of the fem A gene in clinical strains of Staphylococcus aureus].

Methicillin resistance in S. aureus is primarily due to the presence of the mec A gene. However, in addition to mec, the phenotypic expression of methicillin resistance requires the presence of an additional gene(s), fem A which is chromosomally encoded. Previous studies suggest an increase in the biochemical function of fem A gene products due to base substitutions in the region upstream of the fem A gene and in its coding frame. The partial nucleotide sequences of fem A regions in reference and clinical strains of S.aureus were therefore analyzed by PCR-direct solid-phase sequencing and suitable DNA probes. Amplified target DNAs of 251, 330 and 271 bp were resolved on ethidium bromide-stained gels and hybridized with DNA probes conjugated to alkaline phosphatase. In ATCC 12600 strain, a palindromic sequence was conserved in the region upstream of fem A. However, it was destroyed by the occurrence of mutations in other reference, and clinical strains tested regardless of whether they are methicillin-susceptible or resistant. Furthermore, in the coding frame of fem A, two missense mutations were present in MSSA and MRSA without any regularity. These findings suggest that mutations in the fem A region may not be a single factor essential for regulation of methicillin resistance, although fem A probably functions cooperatively with mec A.