Methicillin-resistant Staphylococcus aureus (MRSA) are increasingly responsible for outbreaks of nosocomial infections around the world. Because serious infections due to these organisms currently necessitate use of non-β-lactam antimicrobial
therapy
and because MRSA is often resistant to many antimicrobial agents, infections with this organism are difficult to treat. Methicillin resistance in Staphylococci is mediated by the methicillin resistance determinant mec, which carries the
structural
gene
mecA that encodes for the low-affinity penicillin-binding protein(PBP) PBP2 or PBP2a novel PBP is thought to allow for peptidoglycan biosynthesis at concentrations ofβ-lactams which inactivate the preexisting PBPs 1, 2, 3, and 4 in methicillin
resistant strains. The MICs of β-lactam antibiotics for MRSA are influenced frequently by growth conditions such as temperature, pH, and salt concentration. Thus, the purpose of this study was to explain there is a reasonable correlation between
the
effects of these environmental factors on methicillin resistance expression (MIC) and mecA gene transcription levels. Clinical isolates of S. aureus were classified into three groups; Group I, the isolates which were were methicillin-susceptiblo
and of
which methicillin resistance was not induced by stimulation with nethicillin, GroupⅡ, the isolates which were methicillin-susceptible but of which methicillin resistance was induced by methicillin; GroupⅢ, the isolates which showed methicillin
resistance regardless of stimulation with methicillin. A total of 200 isolates were analyzed by dot blot hybridization with the mecA-spec ific probe to determine whether they carried mecA genes. The isolates belonging to group I were found to be
negative for the mecA probe, and groupⅡand GroupⅢisolates be positive. In order to determine whether mecA gene transcription level is responsible for the altered MIC under various growth conditions, RNA dot blot hybridization with mecA gene
probe
was
performed in the 4 representative isolates of each group. In GroupⅠisolates, no mecA gene transcription was detected under any environmental conditions including control culture condition (37℃, pH 5.2, no addition of NaCl). The level of
transcription
of GroupⅡisolates was elevated in company with the increase of MIC by culturing them at 30℃or at 5% NaG1, and reduced with the decrease of MIC at Ph 5.2 In GroupⅢisolates, the high level of transcription with the very high MIC was observed at
30℃or
5% NaC1 as well as under control culture condition, and the reduced transcription level with decreased MIC at pH 5.2 was observed. These results suggest that (i) methicillin resistance in S. aureus strains which harbor the mecA gene on their
chromosome
but are susceptible to methicillin because of the repression can be induced by stimulation with methicillin, and (ii) in these strains, the regulation of methicillin resistance under different environmental conditions occurs primarily at the
level
of
mecA transcription.
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