Role of the mecA Gene in Oxacillin Resistance in a Staphylococcus aureus Clinical Strain with a pvl -Positive ST59 Genetic Background

Abstract

The most prevalent community-associated methicillin-resistant Staphylococcus aureus (C-MRSA) strains in Taiwan, sequence type 59 (ST59) clones, carry staphylococcal cassette chromosome mec (SCCmec) type V and, to a lesser extent, type IV. These strains show wide variation in sensitivity to oxacillin, but the reasons for this variation are unknown. Here we compared the sequences of the mecA genes from clinical strains of different SCCmec types and found that they contain different mecA promoter mutations. Analysis of mecA promoter activity by reporter gene fusions showed that single base substitutions in the promoter have a strong influence on mecA transcription. The different mecA variants, including promoter sequences, were expressed in the methicillin-sensitive Staphylococcus aureus (MSSA) strain C195 (ST59 background). PBP 2a production among the parental strains and strains with promoter mutant mecA genes showed a close correlation with mecA transcription levels. Furthermore, the quantity of PBP 2a also closely correlated with the level of oxacillin resistance in the C195 background. Our data suggest that mecA promoter mutations play an important role in determining the level of oxacillin resistance. The mecA promoter mutation G-25A (25 bases upstream of the mecA translation start site) was found to be associated with a high oxacillin MIC (256 g/ml), G-7T conferred a moderate oxacillin MIC (32 to 64 g/ml), strains with C-33T showed a low oxacillin MIC (4 to 8 g/ml), and A-38G reversed the effect of the C-33T mutation, restoring the oxacillin resistance level in the A-38G C-33T double mutant. These observations may explain why C-MRSA strains in Taiwan carrying SCCmec type IV or V have such enormous variations in oxacillin MICs.

Genomic identification of cryptic susceptibility to penicillins and β-lactamase inhibitors in methicillin-resistant Staphylococcus aureus

“…The first mutation was a G to T transversion in the mecA ribosomal binding site (RBS), seven nucleotides upstream of mecA start codon (Fig 3a). Isolates with the mecA [-7]:T allele had a median penicillin-clavulanic acid MIC of 0.125 μg/ml (range <0.016 -6) compared to a median of 8 μg/ml (range: 0.023 -96) for isolates with the 'wildtype' mecA [-7]:G. Previous work has demonstrated that despite being in the RBS, the T allele results in lower mecA transcript and PBP2a expression levels 28 .…”

The Global RegulonsarARegulates β-Lactam Antibiotic Resistance in Methicillin-ResistantStaphylococcus aureusIn Vitro and in Endovascular Infections

“…PBP2a, encoded by mecA, is an alternative cell wall cross-linking enzyme with reduced affinity for virtually all β-lactam antibiotics. Previous studies showed that the expression of mecA closely correlated with the level of oxacillin resistance [5]. mecA is part of the staphylococcal chromosome cassette (SCCmec), which is present in MRSA but absent in methicillin (oxacillin)-susceptible S. aureus (MSSA) [6].…”

“…Furthermore, excision of the SCCmec element in MRSA strains results in oxacillin susceptibility [8]. Likewise, inactivation of mecA in MRSA strains reverses the MRSA phenotype to that of MSSA [5].…”

Detection of Oxacillin (Methicillin)-resistant Staphylococcus aureus Isolated from a Tertiary-care Hospital, Georgetown, Guyana

“…It is plausible that these observable variations in the oxacillin MIC's may have been due to mecA promoter mutations, which play an important role in determining the level of oxacillin resistance. For example, a study done by Chen et al [25] found that the mecA promoter mutation G-25A is associated with a high oxacillin MIC (256 μg/ml); G-7T conferred a moderate oxacillin MIC (32 to 64 μg/ml); and strains with the C-33T mutation showed a low oxacillin MIC (4 to 8 μg/ml).…”