Role of mecA transcriptional regulation in the phenotypic expression of methicillin resistance in Staphylococcus aureus

Abstract

The gene required for methicillin resistance in staphylococci, mecA, encodes the low-affinity penicillinbinding protein 2a (PBP2a). Transcriptional regulation of mecA is accomplished in some isolates by mecR1 and mecI, cotranscribed chromosomal genes that encode a putative signal transducer and a transcriptional repressor, respectively. Two Staphylococcus aureus strains that have identical mecR1-mecI nucleotide sequences, BMS1 and N315P, both exhibit low-level, heterotypic expression of methicillin resistance and contain no ␤-lactamase coregulatory sequences. mecR1-mecI was amplified from BMS1 by PCR and was shown to be functional on a high-copy-number plasmid when introduced into an S. aureus strain with a deleted mecR1-mecI locus. Cloned mecR1-mecI repressed phenotypic expression of methicillin resistance, mecA transcription and PBP2a production and mediated PBP2a induction in response to certain ␤-lactam antibiotics. However, mecR1-mecI had different regulatory activities in its native chromosomal location in N315P compared with those in BMS1. Uninduced mecA transcription was markedly repressed in N315P, and mecI inactivation increased mecA transcription and PBP2a production 5-and 40-fold, respectively. Furthermore, the N315P phenotype changed from low-level, heterotypic resistance with intact mecI to high-level, homotypic resistance in strains with disrupted mecI. In contrast, uninduced BMS1 produced abundant mecA transcript and PBP2a, while the disruption of mecI had no effect on phenotype and little effect on mecA transcription or PBP2a production. Thus, mecI-mediated repression of mecA appears to be dysfunctional in BMS1 because of the presence or absence of additional regulatory cofactors. Furthermore, heterotypic resistance expression in this strain is independent of mecA transcriptional regulation.Methicillin resistance, which encompasses resistance to all beta-lactam compounds, is a common phenotype among multiresistant, nosocomial staphylococci. Methicillin-resistant isolates are not effectively treated by most antibacterial agents and are a major challenge for chemotherapy. A low-affinity penicillin-binding protein (PBP), PBP2a, encoded by the chromosomally located gene mecA, mediates methicillin resistance among both Staphylococcus aureus and coagulase-negative staphylococci (13,30,44). PBP2a is expressed in addition to the normal complement of resident PBPs and remains the sole unsaturated PBP in the presence of beta-lactam antibiotics, alone capable of maintaining cell wall integrity. The mecA gene and Ն30 kb of flanking DNA are unique to methicillin resistant staphylococci; there are no allelic equivalents in methicillinsusceptible strains (2,8,23).Two methicillin resistance phenotypes are discernible by efficiency of plating (EOP) studies: homotypic and heterotypic (6,15,32). Homotypic (homogeneous) strains uniformly express high-level resistance, whereas heterotypic (heterogeneous) organisms exhibit a strain-unique variation in resistance expression. In heterotypic isolates, highly resistant s...

Phagocytosis and killing of epidemic methicillin-resistant Staphylococcus aureus by human neutrophils and monocytes

“…It should be emphasized that strains resistant to methicillin/ oxacillin have penicillin-binding proteins (PBPs) in their cell walls (named PBP2a), different from the PBPs of methicillin/oxacillin sensitive strains. These proteins, encoded by the chromosomal mecA gene, are responsible for the formation of S. aureus cell wall peptidoglycan chains, and are also targets for antibiotics such as penicillin [9,35,36]. Considering that these altered PBP2a continue to be accountable for cell wall structure and taking into account the above data about the cell wall, we can hypothesize that resistant strains have a morphology that confers greater protection against destruction by lysosomal enzymes or by oxidative metabolism products of phagocytes.…”

Novel Non- mecA -Containing Staphylococcal Chromosomal Cassette Composite Island Containing pbp4 and tagF Genes in a Commensal Staphylococcal Species: a Possible Reservoir for Antibiotic Resistance Islands in Staphylococcus aureus

“…The mecA gene complex class, A or B, is assigned on the basis of the presence or absence of IS1272, mecI, and two functional regions of mecR1 (17). Sequence variability in the ccrA and ccrB genes determines the assignment to ccr gene complex types 1 to 4 (10,20).…”

Evaluation of Clinical Laboratory Standards Institute Interpretive Criteria for Methicillin-Resistant Staphylococcus Pseudintermedius Isolated from Dogs

“…15 The gold standard for diagnosis of methicillin resistance is detection of mecA via polymerase chain reaction (PCR). 6,7 Veterinary microbiology laboratories rely on a variety of laboratory tests to identify methicillin resistance, and few laboratories at this time, if any, are known to perform PCR for mecA on a routine diagnostic basis.…”