mecA - blaZ Corepressors in Clinical Staphylococcus aureus Isolates

Abstract

The presence and nucleotide sequences of the two mecA repressors, mecI and blaI, were assessed in 73 clinical Staphylococcus aureus isolates. Isolates with mecI mutations were grouped into unique clonal types based on their spa nucleotide repeat patterns. Forty-three of the 45 (96%) isolates with mutant mecI or with a deletion of mecI contained blaI, while blaI was present in only 21 of 28 (78%) isolates with wild-type mecI (P < 0.05). Among 22 additional isolates that did not contain blaI, all had wild-type mecI sequences. We conclude that oxacillin-resistant S. aureus must have at least one of the two functional mecA regulators.Transcription of mecA, the gene mediating oxacillin resistance in staphylococci, is regulated by a repressor, mecI, that is divergently transcribed from mecA as the second gene in a two-gene operon that also includes mecR1 (1,11,21). More than 90% of staphylococcal isolates also produce ␤-lactamase, the product of blaZ, and contain blaZ regulatory sequences (blaI and blaR1) that are similar in sequence and function to mecA regulators (9, 10). In addition to regulating blaZ transcription, BlaI also binds to mecA-mecR1 promoter-operator (P-O) sequences and regulates their transcription (9,13,14,19). Coregulation of mecA by both MecI and BlaI has been demonstrated in defined laboratory strains (14), but neither the presence nor the nucleotide sequences of the two coregulators have been investigated in clinical isolates.Mutations and deletions in mecI and deletions of both mecI and mecR1 appear to be common in clinical Staphylococcus aureus isolates (1,12,16,21,22). In contrast, while there has been no systematic assessment of blaI and blaR1 in clinical isolates, these regulators have been found to be intact in sequence or function whenever they have been examined. To explore the extent of MecI and BlaI coregulation of mecA, we examined a collection of geographically and temporally diverse oxacillin-resistant S. aureus isolates, most of which were also shown to have genetic diversity. We also correlated the clonal type with regulator sequence by using the number of unique nucleotide repeat sequences in the protein A (spa) gene as a typing tool as previously described (20).Source of isolates. The oxacillin-resistant S. aureus isolates examined in this study came from three sources. The first source was the Public Health Research Institute (PHRI) collection, from which 40 isolates were chosen. These isolates were selected because 37 of the 40 isolates had different spa repeat patterns (20), indicating clonal diversity. They were of unknown mecI status, came from five different countries and 19 cities, and were isolated between 1960 and 1997. The second source was the Virginia Commonwealth University (VCU) collection, from which 33 isolates were chosen. These isolates had been previously shown to have sequences that hybridized with a mecI probe (1) and were recovered from patients in 25 different cities in four countries between 1968 and 1993. The final source was the SCOPE collection (6), fro...

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