Abstract
Methicillin-resistant Macrococcus caseolyticus strains from bovine and canine origins were found to carry a novel mecD gene conferring resistance to all classes of β-lactams including anti-MRSA cephalosporins. Association of β-lactam resistance with mecD was demonstrated by gene expression in S. aureus and deletion of the mecD-containing island in M. caseolyticus. The mecD gene was located either on an 18,134-bp M. caseolyticus resistance island (McRImecD-1) or a 16,188-bp McRImecD-2. Both islands were integrated at the 3′ end of the rpsI gene, carried the mecD operon (mecD-mecR1m-mecIm), and genes for an integrase of the tyrosine recombinase family and a putative virulence-associated protein (virE). Apart from the mecD operon, that shared 66% overall nucleotide identity with the mecB operon, McRImecD islands were unrelated to any mecB-carrying elements or staphylococcal cassette chromosome mec. Only McRImecD-1 that is delimitated at both ends by direct repeats was capable of circular excision. The recombined excision pattern suggests site-specific activity of the integrase and allowed identification of a putative core attachment site. Detection of rpsI-associated integrases in Bacillus and S. aureus reveals a potential for broad-host range dissemination of the novel methicillin resistance gene mecD.
Highlights
Macrococcus is evolutionarily closely related to the genus Staphylococcus, but possesses a distinctly smaller genome with a size of 2.1 Mb in the species M. caseolyticus[1]
Whole genome sequencing and tblastn analysis using PBP2a encoded by mecA, mecB and mecC as queries revealed a novel mec gene homologue in the three β-lactam-resistant strains but not in the β-lactam-sensitive M. caseolyticus strain KM1352 (Table 1)
The mec gene was designated mecD according to the staphylococcal cassette chromosome mec (SCCmec) guidelines for reporting new mecA gene homologues which defines a new mec gene type if the nucleotide sequence shares less than 70% identity to any known mec genes[13]
Summary
Macrococcus is evolutionarily closely related to the genus Staphylococcus, but possesses a distinctly smaller genome with a size of 2.1 Mb in the species M. caseolyticus[1]. PBP2a is encoded by a structural mec gene so far identified as mecA, mecB and mecC, and its presence can be predicted phenotypically using a screen for cefoxitin or oxacillin resistance[16]. In Macrococcus, methicillin resistance has been associated with the mecB gene (formerly mecAm), a distantly related mecA homologue[1]. It is present in a mec gene complex (blaZm-mecB-mecR1m-mecIm) which includes the www.nature.com/scientificreports/. This study This study This study This study regulators and the blaZ homologue blaZm. The mecB gene complex was originally found associated with transposon Tn6045 located on either large plasmids or integrated into the chromosome as part of a SCCmec-like element in M. caseolyticus[1,8]. A SCCmec carrying mecB independent of Tn6045 was reported in a hemolytic Macrococcus strain (M. canis sp. nov., formerly identified as M. caseolyticus)[4,12]
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