Mutation of the RNA polymerase M-NM-2-subunit gene promotes hetero-to-homo conversion of M-CM-^_-lactam resistance of methicillin-resistant Staphylococcus aureus

ABSTRACT: Three types of phenotypic expression of M-CM-^_-lactam resistance has been reported in MRSA: heterogeneous-, homogeneous-, and Eagle-type resistance. Heterogeneous to homogeneous (hetero-to-homo) conversion of M-CM-^_-lactam resistance is postulated to be caused by a chromosomal mutation (chr*) together with mecA-gene expression. The Eagle-type resistance is a special pattern of chr* expression in the pre-MRSA strain N315 under the strong mecI-gene mediated repression of mecA gene transcription. Here, for the identification of chr*, experiments were conducted using an in-vitro derived homogeneously imipenem-resistant MRSA strain N315M-bM-^HM-^FIPH5 (M-bM-^HM-^FIPH5). The strain was selected with imipenem 8 mg/L from the heterogeneously imipenem-resistant MRSA strain N315M-bM-^HM-^FIP (M-bM-^HM-^FIP). The whole genome sequencing of M-bM-^HM-^FIPH5 revealed the presence of a unique mutation in the rpoB gene, rpoB(N967I), causing the amino-acid (AA) substitution of Asp by Ile at the 967th AA position of the RNA polymerase M-CM-^_ subunit. The effect of the mutation on the phenotypic change was confirmed by constructing and studying the phenotype of the strains H5rpoB(I967N), a M-bM-^HM-^FIPH5-derived strain cured of the rpoB mutation, and N315rpoB(N967I), a N315-derived strain introduced with the mutated rpoB gene. H5rpoB(I967N) regained the hetero-MRSA phenotype, and the mutant strain N315rpoB(N967I) showed an Eagle-type phenotype similar to that of N315h4. Furthermore, subsequent whole genome sequencing revealed that N315h4 also had a missense mutation in the rpoB gene rpoB(R644H). The rpoB mutations caused decreased autolysis, prolonged doubling-time, and tolerance to bactericidal concentrations of methicillin. We concluded that the certain rpoB mutations are chr* responsible for the hetero-to-homo phenotypic conversion of MRSA. We compared the gene expression profiles of the wild-type strain and rpoB mutant (N967I) using a 60mer oligo array.

ORGANISM(S): Staphylococcus aureus

SUBMITTER: tomomi hishinuma 

PROVIDER: E-GEOD-42218 | biostudies-arrayexpress |

REPOSITORIES: biostudies-arrayexpress