Mutational resistance to linezolid and other anti-gram-positive antibiotics in staphylococci

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) are endemic in UK hospitals. Treatment options for the infections caused by these multiresistant strains include vancomycin, teicoplanin and linezolid. Mutational resistance to these antibiotics, although rare in the clinic, has been detected. Resistance to linezolid requires at least one chromosomal mutation followed by subsequent internal recombination. The hypothesis tested was that resistance to linezolid or glycopeptides could emerge in hypermutable strains and that hypermutability could be co-selected with resistance to linezolid or glycopeptides. Accordingly, this work studied linezolid and teicoplanin-resistant clinical isolates and laboratory-selected linezolid-resistant S. aureus mutants for evidence of hypermutability. Mutation frequencies to a range of antibiotics were determined to evaluate the occurrence of hypermutability. Linezolid-resistant mutants were selected by serial passage in the presence of increasing concentrations of linezolid. Mutations conferring linezolid resistance were characterized by PCR-RFLP and sequencing. Pyrosequencing and hybridization were used to detect and quantify six mutations known to confer linezolid resistance. Pre-existing hypermutability increased the ability of a laboratory strain to generate linezolid resistant mutants. However, few clinical linezolid or teicoplanin-resistant isolates or mutants were found to be hypermutable, indicating that stable hypermutability is not a prerequisite for the emergence of these resistances. Likewise, no laboratory-selected linezolid-resistant mutants were hypermutable, demonstrating the lack of co-selection of linezolid resistance with hypermutability. Most linezolid-resistant laboratory-selected mutants were unstable. There was a direct correlation between the number of mutated 23 S rRNA genes and linezolid MIC in clinical and laboratory mutants, although analysis was complicated by the fact that the 23S rRNA gene copy number was variable among the laboratory mutants. Cross-resistance between linezolid and chloramphenicol was detected in some laboratory mutants. In conclusion, there was no evidence for the co-selection of hypermutability and linezolid or teicoplanin resistance, and hypermutability was not found to be a prerequisite for the emergence of linezolid resistance