First study on detection of cryptic resistance to linezolid among clinical isolates of methicillin resistant Staphylococcus aureus from India.

PURPOSE: Linezolid is an oral antibiotic which is widely used for serious infections caused by Methicillin Resistant Staphylococcus aureus (MRSA). With emergence of vancomycin MIC creep among clinical strains of MRSA, it is essential to know the possible emergence of subclinical resistance against linezolid as well. With this background, we aimed to detect evident (phenotypic) and cryptic (hidden or genotypic) linezolid resistance among MRSA isolates.

METHODS

250 clinical isolates of MRSA were collected and their susceptibility patterns were determined. Every third MRSA isolate was subjected to PCR for domain V of the 23S rRNA for the mutation hotspot in the 746bp segment which harbors the classical mutation for linezolid resistance. Restriction Fragment Length Polymorphism was done to confirm presence of the G2576U mutation.

RESULTS

Six isolates (2.4%) were phenotypically resistant to linezolid. Among these six LRSA isolates, 5 demonstrated the G2576U mutation by PCR - RFLP. Cryptic resistance to Linezolid was identified in two isolates among linezolid susceptible isolates.

CONCLUSIONS

In the present study, hidden resistance to linezolid was observed in linezolid susceptible clinical isolates. Emergence of resistance against over-the-counter drugs like linezolid is major challenge. Identification of cryptic resistance among patients implies impending resistance to linezolid. Judicious use of antimicrobials, application of strict infection control practices and prescription audit needs to be made mandatory to preserve such drugs.