Characterisation of clinical meticillin-resistant Staphylococcus epidermidis demonstrating high levels of linezolid resistance (>256 μg/ml) resulting from transmissible and mutational mechanisms.

Staphylococcus epidermidis (S. epidermidis), one of the leading etiological agents of nosocomial infections poses a significant economic burden globally. Introduced in 2000, linezolid (LZD) has become an important antibiotic, used in nearly seventy countries worldwide to treat infections caused by Gram-positive pathogens such as meticillin-resistant Staphylococcus and Streptococcus species along with vancomycin-resistant enterococci. Resistance to LZD in clinical settings remains rare. Here, we report the emergence of meticillin resistant S. epidermidis (MRSE) clinical isolates from two voluntary general acute hospitals exhibiting higher than typically reported levels of LZD resistance (MIC>256 μg/ml). The MRSE ST-2 clone isolated from eight patients (2010-2011) not only possessed resistance-conferring mutations such as G2576T in domain V of 23S rRNA gene (as determined by HRM-PCR analysis) and R172C substitution in the ribosomal protein L3, but also carried the cfr gene (the only known transmissible mechanism of LZD resistance). All isolates possessed several key biofilm-associated genes (such as icaA, icaD, aap and atlE) and resistance to multiple clinically significant antibiotics was recorded. This study reports the earliest incidence (2010) of clinical MRSE in the Republic of Ireland demonstrating multiple LZD resistance mechanisms both mutational and potentially transmissible, and characterises this emerging resistance from a molecular perspective.