Prevalence of linezolid-resistant organisms among patients admitted to a tertiary hospital for critical care or dialysis

Linezolid is an oxazolidinone antimicrobial regarded as a “last resort” antimicrobial, used typically for treatment of Gram-positive bacterial infections. It is acknowledged that prevalence of resistance to linezolid is increasing in Europe. In Ireland, a number of outbreaks of linezolid-resistant isolates have been reported, including an outbreak at the location for this study, the Intensive Care Unit (ICU) of University Hospital Limerick (UHL). The Chromagar™ Lin-R selective medium was validated using a panel of linezolid-sensitive and linezolid-resistant strains. Subsequently, the prevalence exercise focused on a convenience sample of patients (n = 159) in critical care wards, ICU (n = 23) and High-Dependency Unit (HDU, n = 51), in addition to patients undergoing dialysis therapy (n = 77). Eight additional patients had specimens collected when attending more than one location. Growth on Chromagar™ Lin-R agar was followed by drug sensitivity testing by disc diffusion and minimum inhibitory concentration (MIC) testing. A validation exercise was performed on 23 isolates: seven target and sixteen non-target organisms. Isolates performed as intended (100% sensitivity, 100% specificity). For the prevalence study, of 398 tests, 40 resulted in growth of non-target organisms (specificity approx. 90%). A sole patient (1/159) was identified as colonized by a linezolid-resistant Staphylococcus epidermidis, a prevalence of 0.63%. Molecular investigation confirmed presence of the G2576T mutation in the 23S rRNA. While this point prevalence study identified extremely low carriage of linezolid-resistant bacteria, it remains prudent to maintain vigilance as reports of outbreaks associated with linezolid-resistant S. epidermidis (LRSE) in European critical care units are increasing.