Linezolid resistance genes in enterococci isolated from sediment and zooplankton in two Italian coastal areas

Linezolid is a last resort antibiotic for the treatment of severe infections caused by multi-resistant Gram-positives; although linezolid resistance remains uncommon, the number of linezolid-resistant enterococci has increased during recent years due to worldwide spread of acquired resistance genes (cfr, optrA and poxtA) in clinical, animal and environmental setting. In this study we investigated the occurrence of linezolid-resistant enterococci in marine samples from two coastal areas in Italy. Isolates grown on florfenicol-supplemented Slanetz-Bartley were investigated for their carriage of optrA, poxtA and cfr genes: optrA was found in one E. faecalis, poxtA in three E. faecium and two E. hirae and cfr was not found. Two of the three poxtA-carrying E. faecium and the two E. hirae showed related PFGE profiles. Two E. faecium belonged to the new ST1710, which clustered in the clonal complex CC94, encompassing nosocomial strains. S1-PFGE/hybridization assays showed a double (chromosome and plasmid) location of poxtA and plasmid location of optrA WGS revealed that poxtA was contained in a Tn6657-like element carried by two plasmids (pEfm-EF3 and pEh-GE2) of similar size, found in different species, and that poxtA were flanked by two copies of IS1216 in both plasmids. In mating experiments all but one (E. faecalis EN3) strains were able to transfer the poxtA gene to E. faecium 64/3. The occurrence of linezolid resistance genes in enterococci from marine samples is of great concern and highlights the need to improve practices aimed at limiting the transmission of linezolid resistant strains to humans from the environmental reservoirs.Importance Linezolid is one of the few antimicrobials available to treat severe infections due to drug-resistant Gram-positive bacteria, thus the emergence of linezolid-resistant enterococci carrying transferable resistance determinants is of great concern for public health. Linezolid resistance genes (cfr, optrA and poxtA), often plasmid located, can be transmitted via horizontal gene transfer and have the potential to spread globally. This study highlights the first detection of enterococci carrying linezolid resistance genes from sediment and zooplankton samples in two coastal urban areas in Italy. The presence of clinically relevant resistant bacteria, such as linezolid-resistant enterococci, in marine environment could reflect their spillover from human and/or animal reservoirs and could indicate that also coastal seawaters could represent a source of these resistance genes.