Linezolid Resistance Genes in Enterococci Isolated from Sediment and Zooplankton in Two Italian Coastal Areas

Abstract

Linezolid is a last-resort antibiotic for the treatment of severe infections caused by multidrug-resistant Gram-positive organisms; although linezolid resistance remains uncommon, the number of linezolid-resistant enterococci has increased in recent years due to worldwide spread of acquired resistance genes (<i>cfr</i>, <i>optrA</i>, and <i>poxtA</i>) in clinical, animal, and environmental settings. 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 agar plates were investigated for their carriage of <i>optrA</i>, <i>poxtA</i>, and <i>cfr</i> genes; <i>optrA</i> was found in one <i>Enterococcus faecalis</i> isolate, <i>poxtA</i> was found in three <i>Enterococcus faecium</i> isolates and two <i>Enterococcus hirae</i> isolates, and <i>cfr</i> was not found. Two of the three <i>poxtA</i>-carrying <i>E. faecium</i> isolates and the two <i>E. hirae</i> isolates showed related pulsed-field gel electrophoresis (PFGE) profiles. Two <i>E. faecium</i> isolates belonged to the new sequence type 1710, which clustered in clonal complex 94, encompassing nosocomial strains. S1 PFGE/hybridization assays showed a double (chromosome and plasmid) location of <i>poxtA</i> and a plasmid location of <i>optrA</i> Whole-genome sequencing revealed that <i>poxtA</i> was contained in a Tn<i>6657</i>-like element carried by two plasmids (pEfm-EF3 and pEh-GE2) of similar size, found in different species, and that <i>poxtA</i> was flanked by two copies of IS<i>1216</i> in both plasmids. In mating experiments, all but one strain (<i>E. faecalis</i> EN3) were able to transfer the <i>poxtA</i> gene to <i>E. faecium</i> 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 environmental reservoirs.<b>IMPORTANCE</b> Linezolid is one of the few antimicrobials available to treat severe infections due to drug-resistant Gram-positive bacteria; therefore, the emergence of linezolid-resistant enterococci carrying transferable resistance determinants is of great concern for public health. Linezolid resistance genes (<i>cfr</i>, <i>optrA</i>, and <i>poxtA</i>), often plasmid located, can be transmitted via horizontal gene transfer and have the potential to spread globally. This study highlights the detection of enterococci carrying linezolid resistance genes from sediment and zooplankton samples from two coastal urban areas in Italy. The presence of clinically relevant resistant bacteria, such as linezolid-resistant enterococci, in marine environments could reflect their spillover from human and/or animal reservoirs and could indicate that coastal seawaters also might represent a source of these resistance genes.