Antibiotic resistance profiles of Enterococcus spp. isolates discovered from food-producing mussels (Mytilus galloprovincialis) farmed in Central Italy

Aims This study investigated the antimicrobial resistance (AMR) phenomenon (phenotypical and genotypical profiles) in Enterococcus isolates, enrolled as marine bio sentinels, identified from bivalve lamellibranches (Mytilus galloprovincialis) farmed for human consumption on three different mariculture farms located along the coasts of the Central Adriatic Sea (Italy).Methods and Results A total of 790 samples were collected and microbiologically screened by using culture-dependent methods, which permitted the identification of 118 enterococci (86 Enterococcus faecium, 24 Enterococcus durans, and eight Enterococcus gallinarum). In the antimicrobial susceptibility tests, all the isolates were mainly resistant to tetracyclines (54.54%), vancomycin (45.45%), nitrofurantoin (31.81%), quinupristin-dalfopristin (15.90%), teicoplanin (9.09%), and linezolid (6.82%). Uniplex and multiplex polymerase chain reaction assays amplified many antibiotic resistance genes such as tetA (49.15%), tetK (16.95%), tetL (15.25%), tetD (7.63%), vanD (13.56%), vanN (13.56%), nfsA (14.41%), cfr (11.02%), vgaA and vatD (10.17%), and qnrS (6.78%). The calculated multiple antibiotic resistance index (MAR=0.21) and antibiotic resistance pattern abundance (ARPA=0.58) revealed relevant anthropic pollution from the identified Enterococcus isolates.Conclusions These findings confirm the involvement of the Enterococcus genus in the harboring of AMR genetic forms and highlight the consistent genetic trading that can remain silent in bacterial isolates.