Listeria monocytogenes from salmon processing and ready-to-eat seafood: Antibiotic susceptibility and disinfectant tolerance

Ready-to-eat (RTE) seafood is susceptible to contamination by Listeria monocytogenes , underscoring the need for effective disinfectants in seafood processing environments. Here, we investigate the effectiveness of four disinfectants (DIs) used in the seafood industry and explore DI-tolerance and antibiotic resistance in L. monocytogenes isolates ( n = 44). These originated from RTE-seafood products from the Japanese retail market, and from a Norwegian salmon processing plant. Antibiotic resistance was generally low among the isolates, with full susceptibility for 12 out of 18 antibiotics. Resistance was observed to clindamycin (41%), ciprofloxacin (20%), levofloxacin (18%), daptomycin (9%), oxacillin (9%), and linezolid (7%), none of which are first-line treatments for listeriosis. Susceptibility testing with benzalkonium chloride (BAC), sodium hypochlorite based Saniplan (SAN), peracetic acid based Aqua Des Foam PAA (ADF), and alkyl amine based Aqua San AM Plus (AQS) revealed variations in the minimum inhibitory (MICs) and bactericidal concentrations (MBCs). For example, the geometric mean (GM) MBC of BAC was 6.63 ppm and significantly higher ( p < 0.05) than the GM MIC of 2.27 ppm. Whole genome sequencing revealed that isolates with high phenotypic tolerance to BAC harbored resistance genes bcrABC or Tn6188_qac ( emrC) . Several of these isolates harbored plasmids, stress islets (SSI-1 and SSI-2), genomic islands (LGI-2 and LGI-3), and 18% were predicted as highly virulent. Ultimately, our study contributes to understanding DI-efficacy and provides new insights into antibiotic resistance and virulence of seafood-associated L. monocytogenes . • Overall low antibiotic resistance among seafood related L. monocytogenes • Multidrug resistance was only observed in four isolates (CC1, CC2, CC5, and CC101) from RTE-seafood • Disinfectants require up to an eight-fold higher concentration for eradication compared to inhibition • Presence of BAC resistance genes did not lead to increased tolerance to peracetic and sodium hypochlorite-based disinfectants • 18% of the L. monocytogenes isolates were predicted as highly virulent