Antimicrobial Susceptibility Test

All strains collected were analyzed by the KB method using breakpoints recommended by the National Committee for Clinical Laboratory Standards (Clinical and Laboratory Standards Institute, 2014) for Staphylococcus, except for ampicillin and penicillin G where specific Listeria breakpoints are defined (M45-A2 Vol. 30 No. 18) (Supplementary Table S3). In total, 16 antibiotic agents, including those used to treat human listeriosis, were tested by the KB method as follows: ampicillin (AMP; 10 μg), chloramphenicol (C; 30 μg), erythromycin (E; 15 μg), gentamicin (CN; 10 μg), kanamycin (K; 30 μg), rifampin (RD; 5 μg), doxycycline (DO, 30 μg), penicillin (P, 10 U), tetracycline (TE; 30 μg), vancomycin (VA; 30 μg), sulfamethoxazole with trimethoprim (SXT; 23.75/1.25 μg), sulbactam/ampicillin (SAM; 10/10 μg), meropenem (MEM; 10 μg), linezolid (LZD, 30 μg), and amoxycillin/clavulanic acid (AMC; 10 μg) (Oxoid, Basingstoke, United Kingdom). Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922 were used as quality control strains. Zones of inhibition were measured with precision calipers to the nearest 0.01 mm. Isolates exhibiting resistance to at least three classes of the antimicrobial agents tested, were considered multidrug-resistant strains.