Food Protection Trends - September/October 2016 - 360

Table 2. MICs of antimicrobials for L. monocytogenes environmental isolates ID Antimicrobial (mg/L) 1 AMP CF FOX CRO FEP GEN KAN STR TET VAN RIF CIP SXT NCTR_LM16 NCTR_LM20 NCTR_LM32 NCTR_LM39 NCTR_LM42 NCTR_LM43 NCTR_LM44 NCTR_LM45 NCTR_LM46 NCTR_LM47 NCTR_LM50 NCTR_LM51 NCTR_LM52 NCTR_LM53 NCTR_LM54 NCTR_LM55 NCTR_LM56 NCTR_LM61 1 1 1 2 1 2 4 2 2 4 < 0.5 4 1 4 1 2 1 2 8 8 16 16 8 16 16 16 16 16 1 16 16 16 8 16 16 16 32 > 32 > 32 > 32 32 > 32 > 32 > 32 > 32 > 32 8 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 8 > 32 > 32 > 32 > 32 > 32 > 32 > 32 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 > 32 4 > 32 > 32 > 32 > 32 > 32 > 32 > 32 2 8 1 1 4 2 2 2 1 2 4 2 2 2 4 2 2 2 4 8 2 4 16 4 8 4 2 8 16 8 8 16 16 8 8 8 NCTR_LM62 1 8 > 32 > 32 > 32 2 S. aureus < 0.5 < 0.5 2 4 2 2 CHL 16 32 16 16 32 32 32 32 16 32 16 16 16 16 16 16 16 32 1 1 1 1 < 0.5 64 64 64 1 1 1 2 64 4 2 32 1 64 1 1 1 1 1 0.5 1 1 1 1 2 1 1 1 1 1 1 1 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 < 0.12 0.25 < 0.12 < 0.12 < 0.12 < 0.12 0.5 0.25 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.25 1 1 0.5 0.5 0.25 0.5 0.5 < 0.25 2 < 0.25 2 0.5 2 0.5 2 0.5 4 < 0.25 2 0.5 2 < 0.25 2 < 0.25 2 0.5 < 0.5 0.5 4 0.5 2 < 0.25 2 0.5 2 < 0.25 4 < 0.25 2 < 0.25 2 < 0.25 2 16 32 1 1 < 0.12 1 0.5 2 16 4 < 0.5 2 < 0.12 0.5 2 8 AMP, ampicillin; CF, cephalothin; FOX, cefoxitin; CRO, ceftriaxone; FEP, cefepime; GEN, gentamicin; KAN, kanamycin; STR, streptomycin; TET, tetracycline; VAN, vancomycin; RIF, rifampin; CIP, ciprofloxacin; SXT, sulfamethoxazole-trimethoprim; CHL, chloramphenicol. All strains were resistant to nalidixic acid (>32 μg/mL) and susceptible to erythromycin (< 0.12 μg/mL). 1 2). Eight and six strains were resistant to streptomycin and tetracycline, respectively (Table 2). No strain was resistant to 3 or more antimicrobial classes except for cephalosprins (Table 2). All of the environmental isolates were susceptible to the first-choice antibiotics ampicillin and gentamicin in the treatment of listeriosis. Of the antimicrobial resistance determinants (Table 1), only the tetM gene was amplified from the total genomic DNA of strains NCTR_LM43, NCTR_LM45, NCTR_LM52, NCTR_LM55, and NCTR_LM61, all of which were resistant to tetracycline (Table 2). We suggest that tetM may be a major determinant of tetracycline resistance in L. monocytogenes, as shown in a previous study (21). Interestingly, all tetracycline-resistant strains in this study were serotype 1/2a. In our previous study, more serotype 1/2a strains from the environment (five of 19 strains, 26.3%) than from food products (one of 35 360 Food Protection Trends September/October strains, 2.9%) were resistant to tetracycline (1). The higher resistance to tetracycline seen in food environmental strains may be caused by exposure to a sanitizer, such as benzalkonium chloride (BC), which increased the MIC of tetracycline in BC sensitive L. monocytogenes strains (17). Strains of L. monocytogenes repeatedly exposed to sanitizers or disinfectants in the food processing environment may show increased resistance to tetracycline. Further study may be needed for determining the adaptation or acquisition of tetracycline resistance in L. monocytogenes isolates by using a model exposed repeatedly to a sanitizer or disinfectant. Fewer studies have investigated the distribution of antimicrobial resistance of food processing environment strains of L. monocytogenes than of strains isolated directly from food products (16). Data from food-related environmental isolates may be useful for comparative

Table of Contents for the Digital Edition of Food Protection Trends - September/October 2016