Antibacterial Susceptibility of Enterobacteriaceae Isolated from Raw Horsemeat Intended for Human Consumption (Basashi)

: Drug susceptibility testing was carried out using 14 antibiotics in order to identify trends in the antibiotic tolerance of 142 strains of Enterobacteriaceae isolated from horsemeat commercially available for raw consumption (basashi). A comparison of the sensitivity to the 14 antibiotics using the 90% MIC (minimum inhibitory concentration) values (MIC90) showed the strongest tolerance to ampicillin (ABPC) at a concentration of > 128 μg/mL, followed by that to fosfomycin (FOM) at a concentration of 128 μg/mL. When the sensitivity to these antibiotics was examined for each individual genus of tested bacteria, Hafnia spp. exhibited relative tolerance to ceftazidime (CAZ) and ceftriaxone (CTRX) at a concentration of 4 μg/mL and 2 μg/mL, respectively, which was high in comparison to that observed for the other strains. Furthermore, Raoultella spp. and Serratia spp. were found to be highly resistant to tetracycline (TC) at a concentration of 128 μg/mL and 64 μg/mL, respectively. Of the 142 strains of test bacteria, 140 (98.6%) demonstrated resistance to ABPC, with the exception of Hafnia alvei and Klebsiella pneumonia. In addition, a total of eight strains (5.6%), seven Serratia marcescens strains and one Raoultella terrigena strain, were found to be resistant to TC. Furthermore, one strain of Citrobacter freundii exhibited resistance to nalidixic acid (NA), while another displayed resistance to ofloxacin (OFLX) (0.7% each), and one strain (0.7%) each of Enterobacter cloacae, Serratia marcescens and Citrobacter youngae demonstrated resistance to fosfomycin (FOM), streptomycin (SM) and kanamycin (KM), respectively. A single strain of C. freundii was found to be resistant to three antibiotics, ABPC, NA and OFLX. Resistance to two antibiotics was confirmed in 11 strains, including seven strains of S. marcescens and one strain of R. terrigena (a total of eight strains) resistant to ABPC and TC, and one strain each of C. youngae, S. marcescens and E. cloacae resistant to ABPC and KM, ABPC and SM, and ABPC and FOM, respectively. In addition, 128 strains were resistant to the single antibiotic of ABPC alone. Of the 140 strains demonstrating antibiotic resistance, 137 (97.9%) retained the conjugative R-plasmid transfer factor, excluding three strains of S. marcescens. All transfer factors were ABPC and retained by a high proportion of the bacterial groups, with one strain (100%) being resistant to three antibiotics, nine (81.8%) of the 11 strains being resistant to two antibiotics, and 127 (99.2%) of the 128 strains being resistant to a single antibiotic. In addition, we examined ESBL productivity in the 140 strains of bacteria demonstrating drug tolerance; however, no strains exhibited this characteristic. Therefore, further observation is required to ascertain trends in antibiotic-tolerant bacteria.