Serotyping, antimicrobial susceptibility, pulse field gel electrophoresis analysis of Salmonella isolates from retail foods in Henan Province, China

A total of 152 Salmonella isolates recovered from retail foods in Henan Province in China were characterized using serotyping and antimicrobial susceptibility testing. Nalidixic acid and/or ciprofloxacin-resistant isolates were further examined for amino acid substitution of gyrase and topoisomerase Ⅳ, and for the presence of qnr (qnrA, qnrB and qnrS) and aac(6′)-Ib genes. Selected serovars were subtyped using pulse field gel electrophoresis (PFGE). Among all isolates tested, Salmonella Enteritidis (34.2%) was the most common serotype detected, followed by S. Indiana (9.9%), S. Derby (9.9%), S. Agona (6.6%), S. Typhimurium (5.3%) and S. Albany (4.6%). Twenty-eight percent of the isolates were resistant to 1–3 antimicrobials, 37% to 4–6 antimicrobials, 23% to 7–9 antimicrobials, and 12% to more than 10 antimicrobials. Resistance was most frequently detected to sulfamethoxazole (95.3%), followed by trimethoprim/sulfamethoxazole (80.5%), tetracycline (75.8%), nalidixic acid (75.8%), ampicillin (45.6%), chloramphenicol (35.6%), streptomycin (32.9%), kanamycin (24.8%), gentamicin (21.5%) and amoxicillin/clavulanic acid (14.8%). Resistance was also observed to ciprofloxacin (12.1%), cefoperazone (18.8%), ceftriaxone (6.0%) and cefoxitin (4.0%). A total of 45 amino acid substitutions were identified in quinolone resistant determination region (QRDR) of gyrA and parC of 19 ciprofloxacin-resistant isolates. The most common mutations in gyrA were Ser83Phe and Asp87Gly, in parC was Ser80Arg. QnrA, qnrB, qnrS and aac(6′)-Ib genes were identified in 55 (46.6%), 15 (12.7%), 23 (19.5%) and 16 (13.6%) of 118 nalidixic acid resistant but ciprofloxacin susceptible or less resistant isolates, respectively. A total of 108 PFGE patterns were generated among 146 selected isolates. Our findings indicate that foodborne Salmonella isolates in Henan, China were phenotypically and genotypically diverse and many isolates demonstrated multidrug resistance.