Occurrence and Phenotypic and Molecular Characterization of Antimicrobial Resistance of Salmonella Isolates from Food in Tunisia

Salmonella is a leading cause of foodborne diseases worldwide. The use of antibiotics in food-producing animals may contribute to the development of antimicrobial resistance in nontyphoidal Salmonella. The development of resistance to potent antimicrobials such as fluoroquinolones and extended-spectrum beta-lactamases is a significant public health problem. The present study was conducted to examine the occurrence and antimicrobial resistance of Salmonella isolates obtained from food samples. Salmonella was cultured according to ISO 6579: 2002 method, and antimicrobial resistance was evaluated with the Kirby-Bauer disk diffusion method. Forty-five Salmonella isolates were recovered, and a high Salmonella prevalence was detected in clams (7 of 20 samples, 35%), chicken (28 of 97 samples, 28.9%) and cow's milk (10 of 80 samples, 12.5%). Salmonella Enteritidis (n = 19) and Salmonella Kentucky (n = 18) were the most prevalent isolates. Multidrug resistance was found in 31.1% of the isolates (14 of 45); 84, 46, 28, and 17% of the isolates were resistant to nalidixic acid, ciprofloxacin, amoxicillin-clavulanic acid, and both ofloxacin and cefotaxime, respectively. The isolates resistant to cefotaxime were screened by PCR for the genes for TEM beta-lactamase, extended-spectrum beta-lactamases (CTX and OXA), and AmpC beta-lactamases (FOX, MOX, DHA, ACC, CIT, and EBC). One Salmonella Kentucky isolate from milk harbored an AmpC gene (FOX), and the same serotype isolated from chicken carried the EBC AmpC determinant. The blaTEM gene was detected in all nonsusceptible isolates. We also screened isolates with reduced fluoroquinolone susceptibility for the presence of transferable plasmid-mediated quinolone resistance determinants. Three qnr genes (qnrB, qnrD, and qnrS) were detected in four isolates (two from milk and two from chicken). To our knowledge, this is the first report of the AmpC FOX and EBC gene families and the qnrD gene within a foodborne pathogen in Tunisia. These findings highlight the emergence of multidrug-resistant Salmonella isolates with decreased susceptibility to fluoroquinolones and third-generation cephalosporins, which are drugs commonly used for the treatment of Salmonella infections.