Molecular clonality and antimicrobial resistance in Salmonella enterica serovars Enteritidis and Infantis from broilers in three Northern regions of Iran

BACKGROUND: Multidrug-resistant Salmonella strains are frequently encountered problems worldwide with considerable increased occurrences in recent years. The aim of this study was to investigate the occurrence and frequency of antimicrobial resistance and associated resistance genes in Salmonella isolates from broiler farms in different regions of Iran covering a time period of four years. RESULTS: From 2007 to 2011, 36 Salmonella strains were isolated from broiler farms located in three northern provinces of Iran. The isolates were serotyped, antimicrobial susceptibility tested, and characterized for antimicrobial resistance genes associated to the phenotype. Pulsed-field gel electrophoresis (PFGE) was applied for comparison of genetic relatedness. Two serovars were detected among the isolates; Salmonella enterica serovar Infantis (75%) and S. Enteritidis (25%). Thirty-four (94%) of the isolates exhibited resistance to nalidixic acid and ciprofloxacin caused by a single mutation in the quinolone resistance-determining region (QRDR) of gyrA. For all strains this mutation occurred in the codon of Asp(87) leading to a Asp(87-)Tyr, Asp(87-)Gly or Asp(87-)Asn substitutions. All S. Infantis (n = 27) were resistant to tetracycline, spectinomycin, streptomycin, and sulfamethoxazole and harbored the associated resistance genes; tetA, dfrA14, aadA1, and sulI together with class 1 integrons. The isolates revealed highly similar PFGE patterns indicating clonal relatedness across different geographical locations. CONCLUSION: The data provided fundamental information applicable when launching future control programs for broilers in Iran with the aim to conserve the effectiveness of important antimicrobials for treatment in humans.