Antimicrobial drug resistance and genetic properties of Salmonella enterica serotype Enteritidis circulating in chicken farms in Tunisia.

This study focused on 77 isolates of Salmonella enterica serotype Enteritidis collected during 2009 to 2013 from healthy and sick chickens and environmental farm samples in Tunisia. Resistance to 14 antimicrobials and the encoding genes were analyzed. 66, 26, 6.5, 3.9 and 1.3% were pan-susceptible or showed resistance to nalidixic acid (Asp87 to Tyr and Asp87 to Asn substitutions in GyrA), ampicillin (bla and bla), sulfonamides (sul1and sul3) and streptomycin (strB), respectively. A single isolate with intermediate susceptibility to ciprofloxacin was positive for qnrB, whereas qnrA, qnrS or aac(6')-Ib-cr were not detected. The virulotype of the isolates was established by testing ten virulence genes. The orgA, ssaQ, mgtC, siiD, sopB genes, located on Salmonella pathogenicity islands, and spvC of the serotype-specific virulence plasmid, were common to all isolates. In contrast, the prophage-associated sopE-1, sodC1 and gipA genes and the fimbrial bcfC gene were variably represented. All isolates except one contained the virulence plasmid, which appeared either alone or together with one or more additional plasmids. One isolate carried a single plasmid of ca. 90Kb which may be derived from the virulence plasmid (60Kb). Overall, seven resistotypes, six virulotypes and six plasmid profiles were identified. XbaI-PFGE revealed four related pulsotypes (X1-X4), with 80% of the isolates sharing the X1 pattern. The latter isolates exhibited different resistance, virulence and plasmid profiles, suggesting that mobile genetic elements, particularly prophages and plasmids, are of central importance for the evolution and adaptation of S. Enteritidis circulating in chicken farms in Tunisia.