Molecular characteristics of multidrug resistant Salmonella enterica isolated from humans and pork

Abstract

Total of 183 Salmonella enterica isolates from humans (n=52) and pork (n=131) were included. All isolates were detected for antimicrobial susceptibilities, the presence of class 1, 2 and 3 integrons, antimicrobial resistance genes and virulence genes. The ciprofloxacin-resistant isolates (n=24) were tested for the presence of mutations in gyrA, gyrB, parC and parE and AcrAB-TolC expression. Genetic relatedness of the selected strains (n=40) was examined by multilocus sequence typing (MLST). Most isolates were resistant to tetracycline (73%), followed by ampicillin (53%). None were resistant to ceftriaxone. Thirty-nine percent were positive to intl1, of which 15% carried gene cassettes i.e. dfrA12-aadA2, bla[subscript PSE-1] and aadA2. Two Salmonella isolates carrying class 1 integrons with dfrA12-aadA2 could horizontally transfer their integrons. None were positive for int2 and int3. SGI1-G and SGI1-F were detected in an Albany and a Kedougou, respectively. Eighteen resistance genes were found in the Salmonella isolates with corresponding resistance phenotype. All isolates carried the virulence genes invA, msgA, spiA and tolC. The spvC and pefA gene encoded by Salmonella virulence plasmids were present at limited rate. The amino acid substitutions Met-48-Ile and Thr-82-Met were most common in GyrA and ParC, repectively. The acrB expression level varied from 1 to 430 folds and was not associated with ciprofloxacin MICs. Based on the MLST results, most of the pork and human isolates with the same serovars were grouped into the same clusters, providing evidence that pork could serve as a reservoir for Salmonella in humans.