Salmonella Derby from pig production chain over a 10-year period: antimicrobial resistance, biofilm formation, and genetic relatedness.

The aim of this study was to evaluate 140 Salmonella Derby isolates collected over a 10-year period from porcine origins (environment, pig carcass, lymph nodes, intestinal content, and pork) for their phenotypic and genotypic antimicrobial resistance, their ability to produce biofilm, and their genetic relatedness. The minimum inhibitory concentration (MIC) was determined using microdilution broth method and antimicrobial resistance genes were investigated by PCR. The quantification of biofilm formation was performed in sterile polystyrene microtiter plates. Genetic relatedness was determined by Xba-I macrorestriction analysis. The highest frequencies of non-wildtype (nWT) populations were observed against tetracycline (75.7%), streptomycin (70%), and colistin (11.4%), whereas wildtype populations were observed against ciprofloxacin, ceftazidime, and gentamicin. The resistance genes found were blaTEM (ampicillin), aadA variant (streptomycin/spectinomycin), tetA (tetracycline), and floR (florfenicol). On 96-well polystyrene microtiter plate, 68.6% of the isolates proved to be biofilm producers. Among 36 S. Derby isolates selected to PFGE analysis, 22 were clustered with 83.6% of similarity. Additionally, 27 isolates were clustered in 11 pulsotypes, which presented more than one strain with 100% of similarity. Most of S. Derby isolates were able to form biofilm and were classified as nWT or resistant to tetracycline, streptomycin, and colistin. PFGE allowed the identification of closely related S. Derby isolates that circulated in pig slaughterhouses and pork derived products along a decade.