Co-occurrence of biofilm formation and quinolone resistance in Salmonella enterica serotype typhimurium carrying an IncHI2-type oqxAB-positive plasmid

The objective of this study was to investigate the co-occurrence of biofilms and quinolone resistance in Salmonella enterica serotype Typhimurium mediated by IncHI2-type oqxAB-positive plasmids. Among the 40 Salmonella strains, we found that 27 isolates formed biofilms and displayed identical multidrug-resistance profiles to ciprofloxacin, doxycycline, sulfamethoxazole-trimethoprim, ampicillin and streptomycin, based on biofilm formation assays and antimicrobial susceptibility testing. In particular, a single S. Typhimurium isolate named SC523 produced the thickest biofilms and exhibited the highest-level resistance (MIC = 8 μg/mL) to ciprofloxacin compared to those of the other isolates. The detection of known plasmid-mediated quinolone resistance (PMQR) genes and point mutations in the quinolone resistance-determining region (QRDR) by PCR assay showed that oqxAB genes were present in 27 biofilm-positive isolates. Conjugation experiments, S1-pulse-field gel electrophoresis and biofilm formation assays demonstrated that the conjugative plasmid that encoded biofilms and quinolone resistance in Salmonella SC523 could be transferred to a recipient with a frequency of 4.7 × 10−3 per recipient cell. The results of PCR-based replicon typing (PBRT) showed that the IncHI2-type plasmids accounted for 100% of the biofilm-oqxAB-positive isolates and transconjugants. The sequence analysis of Salmonella SC523 confirmed that the oqxAB cassette and fourteen DNA transfer genes in the IncHI2-type oqxAB-positive conjugative plasmid were genetically responsible for the phenotypic quinolone resistance and biofilm formation. The conclusion is that the IncHI2-type plasmid in S. Typhimurium isolate from chicken farm was identified and sequenced, which contained oqxAB and tra/trh and encoded quinolone resistance and biofilms, and could be transferred to recipients through conjugation. Notably, the prevalence of IncHI2-type biofilm-oqxAB-positive plasmids in animal-origin Salmonella poses a threat to public health, as these Salmonella from poultry farms show a decreased susceptibility to quinolones and could spread to humans.