The Mechanisms Involved in the Fluoroquinolone Resistance of Salmonella enterica Strains Isolated from Humans in Poland, 2018-2019: The Prediction of Antimicrobial Genes by In Silico Whole-Genome Sequencing.

Salmonellosis remains the second most common zoonosis in Europe. Resistance to fluoroquinolones (FQs) in Salmonella has been increasing worldwide, with WHO considering FQ-resistant Salmonella spp. as high-priority pathogens. The aim of this study was a retrospective analysis of the molecular mechanisms of FQ resistance, detected among clinical ciprofloxacin-resistant Salmonella enterica belonging to the most common serotypes. The whole genome sequences (WGS) of tested isolates were also analysed for the occurrence of other antimicrobial resistance determinants. Out of a total of 1051 Salmonella collected in the years 2018-2019, 447 strains belonging to the most common serotypes in Poland were selected were screened for FQ resistance using the pefloxacin disc test according to EUCAST recommendations. All pefloxacin-resistant isolates were confirmed as ciprofloxacin-resistant using the E-test. A total of 168 (37.6%) Salmonella enterica, which belonged to seven serotypes, were resistant to ciprofloxacin (mostly Hadar, Virchow and Newport). A hundred randomly selected Salmonella were investigated by WGS. A total of 127 QRDR mutations in GyrA and ParC were identified in 93 isolates. The qnr genes were the only PMQR determinants detected and were found in 19% of the sequenced isolates. Moreover, 19 additional resistance genes (including: bla,,tet, sul, aad, aac-, ant-, aph-, floR, cmlA) were identified among the FQ-resistant Salmonella tested that confer resistance to clinically important antibiotics such as β-lactams, tetracyclines, sulphonamides, aminoglycosides and phenicol, respectively). In conclusion, FQ resistance of human Salmonella in Poland is rising towards a critical level and needs to be tightly monitored.