Phenotypic and genotypic antibiotic resistance patterns in Salmonella Typhimurium and its monophasic variant from pigs in southern Spain.

The high incidence of human salmonellosis and multi-drug resistant (MDR) strains of Salmonella Typhimurium (ST) is of concern to global public and animal health. Our research, by means of the broth microdilution method, evaluated the Minimum Inhibitory Concentration (MIC) distribution of 12 antimicrobials against a collection of 73 ST and mST and S. typhimurium monophasic variant 4,[5],12:i:- (mST) isolates from slaughtered pigs reared in extensive systems in southern Spain, and also 12 resistance-associated genes or antimicrobial resistance (AMR) determinants using qPCR. Our data revealed that 98.6% of strains were MDR, with resistance to cephalothin/tetracycline/sulfamethoxazole-trimethoprim/ampicillin/chloramphenicol being the most common pattern (55.6%). Regarding AMR determinants, the most significantly (p < 0.05) genes detected by qPCR were sul1 and aadA2 (89% of strains positive), aadA1 and dfrA12 (87.7%), and blaTEM and tet(B) (86.3% and 84.9%, respectively). Up to date information on ST antimicrobial resistance patterns is essential for epidemiological surveillance programs to support animal and public health. The high number of MDR isolates and variability regarding resistance determinants revealed in this study highlights the role of animals reared in extensive systems as a source of resistant Salmonella strains.