First report of PLSA resistance gene sal(B) in multidrug-resistant foodborne Mammaliicoccus lentus: Molecular characterization and food safety implications.

The emergence and dissemination of antimicrobial resistance through the food chain pose a significant threat to food safety and public health. Our nationwide surveillance of retail foods in China revealed alarmingly high resistance rates among foodborne Staphylococcus isolates, particularly to antibiotics targeting the 50S ribosomal subunit. Among these, Mammaliicoccus lentus (formerly Staphylococcus lentus) is a coagulase-negative species commonly found in food-producing animals and retail foods, increasingly recognized as a reservoir of antimicrobial resistance genes. To elucidate the genetic basis of this severe phenotype, we identified M. lentus strain M177 from retail potato flour exhibiting exceptionally high resistance to tiamulin, clindamycin, and lincomycin, a profile unexplained by known resistance genes. Targeted BLASTp analysis identified a putative ARE ABC-F gene sharing 98.71% amino acid identity with Sal(B), which we designated sal(B). Functional validation confirmed that Sal(B) confers specific resistance to PLSA (pleuromutilins-lincosamides-streptogramin A) antibiotics, with tiamulin MIC increased by 128-fold, and clindamycin and lincomycin by 8-fold. Systematic alanine-scanning mutagenesis identified the Loop region as the primary effector of resistance, with Tyr251 playing a predominant role, while Ser262, Pro263, Gly266, Arg275, and Phe276 contributed to varying degrees. Structural analysis revealed an allosteric protection mechanism in which resistance-altering mutations disrupt specific interactions with 23S rRNA nucleotides within the peptidyl transferase center, rather than directly displacing antibiotics. ATP hydrolysis was strictly required for Sal(B) function. Genetic environment analysis indicated that sal(B) is chromosomal and likely intrinsic to M. lentus, with limited horizontal transfer potential. Notably, sal(B) or highly similar homologs were detected in geographically distinct food sources, suggesting a potential distribution that warrants further systematic surveillance. To the best of our knowledge, this is the first report of sal(B) in a foodborne M. lentus isolate, establishing Sal(B) as a mechanochemical allosteric regulator of PLSA resistance. These findings highlight the need for enhanced surveillance of ARE ABC-F genes in foodborne coagulase-negative staphylococci beyond the traditional focus on S. aureus, due to the potential threat these resistance determinants pose to food safety and public health.