Ampicillin-induced resistant variants of Listeria monocytogenes: effects on growth, survival, and virulence.

Listeria monocytogenes is a major foodborne pathogen whose rising antibiotic resistance poses increasing public health concerns. Understanding how resistance acquisition affects bacterial physiology is essential for accurate microbial risk assessment. In this study, five antibiotic-sensitive foodborne L. monocytogenes strains were subjected to ampicillin (AMP) exposure to obtain AMP-resistant variants. Paired susceptible and resistant strains were compared for growth kinetics at 25 °C, 30 °C, and 37 °C; tolerance to heat (60 °C) and acid stress (pH 2.5); biofilm formation; and in vivo virulence using a Galleria mellonella model. Whole-genome sequencing and SNP analysis identified resistance-associated mutations. The AMP-resistant strains exhibited cross-resistance to multiple antibiotic classes, including lipopeptides, lincosamides, tetracyclines, and penicillins. Phenotypically, these strains exhibited a higher growth rate but an extended lag phase at 30 °C, along with reduced thermotolerance, decreased acid resistance, diminished biofilm-forming capacity, and attenuated virulence in the infection model. Whole-genome sequencing (WGS) combined with single nucleotide polymorphism (SNP) analysis identified non-synonymous mutations in five genes (bgsB, pbpA, htrA, uhpT, and mepA), potentially underpinning these phenotypic shifts. These results suggest a trade-off between antibiotic resistance and other biological characteristics in L. monocytogenes, offering valuable insights for microbial risk assessment. Monitoring of resistance-related adaptations and integrating genomic data into food safety strategies will be essential for controlling antibiotics resistant pathogens in food chain and minimizing associated public health risks.