Association Between Alterations in Penicillin-Binding Proteins and β-Lactam Resistance in Ligilactobacillus salivarius

The aim of this study was to elucidate the molecular basis of β-lactam resistance in Ligilactobacillus salivarius, with a focus on mutations in genes encoding penicillin-binding proteins (PBPs). Twelve L. salivarius strains (2 reference strains and 10 turkey-derived isolates) were analyzed. Susceptibility to β-lactams was determined by the broth microdilution method. Whole-genome sequencing (WGS) analyses and polymerase chain reaction (PCR) were used to detect resistance genes. Alterations in 3 PBPs (FtsI, PBP2x-like, and PBP1A) were identified by comparative analysis of presumptive amino acid (aa) sequences. Phylogenetic relationships between strains were assessed using multilocus sequence typing (MLST). Six strains were resistant to ampicillin (MIC ranging from 32 to ≥128 mg/L) and showed elevated MICs for cephalosporins. No β-lactamase genes were detected in any strain. Resistant strains exhibited distinct patterns of aa alterations in the transpeptidase domains of FtsI and PBP2x-like proteins, whereas susceptible strains with elevated ampicillin MICs (2–4 mg/L) showed only partial alteration profiles. PBP1A displayed limited variation and no clear association with resistance phenotype. Taken together, these findings are consistent with an association between β-lactam resistance in L. salivarius and alterations in high-molecular-weight (HMW) PBPs, particularly FtsI and PBP2x-like proteins, in the absence of detectable β-lactamase production. However, further studies, including functional analyses, are needed to clarify the role of these PBP alterations in the observed resistance phenotype.