Background: Previous studies have demonstrated the beneficial effects of Ligilactobacillus salivarius A3iob on honeybee (Apis mellifera) colonies' health and honey production. The present work aimed to assess the genomic characteristics of the A3iob strain to understand its ability to improve bees' health.

Methods: The comparative genomic analysis was performed with the A3iob genome and the genomes of probiotic L. salivarius strains of human, porcine, and chicken origin, as well as bacteria isolated from the bees' gut. The analysis included the examination of metabolic genes and functional genes related to adhesion, the production of bioactive compounds, the modulation of the host's immune system, and antimicrobial substances. Genes associated with antimicrobial resistance and virulence were also analyzed.

Results: In silico studies revealed that L. salivarius A3iob possesses genes for glycosyltransferases (GTs) from the families GT2 and GT4, like Bombella apis and Bombella intestinalis, and glycosylhydrolases (GH) from the families GH1, GH2, GH13, GH36, GH65, and GH177, similar to Apilactobacillus kunkeei, Enterococcus durans, and bifidobacteria isolated from the bee intestine. The A3iob strain also has a unique genetic profile with a high number of secretion system genes and adhesion genes, including the ones coding for the SecA2/Y2 system, the mucus-binding proteins MucBP1, MucBP2, and MucBP3, and a pilus cluster (pilA, SpaA, SpaB, and sorteaseA) that has only been described in five strains of the L. salivarius species and in the intestinal bee-derived strain E. durans EDD2, which could be involved in the successful colonization of the A3iob strain in the bee gastrointestinal tract. Additionally, L. salivarius A3iob showed the presence of exopolysaccharide biosynthesis clusters described in the probiotic L. salivarius UCC118. Genes related to oxidative stress response (thioredoxin and NrdH-redoxin systems) and the bacteriocin genes abp118A and abp118B were found in the A3iob genome. L. salivarius A3iob does not harbor virulence or antibiotic resistance genes.

Conclusions: The genomic characterization of L. salivarius A3iob performed in this work provides some clues about the genetic mechanisms underlying its probiotic properties, paving the way for future research aimed at improving bees' health and productivity in the face of environmental challenges.