Strain-resolved biology of Lactobacillus johnsonii across host systems

Lactobacillus johnsonii is widely described as a probiotic species, but current evidence supports a strain-resolved view. Its biological effects depend on strain background, host niche, diet, challenge context and delivery format, including live cells, cell-derived structures and postbiotic fractions. This review synthesizes comparative genomic, mechanistic and intervention studies on L. johnsonii, using weaned piglets as the main translational reference because barrier injury, inflammation, microbial disruption, diarrhea and impaired growth often coincide after weaning. Piglet studies show that selected strains can reduce diarrhea or pathogen burden, improve barrier-related markers and alter immune, oxidative and metabolic responses. Cross-host work links strain variation to surface architecture, adhesion, exopolysaccharides, redox activity, extracellular vesicles and other non-cellular effectors. These mechanisms recur around colonization resistance, epithelial barrier support, immune regulation, microbial metabolite production and control of cellular stress. The strongest claims connect a named strain with a defined host context and a measurable phenotype. Evidence is weaker when species-level abundance, multistrain formulations or rodent disease models are used to infer direct efficacy in livestock. This review frames L. johnsonii as a host-associated, strain-dependent bacterial lineage rather than a uniform probiotic entity. This review is primarily focused on piglet-centered veterinary applications; therefore, cross-host studies are discussed mainly as mechanistic support rather than as direct evidence of efficacy in piglets or other livestock species.