Genomic architecture and adaptive plasticity of Enterococcus lactis strains isolated from extreme Semi-arid environments

The presence of Enterococcus lactis in semi-arid "resource islands" the remarkable ecological plasticity of a species often associated with host-related environments. Characterizing the genomic mechanisms that facilitate its persistence in extreme edaphic niches is crucial for exploring its biotechnological potential in arid agriculture. This study characterized the genomic architecture, abiotic stress tolerance, and plant growth-promoting (PGP) capabilities of six E. lactis strains isolated from the rhizosphere of Pithecellobium dulce and Haematoxylum brasiletto in La Guajira, Colombia. We compared the pangenomes of the isolates with clinical and environmental reference strains. Genomic predictions were validated through in vitro assays for thermal, saline, and pH stress, PGP traits, and biosafety (hemolysis, biofilm formation). Analysis revealed a pangenome with a conserved 2,113-gene core and a highly plastic 3,134-gene accessory genome. The core genome encodes robust machinery for osmotic stress (e.g., opuA-C operons) and DNA repair (uvrC), while the accessory genome is heavily shaped by Horizontal Gene Transfer, containing abundant Mobile Genetic Elements (6.3%-16.4%). Phenotypically, strains exhibited high resilience to heat (50{degrees}C), salinity (5% NaCl), and alkalinity (pH 12). Adaptation in these isolates favors metabolic parsimony: rather than complex phytohormone synthesis, the strains prioritize inorganic phosphate solubilization (conserved pst system) and harbor a complete 2,3-butanediol cluster for volatile-mediated plant interaction. Notably, strain IS_B39 produced siderophores and carried a specific RiPP-like biosynthetic cluster, indicating niche-specific functional diversification. Genomic and phenotypic screening confirmed a safe profile, lacking key virulence factors. These findings define a robust, low-risk genomic toolkit, supporting the potential of E. lactis as a tailored bioinoculant for sustainable agriculture in extreme, water-limited environments. ImportanceEnterococcus species are traditionally studied as clinical pathogens or dairy-associated bacteria, leaving their ecological role in natural, non-host environments largely overlooked. This study challenges conventional paradigms by exploring Enterococcus lactis strains naturally persisting in the extreme, water-limited soils of semi-arid "resource islands" in La Guajira, Colombia. Through functional genomics and laboratory validation, we demonstrated how these bacteria utilize a specialized genetic toolkit to withstand extreme heat and alkalinity, while actively promoting plant resilience. Rather than relying on complex hormone production, they optimize vital nutrient uptake like phosphorus. These findings significantly advance environmental microbiology by uncovering the hidden survival strategies of lactic acid bacteria in arid lands, showcasing their immense potential as sustainable bioinoculants to support global dryland agriculture under climate change stress.