Selection and evaluation of heavy metal-resistant lactic acid bacteria for exopolysaccharide production and chromium removal

Lactic acid bacteria (LAB) are promising agents for heavy metal bioremediation owing to their capacity to tolerate and sequester toxic metals. This study aims to evaluate metal-resistant LAB strains and assess their potential application in heavy metal remediation. Twenty-one LAB isolates from diverse sources were screened for tolerance to iron (Fe2+), nickel (Ni2+), and chromium (Cr6+) at an initial concentration of 50 mg/L. Surface properties, including hydrophobicity and auto-aggregation, were investigated to identify strains with enhanced adhesive potential. Principal component analysis (PCA) allowed the selection of six strains exhibiting the most favorable traits. Further assessment of exopolysaccharides (EPS) production among the six strains led to the identification of a highly promising isolate, LGS4, identified as Lacticaseibacillus paracasei via 16S rRNA gene sequencing. The strain exhibited remarkable tolerance to Fe2+, Ni2+, and Cr6+, with minimum inhibitory concentration (MIC) reaching 1000 mg/L for each metal. Moreover, LGS4 produced a significant amount of EPS, reaching 290.7 mg/L. Notably, the strain achieved 72.3% Cr6+ removal within 4 h and 97.1% after 24 h. These findings highlight the potential of LAB, particularly EPS-producing strains, as efficient biological agents for the remediation of heavy metal-contaminated environments.