Probiotic and antimicrobial characterization of bacterial isolates from poultry waste-contaminated soil

Soil contaminated with poultry waste may harbor probiotic bacteria with beneficial traits. This study to evaluate proteolytic probiotic bacteria from such soil for potential use in health and biotechnology applications. Six isolates previously isolated, identified and screened for proteolytic potential were used. Probiotic screening included antibiotic susceptibility (disc diffusion), hemolytic activity on blood agar, bile resistance at 0.1–2.0% bile salt, and acid tolerance at pH 1.5 to 3.5. Growth responses were evaluated via CFU count and optical density. Bacillus subtilis strain NBT-15, Bacillus amyloliquefaciens strain FORCN102, LactoBacillus plantarum strains ML05 and B19, and LactiplantiBacillus plantarum strains KCB4 and S4 were identified. The acid tolerance test revealed that all isolates were able to survive highly acidic conditions ranging from pH 1.5 to 3.5. The highest viable count was observed in LactoBacillus plantarum strain ML05 at pH 3.5 with 1.5 × 10⁶ CFU/mL, while Bacillus subtilis strain NBT-15 had 2.3 × 10³ CFU/mL at pH 1.5, indicating strong acid resistance. In the bile resistance assay, isolates demonstrated moderate to high survival at bile concentrations from 0.1% to 2.0%. LactoBacillus plantarum strain B19 exhibited the highest resistance with 98.99% at 0.1% bile and 44.21% at 2.0%, while LactiplantiBacillus plantarum strain S4 showed 94.29% resistance at 0.1% and 28.76% at 2.0%. Antibiotic susceptibility profiling showed that Bacillus amyloliquefaciens strain FORCN102 was susceptible to all ten antibiotics tested, with inhibition zones of 16 mm for levofloxacin, 15 mm for rifampicin and erythromycin, and 14 mm for gentamicin, suggesting low antibiotic resistance. In contrast, LactoBacillus plantarum strains ML05 and S4 were completely resistant to all antibiotics tested. Hemolysis tests revealed that LactoBacillus and LactiplantiBacillus strains were non-hemolytic, while Bacillus subtilis strain NBT-15 and Bacillus amyloliquefaciens strain FORCN102 exhibited alpha-hemolysis. Antibacterial assays showed varying degrees of inhibition against Escherichia coli, Staphylococcus aureus, and Salmonella typhi. LactoBacillus plantarum strains, produced the highest zone of inhibition of 12 mm against E. coli, S. aureus and S. typhi, while Bacillus amyloliquefaciens strain FORCN102 showed inhibition zones of 2 mm against E. coli which was the lowest among all. The findings suggest that poultry waste-contaminated soil is a viable source of proteolytic probiotic bacteria with promising functional properties for potential therapeutic and industrial applications.