Probiotic and aroma-producing Bacillus sp. isolates from teles, a traditional fermented fish from Assam, India: functional characterisation and potential application in fermented foods.

Background: Teles is a traditional fermented food native to Assam, India, valued for its unique aroma and potential health benefits. Despite its cultural importance, the beneficial microbial communities in Teles remain poorly characterised, particularly with respect to probiotic functionality and food-enhancing properties. This study aimed to isolate and characterize microbial strains from Teles for their probiotic potential and aroma-producing capabilities, targeting their application as functional starter cultures in food fermentation.

Methods: Bacterial strains were isolated from Teles and evaluated for essential probiotic traits, including tolerance to acid (pH 2.0), salt (8% NaCl), bile (2.0%), and phenol (0.4%). Phenol tolerance-the ability to survive toxic compounds generated during digestion-was used to assess intestinal robustness. Auto-aggregation-the ability of bacteria to clump together-was measured as an indicator of their potential to form stable colonies in the gastrointestinal tract. Adhesion to intestinal surfaces was assessed using chicken crop epithelial cells. Pathogenicity of the isolates was confirmed by screening for haemolytic activity, DNase production, and antibiotic susceptibility. Functional validation involved fermenting milk and rice substrates (to produce curd and rice beverage), followed by gas chromatography-mass spectrometry (GC-MS) analysis of flavour compounds. Strain identification was conducted via 16 S rRNA gene sequencing.

Results: Out of 35 isolates, strain TL/NA/2 (Bacillus sp.; GenBank Accession No. PQ473688.1) emerged as the most functionally promising strain. It exhibited high tolerance to multiple stressors: acid (pH 2.0), 8% NaCl, 2.0% bile, and 0.4% phenol. TL/NA/2 showed a strong self-clumping ability (27.88 ± 1.04% auto-aggregation after 5 h) and adhered well to epithelial cells (84.19 ± 1.28%). The strain was non-haemolytic, DNase-negative, and sensitive to several clinically relevant antibiotics. GC-MS profiling revealed production of beneficial flavour compounds including pyrazine, heptane, and oxalic acid. Sensory evaluation of the fermented products confirmed improved flavour and overall acceptability.

Conclusion: Strain TL/NA/2 (Bacillus sp.) demonstrates strong probiotic potential along with the capacity to enhance flavour, making it a suitable multifunctional starter culture for developing functional fermented foods and beverages. Its stress resilience, safety profile, and sensory benefits position it as a promising candidate for industrial probiotic applications.