Comparative characterization of Curd-derived Bacillus coagulans and Bacillus clausii as Robust Probiotic Candidates

Traditional fermented foods are valuable reservoirs of robust probiotic microorganisms. In the present study, curd was explored as a natural source for isolating spore-forming probiotic bacteria with enhanced survivability. Two Bacillus isolates were obtained from curd samples following heat treatment and selective cultivation. Morphological, microscopic, and biochemical characterization, supported by 16S rRNA gene sequencing, identified the isolates as Bacillus coagulans (99.2% similarity) and Bacillus clausii (99.5% similarity). Both isolates exhibited key probiotic attributes in accordance with FAO/WHO guidelines. Acid tolerance assays revealed that Bacillus coagulans demonstrated superior survivability at low pH, retaining 80.3% viability at pH 3.0, while Bacillus clausii showed 70.6% survival. In contrast, Bacillus clausii exhibited greater bile salt tolerance, maintaining growth at bile concentrations up to 1%. Both isolates displayed significant antagonistic activity against common enteric pathogens, including Escherichia coli, Salmonella typhi, and Staphylococcus aureus, indicating strong antimicrobial potential. High spore-forming efficiency was observed,
particularly in Bacillus coagulans (83.9%), confirming their resilience to adverse environmental conditions. Additionally, both strains produced extracellular digestive enzymes such as amylase, protease, and lipase, highlighting their functional role in nutrient assimilation. Overall, the findings demonstrate that curd-derived Bacillus coagulans and Bacillus clausii possess complementary probiotic traits, combining acid resistance, bile tolerance, antimicrobial activity, enzymatic potential, and spore-forming capacity. These indigenous strains represent promising candidates for incorporation into functional foods, dietary supplements, and food biotechnology applications.