Lactobacillus acidophilus, a species of lactic acid bacteria naturally found in the human gastrointestinal tract, mouth, and vagina, and widely used as a probiotic. It ferments sugars into lactic acid, helping to maintain an acidic environment that discourages harmful organisms. The food industry adds L. acidophilus to yoghurts, fermented milks, and dietary supplements marketed for digestive health, immune support, and restoring gut flora after antibiotics. Different strains vary in their documented effects, and regulatory authorities require evidence before specific health claims can be made. As one of the most studied probiotic organisms, it remains a cornerstone of the functional dairy category.