Use of The Metabolic Extract of Locally Isolated Streptococcus thermophilus Bacteria in Inhibiting Pathogenic Bacteria

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Abstract
Research into bio preservation using lactic acid bacteria (LAB) as alternatives to synthetic additives has seen significant growth in response to the increasing consumer demand for natural food preservatives. In this study, the biochemical properties and antibacterial activity of Streptococcus thermophilus isolates isolated from commercial yogurt brands were thoroughly evaluated for use in bio preservation. Twenty isolates were obtained from local markets and various yogurt brands and subjected to comprehensive biochemical characterization. Their antimicrobial activity against twelve pathogenic bacterial strains was evaluated. Biochemical analysis demonstrated that all isolates exhibited the intrinsic characteristics of S. thermophilus : catalase-negative, oxidase-negative, and alpha-hemolytic, with optimal growth conditions ranging from 30 to 45°C and remarkable tolerance to a wide range of pH (4.2–9.6) and salt concentrations (2–4% NaCl). All strains demonstrated the ability to ferment lactose, sucrose, glucose, and maltose, but were unable to metabolize mannitol, melibiose, raffinose, and sorbitol. Antimicrobial screening showed that 70% of the isolates (14/20) exhibited significant inhibitory effects, with preferential activity against Gram-positive bacteria (overall inhibition rate: 33.3%). Listeria monocytogenes showed moderate sensitivity (inhibition zone of 1.7 cm), while S. aureus showed the highest sensitivity, with inhibition zones extending to 2.3 cm. A secondary screening based on acidification capacity found strain S5 from Kefil yogurt as superior, exhibiting the highest antibacterial activity (5.7 cm inhibition zone) along with the highest acidity yield (1.14%). These results confirm the enormous potential of yogurt-derived Streptococcus thermophilus strains as natural biopreservative. Furthermore, they address consumer demands for clean alternatives to synthetic preservatives by providing a systematic framework for strain selection and optimization in food preservation applications.

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