Abstract

*Esayas Assefa1, Fekadu Beyene2 and Amutha Santhanam3

Lactic acid bacteria (LAB) predominates the microflora of milk and milk products. They produce antimicrobial metabolites that inhibit growth of food-borne pathogenic and spoilage microorganisms, thereby enhancing the shelf-life of the food. To evaluate the antimicrobial activity and the effect of temperature and pH on the culture supernatant of LAB, 112 strains of LAB, belonging to Lactococcus, Lactobacillus, Entorococcus, Streptococcus, Leuconostoc and Pediococcus species, were isolated from Ergo. The culture supernatants of all isolates were examined for antimicrobial activity on indicator strains, that is, Salmonella typhi, Shigella flexneri, Staphylococcus aureus (ATCC-25923) and Escherchia coli (ATCC-25922), using the disc diffusion assay. Twelve strains of LAB that produced antimicrobial substances were subsequently selected and their culture supernatants were evaluated for stability at different temperatures and pH values. The antimicrobial activity of the culture supernatants of the LAB was completely inactivated when treated at 121o C for 15 min, whereas at 30, 60 and 80o C, there was no significant (P>0.05) difference in the diameter of the inhibition zones compared to the control. The culture supernatants was stable over a wide pH range (pH 2 - 10) and no significant (P>0.05) difference was observed between the treated and untreated (control) supernatants. However, the diameter of the inhibition zones was significantly (P <0.05) decreased at pH 12. Thus, Ergo may be a promising source of antimicrobial substance-producing LAB that may in future be applied to food.