Isolation of Lactic Acid Bacteria from Fermented Milk And Evaluation of Their Antimicrobial Activity against Selected Pathogenic Bacteria, Jimma Town Southwest Ethiopia

Abstract

Background: Contamination of raw milk by pathogenic bacteria is still a difficult issue to regulate. However, fermentation with lactic acid bacteria will stifle the endurance of pathogenic and spoilage bacteria through production of lactic acid and other antimicrobial compounds. The objective of this study was to isolate lactic acid bacteria from fermented milk and evaluate their antimicrobial activity against selected pathogenic bacteria. Methods: Laboratory based experimental study design was conducted from May - July, 2021. Three samples of yogurt locally called ‘Ergo’ (each of 250 ml) were collected from Jimma town. Lactic acid bacteria from Ergo were investigated through morphological, physiological and biochemical characteristics using standard bacteriological techniques. Test (indicator) bacteria such as E. coli, S. aureus and Salmonella species were obtained from Jimma University Medical Center (JUMC). Antimicrobial activities of Lactic acid bacteria against indicator bacteria were performed with Agar well diffusion antimicrobial assay. Data was recorded, entered and managed in Microsoft Excel for further analysis. The data was further analyzed by using Statistical Package for Social Science (SPSS) software version 21 and Microsoft Excel Spread Sheet. Descriptive statistics such as tables and figures were applied to describe characteristics of data. Results: Twelve lactic acid bacteria (LAB) isolates were identified phenotypically. Those LAB isolates include six Lactococcus species, five Lactobacillus species and one Leuconostoc species. All of the LAB isolates showed antimicrobial activity against Staphylococcus aureus, Escherichia coli and Salmonella species. The most active LAB isolates against three indicator bacteria were ESCIa - 9 for S. aureus, ESBIa - 5 for E. coli and ESCIc-11 for Salmonella species with zone of inhibition of 13.6 ± 3.1 mm, 12 ± 1.8 mm and 11.6 ± 3.6 mm respectively. Average inhibition zones produced by isolates belonging to each LAB genus were compared and differences were significant for some LAB isolates. LAB isolate two (ESAIb) and LAB isolate nine (ESCIa) were statistically significant at P < 0.05. However, there were no other significant differences found between other conditions (P > 0.05). II Based on the average zones of inhibition produced by the lactic acid bacteria isolates, Lactobacillus isolates showed the highest antagonistic activity to the test strains followed by Lactococcus and Leuconostoc isolates. Among the indicator bacteria, Staphylococcus aureus was highly sensitive to antimicrobial effects of LAB and followed by E. coli and Salmonella species. Conclusion and Recommendation: All Lactic acid bacteria (LAB) isolates under investigation showed antimicrobial activity against indicator bacteria. However, the spectrum of antagonistic activity was varying for some of the isolates tested. The presence of LAB in the fermented milk allows the milk to be safe for use and serve as bio preservation of food. Utilization of antimicrobial compounds from LAB inhibited indicator bacteria, suggesting that it is of alternative importance during treatment of bacterial infections. However, genotypic identification of LAB isolates, looking for probiotic features, looking in vivo probiotic effects of LAB and characterization of antimicrobial compounds will be considered as recommendations.