ABSTRACT

This study was aimed at establishing the antimicrobial activity of lactic acid bacteria (LAB) on some food spoilage and pathogenic organisms. Serial dilutions of fermented maize and sorghum samples were performed and 0.1ml of the appropriate dilutions were inoculated on de Man Rogosa Sharpe medium for the isolation of lactic acid bacteria. Lactococcus and Lactobacillus species were isolated, identified and characterized. The test organisms were confirmed by biochemical tests to be Pseudomonas sp., Escherichia coli, Serratia marcescens, Bacillus sp. and Staphylococcus sp. The two LAB isolates were further confirmed molecularly to be Lactococcus specie 104.6 and Lactobacillus fermentum strain SN5. Using the agar well diffusion method, the two isolates inhibited the growth of all the test organisms except Pseudomonas aeruginosa which showed no zone of inhibition. The inhibitory activity of Lactococcus sp. 104.6 was the highest, 12.7±2.31 mm on S. marcescens, 11±1.00 mm on Staphylococcus sp. 9.6±1.52 mm on E. coli with the least inhibitory activity 8.3±1.52 mm on Bacillus sp. The highest inhibitory activity of Lactobacillus fermentum was also observed as 17.0±1.00mm on S. marcescens, followed by 14.3±1.15 mm on E. coli, 9.0±1.73 mm on Bacillus sp. with the least inhibitory activity 11±1.00mm on Staphylococcus sp. The co-culturing of lactic acid bacteria and test organisms indicated gradual reduction in the number of test organisms during the period of monitoring. The GC-MS analysis of Lactobacillus fermentum showed five volatile organic acids namely, acetic acid, propionic acid, isobutyric, butyric acid and isovaleric acid. The results of this study suggest that Lactococcus species and Lactobacillus fermentum can be used as biopreservatives in food industries rather than the synthetic chemical preservatives which is commonly used.