ABSTRACT

This study was aimed at analyzing the antimicrobial activities of Lactic Acid Bacteria (LAB) on some food spoilage microorganisms. Changes in pH and titratable acidity (TA) of the samples were investigated for a period of four days (96 hours) and a decrease in pH was associated with an increase in titratable acidity. The isolation and characterization of eleven tentative LAB from fermented maize and cassava (Ogi and Fufu respectively) as well as identification of the spoilage organisms from fish were aseptically performed and the antimicrobial activity was determined by agar well diffusion method against three isolated food spoilage organisms (Pseudomonas spp, Enterobacter spp and Bacillus spp). Out of eleven isolates, six best strains designated as OG1, W₂2, W₁2,Y₁1, F₁2 and F6 showed good inhibitory activities (11.25 ± 1.06 - 22.00 ± 0.71mm) against all the food spoilage organisms with F₁2 having the highest inhibitory diameter of  22.00±0.71mm against Bacillus spp. These species of the isolates were selected and further identified as Lactobacillus amylolyticus strain L6, Lactobacillus plantarum strain ci-4wand Lactobacillus sakei strain MLS1by the aide of genotypic characteristics (16S rRNA gene sequences). These strains were screened for their exopolysaccharide (EPS) producing activity, resistance to low pH and bile salts as well as bacteriocin activity. The investigation in the screening for EPS synthesis showed that Lactobacillus amylolyticus strain L6 is an active producer of exopolysaccharide. These three isolated strains showed good tolerance to pH 3.0 after 3 hours and 6 hours with a range of 52.50±6.36 –90.00±4.24log CFU/ml; however, all the isolates were able to survive in the presence of bile salts (0.1, 0.3 and 0.5 %) where Lactobacillus plantarum strain ci-4w and Lactobacillus sakei strain MLS1 showed the highest survival rates. The bacteriocins produced by these strains were prominent antibacterial, tolerated acidic pH 4 although recorded the lowest inhibitory activities at pH 9 but indicate highest inhibitory activities between pH 6.0 and 7.0. The effects of different temperature on the bacteriocin activity of these Lactobacillus strains exhibits full bacteriocin stability at 40°C, 60°C and 80°C. This present study revealed that the activity of bacteriocin was gradually decreased after 48 hours storage period. There was a significant difference (P < 0.05) between the antimicrobial activities of the isolated Lactic acid bacteria (LAB) species. Therefore, this present study suggested that Lactobacillus amylolyticus strain L6, Lactobacillus plantarum strain ci-4wand Lactobacillus sakei strain MLS1have an excellent probiotic potential and the possibility of using their bacteriocins as a food preservative against food spoilage organisms.