Abstract:
This study was designed to isolate and characterize bacteriocin-producing Lactic Acid Bacteria (LAB) from Kati, a sorghum based fermented food and to evaluate their antimicrobial effect on selected spoilage and pathogenic bacteria. Fifteen samples were collected from four different locations in Akoko area of Ondo State, Nigeria for microbiological analysis. The samples were unprocessed sorghum, fermented sorghum gruel and kati, the control sample was prepared in the laboratory of the Department of Microbiology, FUTA. The microorganisms associated with kati were isolated using nutrient agar, DeManRogosa Sharpe agar and potato dextrose agar and characterized on the basis of cultural, morphological and biochemical characteristics.The LAB were further identified as Lactococcuslactis, Lb. plantarum, Lb.brevis,Lb. fermentum KAT1 andLb. fermentum KAT2 using 16SrRNA gene sequence analysis. Out of 28 LABisolates, five were found to inhibit selected pathogenic bacteria namely; Bacillus cereus, Escherichia coli, Staphylococcus aureus and Salmonella typhimuriumusing agar well diffusion method. The bacteriocinproduced by LAB was extractedby centrifuging a 72-hour old MRS broth inoculated with the LAB. The pH of the test solution was adjusted to 7.0 using NaOH to exclude the inhibitory effect of organic acids after which the supernatant was filtered and the antimicrobial effect of hydrogen peroxide was eliminated by the addition of catalase. The crudebacteriocins produced by the isolated LAB were characterized with respect to the effect of temperature, pH and surfactant. The test organisms exhibited activities of 6400, 6400, 6400, 3200 and 1600AU/ml (AU- Arbitrary unit) respectively. Crude bacteriocin from byLb. brevis and Lc. lactis were the most heat stable at 121oC for 60 minutes, followed by Lb. plantarumstable at 121oC for 10 minutes whileLb. fementum KAT1 andLb. fermentum KAT2 had no activity at 121oC.The activity of bacteriocin demonstrated by the isolated LAB was also pH dependent. The highest antibacterial activity was
exhibited in pH ranged from2.0 to 6.0, while inactivation occurred at pH 8.0 to 12.0. Bacteriocins produced byLb. plantarum, Lc. lactis andLb. fermentumKAT1 showed the highest antibacterial activity and stability at pH 2.0 to 6.0. Exposure to Tween 20 resulted in an increase in the bacteriocintitre of the test LAB isolates except for Lb. fermentum KAT2where loss of activity occurred. The findings from this study suggest that bacteriocinogenic LAB present in katihave potentials to inhibit pathogenic/spoilage microorganisms present in foods.
