Abstract

Lactic acid bacteria (LAB) were isolated from raw and fermented milk samples. They were evaluated for hydrogen peroxide (H₂O₂) production. Sixty–three strains of LAB belonging to the genera Lactobacillus, Leuconostoc, Lactococcus and Streptococcuswere isolated and all the isolates produce H₂O₂ with Lactococcus lactis having the highest yield of 0.4279 mg/L in de Mann Rogosa Sharpe (MRS) broth. Among the high–level H₂O₂-producing strains are Lactobacillus bulgaricus, Lactobacillus casei, L. lactisand Streptococcus thermophiles. They were subsequently selected and their culture supernatants were evaluated at different temperatures, pH values and using different carbon and nitrogen sources. The highest quantity of H₂O₂ (0.6517 mg/L) was produced at 37°C by S. thermophilus, while L. casei produced the lowest quantity of 0.1132 mg/L at 40°C. S. thermophilus produced the highest quantity of 0.5912 mg/L at pH 5.5, while the lowest quantity of 0.1042 mg/L was produced at pH 7. L. bulgaricus produced the highest concentration of0.6512 mg/L when galactose was used as carbon source and lowest quantity of 0.0210 mg/L was produced by L. lactis in basal medium containing sorbitol as carbon source. L. casei produced   both the highest and lowest quantities of 0.1895 and 0.0207 mg/L of H₂O₂ in medium supplemented with yeast extract and ammonium sulphate as nitrogen sources respectively. The antimicrobial activity of the culture supernatants against Escherichia coli K12, Staphylococcus aureus and Candida albicansusing agar well diffusion assay was evaluated. Cell-free supernatant by L. bulgaricus had the highest inhibitory activity against E. coli K12 with 21 mm zone of inhibition and againstS. aureus with 15 mm zone of inhibition but showed no antagonistic activity against C. albicans. The study revealed that lactic acid bacteria isolated from raw and fermented milk in South-West Nigeria are capable of producing hydrogen peroxide which has antagonistic effect on pathogenic organisms, thus, may be promising sources of preservative that may in future be applied to food.

Key words: Hydrogen peroxide, growth conditions, nutrient utilization, antagonistic activity, lactic acid bacteria (LAB).