Elimination of Pathogen Escherichia coli O157:H7 in Ground Beef by a Newly Isolated Strain of Lactobacillus acidophilus during Storage at 5°C

Background and Objective: Constant use of limited number of lactic acid bacteria species in biopreservation can cause genetic degradation and or rising resistance against food pathogens or antimicrobial substances they produce. For this objective, a newly isolated strain of Lactobacillus acidophilus possessing high antimicrobial activity was evaluated as a candidate for use in biopreservation. Materials and Methods: Antibacterial activity was evaluated by agar disk diffusion method. Hydrogen peroxide amount was measured by Merckoquant Peroxide test strips. Microbiological analysis of the ground beef infected by Escherichia coli O157:H7 and treated by Lactobacillus acidophilus GH 201was done by plating of serial dilution in physiological saline on Tryptose agar. Results and Conclusion: The cells (10^9 CFU ml^-1) of Lactobacillus acidophilus produced significant amount of antibacterial substances mainly hydrogen peroxide (28 and 30 μg ml^-1) in sodium phosphate buffer (0.2 M, pH 6.5) and LAPTg at 5°C during submerged cultivation with no growth, respectively. Submerged co-cultivation of Escherichia coli O157:H7 with lactobacilli in LAPTg broth at 5°C reduced the total number of the pathogen more than 2 log for 5 days. In case of solid state cultivation on agar-based medium, the maximum inhibitory zones on Escherichia coli O157:H7 lawn around the disks soaked by different amounts of washed Lactobacillus acidophilus cells appear for one-day cold exposition. The size of inhibition zone depends on the concentration of lactic acid bacteria cells. The cell suspension intended for treatment must contain 10^8-9 CFU ml^-1 of lactic acid bacteria. Lactobacillus acidophilus reduced the initial amount (2×10^5 CFU ml^-1) of Escherichia coli O157:H7 in ground beef up to 2 log for 5 days of solid-state co-cultivation. The application of Lactobacillus acidophilus bacteria expanded the shelf-life of ground beef due to inhibition of psychrophilic spoilage microorganisms.