抄録
To investigate antibacterial action of fermented milk against enterohemorragic Escherichia coli O157:H7 (O157), overnight culture of clinically isolated O157 cells of two strains (NGY10 and 33) were incubated at 37℃ with a liquefied yoghurt product and survival of the cells were monitored by CFU assay with Desoxycholate agar plate. After the incubation, viable O157 cells in the product dramatically decreased by 4-5 logs after 7 h and could not be detected by 31 h. This bactericidal activity was not affected when lactic acid bacteria in the product were completely inactivated by heating or removed by membrane filtration. On the other hand, O157 cells in neutralized samples slightly grew in the early stage of incubation, then reversed themselves to decline by production of organic acids including lactic acid by lactic acid bacteria in the product. The skim milk solution whose pH was adjusted at 4.0 by lactic acid, also exhibited bactericidal activity. However, this phenomenon was not obvious when pH of the skim milk solution was adjusted by hydrochloric acid instead of lactic acid. These results suggest that free form of lactic acid is a major factor of the potent bactericidal activity and affects viability of O157 cells. The molecule may invade in bacterial cells through cell membrane and perturb cellular pH to become acidic. The samples represented a marked zone of inhibition against O157 by an agar diffusion method, indicating the similar effect to that obtained by the CFU counting.
