Acid adaptation of Escherichia coli O157:H7 in fermented goat milk

Abstract

This study was undertaken to determine the effect of acid adaptation on the survival and growth of Escherichia coli in broth and fermented goat milk. In the first phase of the study, acid resistance of E. coli O157:H7 strain UT 10 and E. coli ATCC 25922 was determined in brain heart infusion broth at pH 7.4, 4.5 and 2.5. Variations due to acid stress in the counts of both strains were determined. Alterations in the fatty acid profile due to acid adaptation were also determined. Acid adaptation enhanced the survival of both strains at pH 4.5, but neither strain could survive after 4 h at pH 2.5. At optimum growth conditions (pH 7.4), E. coli ATCC 25922 exhibited increased viability over E. coli UT 10. At pH 4.5, E. coli UT 10 was more tolerant to low pH than E. coli ATCC 25922. An increase in saturated fatty acids (SAFAs) of both AA strains was observed, indicating the importance of lipid modification in enhancing survival at low pH. In the second phase of the study, acid resistance of E. coli O157:H7 strains UT 10 and UT 15 were determined in commercial fermented at 30 °C for 24 h and stored at 7 °C for 2 days and traditional Amasi fermented for 3 days at ambient temperature (ca 30 °C). E. coli O157:H7 counts in traditional Amasi could not be detected after 3 days of challenge while those in commercial Amasi were detected at 2.7 log10 cfu/ml after the same period. The survival of acid adapted (AA) and non-adapted (NA) strains was similar in traditional Amasi while in commercial Amasi, the NA strain survived significantly better than the AA strain. The results of this study indicate that E. coli O157:H7 can survive during fermentation and storage of fermented goat milk Amasi. They also show that survival of the pathogen in a broth model differs from that in a food system. Survival of E. coli O157:H7 in fermented milk is affected by fermentation time, pH and storage temperature. The food industry should therefore adapt their processing/ preservation procedures by taking the most acid tolerant pathogenic E. coli strains into consideration in order to ensure the safety of their products.