SURVIVAL AND GROWTH OF ACID ADAPTED ESCHERICHIA COLI STRAINS IN BROTH AT DIFFERENT PH LEVELS

Acid resistance of Escherichia coli O157:H7 strain UT 10 and Escherichia 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 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 of both AA strains was observed, indicating the importance of lipid modification in enhancing survival at low pH. The results of this study indicated that 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. PRACTICAL APPLICATIONS The study will enlighten the industry on the survival of acid adapted pathogens such as E. coli O157:H7 at low pH. It also indicates that current measures used in the preservation of low pH foods can trigger acid adaptation. For this reason, the effectiveness of current preservation measures in controlling foodborne pathogens should be reassessed. The importance of using cells adapted to different pH values in food challenge studies is also highlighted. Since fermented foods, which are generally regarded as safe, have been implicated in foodborne disease outbreaks, more attention should be given to the prevention of contamination of foods with pathogens such as E. coli O157:H7 since they may survive in low pH foods and cause disease.