Quantifying the Effects of Acid (pH 3.6) Stress on Non-O157 Shiga Toxin-producing Escherichia coli Strains

Introduction: In the US, non-O157 Shiga toxin-producing Escherichia coli (STEC) may account for up to 20%-50% of all STEC infections. Because of the high incidence rate and the relatively little that is known about non-O157 STEC, it is important to study and understand how these pathogens respond to acid stress encountered in acidic foods or used as “hurdles” in the food industry.

Purpose: The focus was to investigate how non-O157 STEC strains namely STEC O26, O45, O145, O111, O121, and O103 strains respond to a pH level of 3.6, normally encountered in berries such as strawberries.

Methods: A high titer (10⁹ CFU) preparation of the different STEC strains was incubated in an acidic (pH 3.6) buffer solution and in a strawberry (pH 3.6) puree for 24 hours at room temperature. The pH 3.6 level was used to emulate acid stress routinely occurring in fresh berries. The number of survivors after 24 hours of acid stress exposure was then determined.

Results: The pH 3.6 acid buffer resulted in approximately 6 ± 1.1 log reductions of the STEC strains while the pH 3.6 strawberry puree only caused a 1.4 ± 0.67 log reduction of the STEC strains. STEC O45 and O145 strains were the most resistant to acid stress, while O26 and O121 strains were the most sensitive to acid stress.

Significance: The pathogens are significantly more resistant to pH 3.6 in a fruit matrix compared to a pH buffer. These results suggest that acid stress response in STEC strains is mechanistically different depending on the matrix in which the exposure occurs. The underlying metabolomic mechanisms need to be elucidated.