Metabolomic Analysis of Acid Stress Response in Shiga Toxin-producing E.coli O26:H11

Introduction: Shiga toxin-producing non-O157 E.coli (STEC) strains such as E.coli O26 are responsible for a growing number of food-related illnesses around the world. Bacterial pathogens usually encounter pH levels around 3.6 in fruits and juices and pathogen outbreaks associated with such foods occur. A mechanistic understanding of acid stress in non-O157 STEC can help refine the “hurdles” used in the food industry.

Purpose: Metabolomic biomarkers accumulating in E.coli O26:H11 when exposed to acid (pH 3.6) conditions in strawberries was compared to a non-acidic (pH 7.5) control buffer.

Methods: Synthetic (pH 7.5) buffer and strawberry puree (pH 3.6) were used to identify the metabolites accumulating in a high titer preparation of E.coli O26:H11 cells during a 24-hour exposure to pH 3.6 in the strawberry puree compared to the control buffer. Culture methods and viability assays monitored the pathogen’s survival while TEM was used to verify the structural integrity.

Results: Only a 1-log decline in the pathogen numbers occurred when exposed to the strawberry (pH 3.6) puree. Untargeted metabolomic analysis identified 293 metabolites out of which 155 were differentially (P < 0.01) expressed in the strawberry puree. Six different metabolic pathways were differentially triggered (P < 0.01) after 24 hours of exposure in the strawberry puree  The pathways were  D-Glutamine/D-glutamate, purine, pyrimidine, peptidoglycan biosynthesis, citric acid cycle, and Glyoxylate/dicarboxylate metabolism. Viability assays and TEM confirmed the structural integrity of the pH 3.6 exposed cells.

Significance: The results demonstrate that exposure of E.coli O26 to a strawberry (pH 3.6) matrix elicits a unique set of metabolic biomarkers. The metabolic pathways are preserving the structural integrity and viability when exposed to acid (pH 3.6) stress in the strawberry matrix.