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Title: Modeling the effects of sodium chloride, acetic acid and intracellular pH on the survival of Escherichia coli O157:H7

Author
item HOSEIN, ALTHEA - North Carolina State University
item Breidt, Frederick
item SMITH, CHARLES - North Carolina State University

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2010
Publication Date: 2/1/2011
Citation: Hosein, A.M., Breidt, F., Smith, C.E. 2011. Modeling the effects of sodium chloride, acetic acid and intracellular pH on the survival of Escherichia coli O157:H7. Applied and Environmental Microbiology. 77(3):889-895.

Interpretive Summary: Acid resistant bacteria such as Escherichia coli and Salmonella have caused disease outbreaks and death in some acid foods such as apple cider and orange juice. It is known that some disease causing E. coli strains are the most acid resistant organisms of concern in acidified pickles. While studies to determine survival times in acid solutions have been done for these bacterial pathogens, little is known about the mechanism by which acid solutions kill bacteria, and what changes in the internal physiology of the bacterial cells lead to cell death. We examined how the internal cell acid conditions (pH and acid concentration) change during acid challenge of a selected disease causing E. coli strain under a variety of acid and salt conditions. The pH and acid concentration inside the cell was found to decrease in respond to external acid conditions. The drop in internal pH was directly correlated a specific form of the acid (protonated acid) and with the time needed to kill the organism. Understanding how food acids work to kill pathogenic bacteria may help researchers develop novel acid treatments and help estimate the safety of existing food products.

Technical Abstract: Microbiological safety has been a critical issue for acid and acidified foods since it became clear that acid-tolerant pathogens such as Escherichia coli O157:H7 can survive (even though they are unable to grow) in a pH range of 3 to 4, which is typical for these classes of food products. The primary antimicrobial compounds in these products are acetic acid and NaCl, which can alter the intracellular physiology of E. coli O157:H7, leading to cell death. For combinations of acetic acid and NaCl at pH 3.2 (a pH value typical for non-heatprocessed acidified vegetables), survival curves were described by using a Weibull model. The data revealed a protective effect of NaCl concentration on cell survival for selected acetic acid concentrations. The intracellular pH of an E. coli O157:H7 strain exposed to acetic acid concentrations of up to 40 mM and NaCl concentrations between 2 and 4% was determined. A reduction in the intracellular pH was observed for increasing acetic acid concentrations with an external pH of 3.2. Comparing intracellular pH with Weibull model predictions showed that decreases in intracellular pH were significantly correlated with the corresponding times required to achieve a 5-log reduction in the number of bacteria.