Purpose: To determine if differences in lethality between acetic and sorbic acids are influenced by changes in internal cell pH and membrane potential (delta psi), using acid sensitive and acid resistant E. coli O157:H7 strains.
Methods: Acid sensitive (B201) and resistant (B241) strains of E. coli O157:H7 were grown in glucose containing minimal medium (M9GT) and Luria broth (LBG) to induce acid resistance, than incubated with acetic or sorbic acid at pH 3.5. Internal pH and delta psi were measured with radiolabeled benzoate (internal pH) or tetraphenylphosphonium bromide (delta psi) and tritium (cell volume). Partitioning of labeled compounds between the cell pellet and supernatant were determined using a scintillation counter.
Results: The internal pH of B201 and B241 were not significant between acid treatments for cells grown in LBG, but were (P>0.05) for M9GT grown cells. Sorbic acid treatments (but not acetic acid) decreased internal pH to 6.4 and 6.6 from 6.8 and 7.1 controls, and decreased delta psi to -81.9 mV and -72.1 mV (for B201 and B241, respectively) in M9GT. A positive correlation was seen between decreasing internal pH and delta psi, r2=0.8088.
Significance: Sorbic acid may diffuse across the cell membrane more efficiently than acetic acid which would reflect the lower internal pH reported. Additionally, the negative delta psi likely attracts extracellular protons to cross the cell membrane, killing cells faster. Although acetic and sorbic acid should affect cell death the same, these acids behave differently. Understanding the mechanisms by which organic acid preservatives kill pathogens may help define additional safety parameters for acid and acidified foods.