Submitted to: Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/1996
Publication Date: N/A
Interpretive Summary: E. coli O157:H7 is a highly toxic strain that kills more than 500 people per year. The growth of E. coli in the rumen and in the intestines is inhibited by a group of compounds known as volatile fatty acids. The primary inhibitor is acetate, a two-carbon, organic acid. Other workers reported that E. coli O157:H7 had a greater resistance to acetate than other strains of E. coli, but the mechanism of this resistance was not known. We found that E. coli O157:H7 can let its intracellular pH decline (a decline in pH corresponds to increased acidity). This decline in intracellular pH prevented E. coli O157:H7 from accumulating high and toxic concentrations of intracellular acetate. This and subsequent work may provide a means of inhibiting growth of E. coli in animals and man, thereby reducing the ill effects of exposure to this strain of E. coli.
Technical Abstract: Batch cultures of E. coli K-12 grew well in anaerobic glucose medium at pH 5.9, & the final pH was 4.3. The growth of K-12 was inhibited by as little as 20 mM sodium acetate, & this inhibition caused an increase in final pH. E. coli O157:H7 was at least 4-fold more resistant than K-12, & it tole- rated more than 80 mM sodium acetate before growth was inhibited. Contin- uous cultures of E. coli K-12 (pH 5.9, dilution 0.085 h-1) did not wash ou until the sodium acetate concentration was 80 mM. E. Coli O157:H7 persist in continuous culture until the sodium acetate concentration was 160 mM. E. coli K-12 accumulated as much as 500 mM acetate, but intracellular acetate concentration of O157:H7 was never greater more than 300 mM. Differences in acetate accumulation could be explained by intracellular pH & the trans- membrane pH gradient (delta pH). E. coli K-12 maintained a more or less constant delta pH (intracellular pH 6.8), but E. coli O157:H7 let its delta apH decrease from 0.9 to 0.2 units as sodium acetate was added. Sodium acetate caused a decrease in YATP, but there was little evidence to support the idea that acetate was creating a futile cycle of protons. Intracellular acetate anion concentrations approached amounts predicted by delta pH, & decreases in YATP could be explained by decreases in the delta G of acetate production & shifts to D-lactate production. Both strains exhibited an acetate-dependent increase in lactate production, but O157:H7 had at least 2.5-fold greater capacity to convert glucose to lactate. Based on these results, the acetate tolerance of O157:H7 can be explained by fundamental differences in metabolism & intracellular pH regulation. By producing lac- tate & decreasing intracellular pH, O157:H7 was able to decrease delta pH and prevent toxic accumulations of intracellular acetate anion.