Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Nutrition, Dietetics and Food Science

Committee Chair(s)

Charles E. Carpenter


Charles E. Carpenter


Jeff Broadbent


Brian Nummer


Listeria monocytogenes and Escherichia coli are both among the most common microbial pathogens that cause foodborne illnesses and death. They both are capable of growing over a wide range of conditions. Organic acids are widely employed in the food industry to control growth of these pathogens to help prevent foodborne illnesses. There is substantial evidence that intracellular accumulation of organic acid anions is a major inhibitor to cell growth, and that many bacteria may combat anion accumulation by lowering their intracellular pH (pHi). In this study, we followed the accumulation of acid anion into the cell pellet and parallel changes in pHi in two human pathogenic strains of L. monocytogenes (N1-227 and R2-499) and in E. coli O157:H7 after exposure to sub-bacteriostatic levels of lactic and acetic acids at mildly acidic pH 6.

Cells were exposed to two controls, or to acetic or lactic acid. The accumulation of anions into the cell pellet and pHi was measured after 60 minutes habituation in these treatments. It is noteworthy that our measures of anion accumulation and pHi were independent rather than being confounded as when employing the common practice of calculating intracellular concentration of anions based on difference in internal and external pH, or conversely calculating pHi based on intracellular accumulation of acid anion.

All three bacteria accumulated up to 3 times more acetate anion into the cell pellet compared to their external environment. L. monocytogenes accumulated less lactate than acetate anion, and E. coli did not significantly accumulate lactate. The values for anion accumulation into the pellet were much less than expected based on previous reports and predicted by the measured difference in internal and external pH. Exposure of E. coli to organic acids did not induce a change in pHi from its baseline pHi. In contrast, exposure of L. monocytogenes N1-227 and R2-499 to the organic acids induced lowered pHi as compared to baseline in both strains.

In summary, Listeria and E. coli were exposed to non-bacteriostatic levels of external organic acids at mildly acidic pH 6. Listeria accumulated more acetate than lactate while mounting a defense against anion accumulation that included lowering its pHi, while E. coli accumulated only acetate and apparently made use of combat mechanisms other than lowering pHi not explored in this study. The methodology employed in these studies was based on independent measures of pHi and intracellular anion accumulation, and the resulting data brings into question the common, but confounding, practice of using intracellular anion accumulation as a measure of pHi, and vice versa.