Anti-Listeria Action of Organic Acids

NON-TECHNICAL SUMMARY: Organic acids are natural antimicrobials that are widely used in the food industry to inhibit growth of microbial pathogens such as Listeria monocytogenes and Escherichia coli O157:H7. The mechanisms by which organic acids inhibit bacteria are not fully understood, although a growing body of evidence suggests that the accumulation of acid anions may be the most inhibitory to cell growth in the mildly acidic environments that characterize many foods. This research will provide a more fundamental understanding of how intracellular anion accumulation inhibits growth of L. monocytogenes, and thereby uncover new strategies to maximize the efficacy of organic acids for control of this and other important bacterial pathogens in human food. <P>

APPROACH: L. monocytogenes 4b F2365 will be grown in mildly acidic medium (pH = 5.8 or 6.5) containing no added organic acid or containing d- and l-lactate, acetate, or levulinate added at concentrations calculated to similarly drive internalization of organic anions. Targeted measures will then be made of the growing cells to elucidate general and specific metabolic impacts of anion accumulation, and to identify cellular strategies to attenuate inhibition. Experiments will also be performed to collect and compare transcriptional profiles of cells grown under the same conditions, and superimpose those changes with observed physiological changes. Physiologic measures of the cells will include internal pH, anion and cation accumulation, anion efflux, membrane fatty acid content, ATP concentration, cell doubling time, and induction of acid tolerance response. DNA microarrays will be designed and synthesized based on the the complete genome sequence of L. monocytogenes 4b F2365, and the transcriptional profiles will be determined for cells grown in each treatment media. Key genes and operons identified in the microarray analysis will be studied using quantitative real time RT-PCR.