Effects of Organic Acids on Escherichia coli O157:H7 Independent of pH under Acidified Food Conditions
Outbreaks of illness caused by acid-tolerant bacterial pathogens in acid foods have raised questions about the safety of acidified foods. However, acidified foods have an excellent safety record which is in part due to the presence of organic acids. The objective of this research was to (1) a develop method that allowed rapid comparison of test acids on E. coli, (2) measure the effectiveness of acetic, malic, citric, and lactic acids to kill Escherichia coli O157:H7 independent of pH effects and (3) examine the mechanism of protection of D-lactic acid and on E. coli O157:H7. Gluconic acid (20 mM), which has been shown to be non-inhibitory to E. coli, was used to buffer solutions with 0 mM to 420 mM fully protonated concentrations of the test acids at various pH?s and temperatures, with ionic strength adjusted to 0.60-0.68. The effect of anaerobic incubation during acid challenges was examined. We found that using the 96 well microplate and determining viability by plating before and after acid challenge was consistent reproducible and efficient method which allowed us to evaluate up to 96 solution simultaneously. Compared to pH, acetic, malic, pyruvate, and particularly D-lactic acids had a significant protective effect on E. coli O157:H7 at low (1?20 mM) protonated acid concentrations. Citric acid was not found to exhibit any protective effect at similar concentrations. At higher concentrations L-lactic acid was the most lethal acid followed by acetic, citric and malic acids which had about equal degree of lethality against the O157:H7 strains. To our knowledge this is the first time organic acids have been observed to have a protective effect on pathogenic bacteria. D-lactic acid and pyruvate protected cells against 40 mM protonated acetic acid which has previously shown to reduce cells beyond detection during the acid challenge. These acids may protect the cells by inactivation of reactive oxygen radical species. These results showed that oxygen may play a major role in the effect of organic acids on E. coli O157:H7. It is important to understand the mechanism of this protective effect to be able to assure safety of acid and acidified foods. Additional research will be needed to understand acid specific effects on the survival of E. coli O157:H7 and other acid-resistant food pathogens in acid and acidified foods. In order to reliably and efficiently eliminate acid tolerant pathogens from acid and acidified foods we should have a clear understanding of the environmental factors that affect killing of these microorganisms. Thus, the role of oxygen during acid challenge studies must be determined to assure the safety of acid and acidified products.
Advisor:Dr. Fred Breidt; Dr. Hosni M. Hassan; Dr. Roger F. McFeeters
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Date of Publication:08/10/2005