Submitted to: Federation of European Microbiological Societies Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2003
Publication Date: 9/4/2003
Citation: Bhagwat, A.A. 2003. Regulation of the glutamate-dependent acid-tolerance system of diarrheagenic Escherichia coli strains. Federation of European Microbiological Societies Microbial Letters. 227:39-45. Interpretive Summary: The ability of human pathogens to survive in an acidic environment plays a crucial role in food and water borne diseases. Acidic produce, such as apple cider and apple juice, as well as fresh-cut melons and tomatoes have been implicated in recent outbreaks of infections caused by E. coli O157:H7. The mechanism by which human pathogens survive on acidic produce is not fully understood. This study demonstrates that the availability of glutamate is beneficial to the pathogenic strains of E. coli to overcome acid challenge. The acid resistance of bacteria was influenced by several environmental factors. The pH of the bacterial growth media also played an important role in activating acid-tolerance pathway of diarrheagenic E. coli strains. Understanding the acid tolerance pathways will advance our knowledge of how enteric human pathogens survive on fresh-cut produce. The research will benefit the fresh produce industry, as well as increase the microbial food safety of Americans food supply.
Technical Abstract: The ability to withstand an acid-challenge of pH 2.5 or less by Escherichia coli strains is a trait generally believed to be restricted to their stationary-phase of growth. Of the three distinct acid resistant systems that have been identified in E. coli, the glutamate-dependent acid resistance (GDAR) system provides the highest level of acid resistance. Earlier reports indicated that in E. coli K-12 strains the GDAR-system is not active in the logarithmic-growth phase cells even when both glutamate decarboxylase isozymes (gadA and gadB) are highly expressed. The present study reports that several diarrheagenic E. coli strains have a complete set of induced genes necessary for GDAR in the logarithmic-growth phase to overcome the acid-challenge of pH 2.5 for several hours. The glutamate decarboxylase isozymes were inactivated differentially in the rich vs. minimal growth media. In the stationary growth phase, the functional GDAR-system was present regardless of pH and growth media. During the transition from the stationary-growth phase to the logarithmic-growth, glutamate decarboxylase isozymes were degraded in Luria-Bertani broth independent of pH, while in the minimal growth medium the degradation was pH dependent.