Submitted to: Infection and Immunity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 9, 2005
Publication Date: August 20, 2005
Citation: Bhagwat, A.A., Lynn, C., Shufen, R., Tan, J., Kothary, M., Jean-Gilles, J., Tall, B.D. 2005. Characterization of pathogenic escherichia coli strains based on acid-resistance phenotypes. Infection and Immunity. 73:4993-5003.
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 has been implicated in recent outbreaks of infections caused by E. coli O157:H7. The acid resistance of bacteria is influenced by the environmental conditions they were subjected to before and during acid challenge. In this study we show that pathogenic E. coli strains are able to utilize as many as three different pathways to overcome gastric acidity. The type of growth media and availability of oxygen played significantly important roles in activating acid-tolerance pathways of enteropathogenic 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 increasing the microbial food safety of the American food supply.
The acid-resistance is perceived to be an important property of diarrheageneic Escherichia coli strains, enabling the organisms to survive passage through the acidic environment of the stomach so that they may colonize the mammalian gastrointestinal tract and cause disease. Accordingly, the organism has developed at least three genetically and physiologically distinct acid-resistance systems which provide different levels of protection. The glutamate-dependent acid-resistance (GDAR) system utilizes extracellular glutamate to protect cells during extreme acid challenges and is believed to provide the highest protection from stomach acidity. In this study, the acid-resistance systems of 82 pathogenic E. coli isolates from 34 countries and 23 states within the U.S.A. were examined. Twenty nine isolates were found to be defective in inducing GDAR under aerobic growth conditions, while 5 other isolates were defective in GDAR under aerobic as well as fermentative growth conditions. We introduced rpoS on a low-copy plasmid in 26 isolates and were able to restore GDAR in 20 acid-sensitive isolates under aerobic growth condition. Four isolates were found to be defective in the newly discovered LuxR-like regulator GadE (formerly YhiE). Defects in other isolates could be due to mutation(s) in gene(s) with a yet undefined role in acid-resistance since GadE and/or RpoS could not restore acid-resistance. These results show that in addition to mutant alleles of rpoS, mutations in gadE exist in natural populations of pathogenic E. coli. Such mutations most likely alter infectivity of individual isolates and may play a significant role in determining the infective dose of diarrheagenic E. coli.