Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 29, 2001
Publication Date: October 20, 2001
Interpretive Summary: The ability of human pathogens to survive in an acidic environment plays a crucial role in food and water borne diseases. On the other hand, low pH in the human stomach has long been recognized as the first line of defense against food-borne pathogens. 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. Survival of pathogens such as E. coli O157:H7 for extended periods of time on fresh-cut produce is a serious cause of concern. Understanding the biochemical and genetic basis of acid tolerance mechanisms of E. coli O157:H7 will help us design better intervening agents. The acid resistance of bacteria varies from organism to organism and on the environment they were subjected to before and during acid challenge. In this study we show that acid sensitivity of E. coli O157:H7 is dependent on cell density. The availability of certain amino acids, such as glutamate and arginine, durin acid challenge play a crucial role in determining cell survival. Acid tolerance of Salmonella strains, which do not utilize glutamate or arginine during acid challenge, was found to be independent of cell density. Understanding various acid tolerance pathways utilized by human pathogens will advance our knowledge of microbial food safety, and therefore benefit the fresh produce industry, as well as increase the microbial food safety of American food supply.
The cell density-dependent acid sensitivity phenotypes of Escherichia coli strains K-12 and O157:H7, were examined during stationary growth phase. High cell density suspensions at > 9-log cells/ml survived acid-challenge very poorly (< 0.004% survival) in Luria-Bertani broth (pH 2.5, 37 C, 2h). The same cultures, when subjected to an identical acid-challenge at lower cell density (< 6-log cells/ml), showed higher survival (> 15%). We analyzed the cell density-dependent acid sensitive phenotype of E. coli, with reference to three possible mechanisms of acid resistance. There was no evidence of any diffusible substance released from dead cells, which could influence the cell density-dependent acid-survival phenotype. Nor was there evidence for the involvement of the alternative sigma- transcription factor (rpoS). Instead, cell-density dependent acid-survival phenotype was associated with induction of glutamate- and arginine- decarboxylase acid-survival pathways and concomitant availability of glutamate and arginine during acid-challenge. The date revealed that during acid-challenge at high cell density, the limited availability of glutamate and/ or arginine creates an illusion of cell density-dependent acid sensitive phenotype.