Submitted to: Federation of European Microbiological Societies Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 10, 2004
Publication Date: March 20, 2004
Citation: Bhagwat, A.A., Bhagwat, M. 2004. Comparative analysis of transcriptional regulatory elements of glutamate-dependent acid-resistance systems of Shigella flexneri and Escherichia coli O157:H7. Federation of European Microbiological Societies Microbiol Letters. 234:139-147.
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, tomatoes and cilantro have been implicated in recent outbreaks of infections caused by Shigella sp. and 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 have compared acid resistance capabilities of two enteric pathogens, E. coli O157:H7 and Shigella flexneri. Several unique characteristics were noticed in the regulatory mechanisms of the glutamate-dependent acid resistance pathway of S. flexneri. The pH of the bacterial growth media also played an important role in activating acid-tolerance pathway of S. flexneri and 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.
The ability to withstand an acid-challenge of pH 2.5 or less by Shigella flexneri is a necessary trait for virulence and is generally believed to be restricted to the stationary-phase of growth. Earlier reports indicated the glutamate-dependent acid-resistance (GDAR) system of S. flexneri is under the regulation of rpoS, the gene encoding alternative sigma factor that is induced in the stationary growth phase. The present study reports that unlike E. coli O157:H7, S. flexneri cells when grown on minimal medium, require acid-induction in the stationary growth phase for a functional GDAR. No acid induction was required for cells grown on complex media. Distinct differences in the gadA, gadBC, gadE, and hdeA (but not in rpoS) transcript levels were observed in the stationary growth phase cells between the two pathogens grown on minimal medium. Additionally, rpoS-independent acid induction of GDAR in the logarithmic growth phase that has been recently observed in E. coli strains [Bhagwat, A.A. 2003. FEMS Microbiol. Lett. 227: 39-45] was not detected in the S. flexneri rpoS mutant. On the other hand, when grown under anaerobic growth conditions in complex media, GDAR of S. flexneri was independent of the rpoS regulon. In spite of these differences in the GDAR system, upstream regulatory elements of gadA, gadBC and gadE were identical when compared with Escherichia coli O157:H7.