Date of Award

5-16-2013

Document Type

Thesis

Degree Name

Bachelor of Arts

First Advisor

Slonczewski, Joan

Abstract

Pathogenic bacteria such as E. coli O157:H7 can cause serious illness such as hemolytic uremic syndrome if they successfully produce toxins in the lower intestines. Before reaching this point, these bacteria face a rapid transition from extremely acidic pH in the stomach (pH 2.5) to pH 10 at the pancreatic duct. While mechanisms of acid resistance are well understood, particularly under aerobic conditions, alkaline resistance mechanisms remain largely uncharacterized. Recent expression studies have identified candidate genes (gadC, rpoS, nhaA, tnaA) that may contribute to survival at high pH. We found that rpoS and gadC are required for optimal survival of stationary-phase E. coli exposed to pH 10 minimal media for 2 hours. Wild-type survival (of W3110) was 10-fold higher than the survival rate of gadC and at least 100-fold higher than the survival rate of rpoS. Strains tnaA and nhaA survived at rates comparable to W3110. nhaA was required for optimal survival when sodium and high pH stress were combined. Growth at pH 8 optimized survival of E. coli when shocked at pH 10. E. coli can compensate for the loss of nhaA when grown at pH 8 but not at pH 7. Growth at pH 5.5 minimized survival at pH 10. Survival in moderately basic, carbonate-rich media (pH 8-9) under anoxia did not require rpoS or gadC. We hypothesize that RpoS cross-protection triggered by transition to stationary phase yields a protective effect at pH 10. GadC plays an important role in this protective effect, though we found evidence that other elements of the RpoS regulon are contributing. E. coli H+ influx antiporters are niche-specific, but partially overlap in their capability to acidify the cytoplasm.

Comments

Includes bibliographical references (p. 45-51)

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