Hussain, Noor Hana; (1996) Inducible tolerance and sensitivity to stress responses in Escherichia coli with particular reference to copper and pH. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Stress responses to copper and alkali were studied in Escherichia coli. E. coli 1829 and its derivatives, were able to tolerate lethal doses of CUSO4 (58.92 μg/ml and 117.84 μg/ml) after pre-exposure to sublethal doses of CUSO4 (14.73 μg/ml and 29.46 μg/ml). The observed copper tolerance was due to a phenotypic change induced during the pre-exposure period which depends on de novo protein synthesis. Cytoplasmic membrane proteins of molecular weights 26 and 24.5 kDas and outer membrane proteins of molecular weights 16.5, 18, 31.5 and 65 kDas were overexpressed in the copper-induced cells. The DNA from the copper-induced cells was also less damaged than that from the uninduced cells. Pre-exposure to 14.73 μg/ml CUSO4 also confers cross-protection to heat, acid, alkali and cadmium sulphate but not to hydrogen peroxide. Pre-exposure to mildly acidic pH which would normally induce acid tolerance was shown to also induce alkali sensitivity. When E. coli 1829 cells were transferred from pH 7.0 to pH 5.5 for one hour they became alkali sensitive upon challenge with pHs 9.5 and 9.75 for 30 minutes. Substantial induction also occurs at pH 6.0 but there was less at pH 5.0 and practically none at pH 6.5. The response was triggered by cytoplasmic acidification by protons entering the cells possibly via OmpC, LamB, PhoE, NhaA and NhaB. The induction of alkali sensitivity also depends on de novo protein synthesis of components involved in alkali sensitization. Cytoplasmic membrane proteins of molecular weights 14 and 18 kDas were overexpressed in the pH 5.5 induced cells. The induction of the alkali sensitization components is not subject to catabolite repression nor affected by deletion in rpoS but appeared to be under the control of Fur, RelA, CysB and Lrp. Mutants with a deletion in tonB showed derepressed alkali sensitivity; the response being observed in pH 7.0 induced cells instead. The expression of the alkali sensitization components also appeared to be affected by changes in the DNA supercoiling and is influenced by HimA, HimD and H-NS.

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