An extracellular acid stress-sensing protein needed for acid tolerance induction in Escherichia coli

An extracellular induction component (EIC), needed for acid tolerance induc tion at pH 5.0 in Escherichia coli, arises from an extracellular precursor which senses acid stress and is activated (forming the EIC) by such stress. The precursor, which is a heat-stable protein, was formed by cells which h ad not been subjected to acid stress, being present in culture media after growth at pH values from 7.0 to 9.0. This stress-sensing molecule was activ ated to the EIC at pH values from 4.5 to 6.0 but not at pH 6.5 and did not form EIC on incubation at an extremely acidic pH e.g. 2.0. The precursor wa s not inactivated at pH 2.0. Precursor activation might be reversible, as t he EIC lost its ability to induce acid tolerance after incubation at pH 9.0 , but regained it if subsequently incubated at pH 5.0. Whereas the sensor f ormed at pH 7.0 can only be activated at pH 5.0 to 6.0, that synthesized at pH 9.0 can be activated at pH 5.0 to 7.5. Accordingly, this work shows tha t the acid stress sensor is extracellular, and it is proposed that its pres ence in the medium, rather than in the cells, allows more sensitive and rap id responses to acid stress. (C) 1999 Federation of European Microbiologica l Societies. Published by Elsevier Science B.V. All rights reserved.