Figure 7. Model of total LT production and secretion.

A) The total production defined at LT both in the pellet fraction and in the supernatant fraction was higher in E2863ΔCRP than in the wild-type E2863 confirming that transcription of the eltAB operon is induced in the absence of cAMP-CRP. Secretion was decreased in E2863ΔCRP demonstrating that CRP is a positive regulator of LT secretion. B) At pH 5 secretion of LT is inhibited and the outer environment and the periplasmic space is acidic and the proton gradient favors an influx of protons into the cytosol. In order to keep the cytosol close to neutral there is a net outflux of protons from the cytosol. At neutral pH the cytosol is kept slightly more alkaline than the periplasm favouring a normal proton gradient and some LT is secreted At pH 9 the proton concentration is lower in the periplasm than the cytosol creating an inverted proton gradient and LT production and secretion is induced in response to the alkaline conditions in the periplasm. C) The inverted proton gradient poses a stress for the bacterial cell resulting in lower growth rate at pH 9 while growth at pH 5 supported growth to higher OD values of bacterial cells than pH 9.