In alkaline environments, MdtM functions to maintain a cytoplasmic pH that is acidic relative to external pH Taken together, all the previous data strongly support the idea that MdtM contributes to cytoplasmic pH homeostasis under conditions of alkaline stress. Therefore, to demonstrate directly a role for MdtM in this process, in vivo measurements of the intracellular pH of E. coli BW25113 ΔmdtM transformed with pMdtM or pD22A at different external alkaline pH values between pH 7.5 and pH 9.5 were performed in the presence of NaCl using fluorescence measurements of the free acid of the pH-sensitive probe 2,7-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM). Calibration

of our system resulted in a reasonably linear correlation between intracellular pH and the 490 nm/440 nm fluorescence ratio Selleckchem EVP4593 over a range of pH values from 7.5 to 9.5 (Figure 10A) thereby making internal cellular pH measurements over this range amenable. The intracellular pH of cells that overexpressed wild-type MdtM from a multicopy plasmid remained relatively constant (at between pH 7.5 and 8.0) over

the range of external alkaline pH values tested (Figure 10B; filled symbols). In contrast, cells expressing the dysfunctional D22A mutant of the transporter were unable to maintain a stable cytoplasmic pH, acidic relative to the outside; as the external pH increased there was a concomitant alkalinisation of the cell cytoplasm (Figure 10B; empty symbols). These results uphold our contention selleck that MdtM contributes to alkaline pH homeostasis in E. coli. Figure 10 Measurements of cytoplasmic pH. (A) Calibration plot that correlates the 490 nm/440 nm fluorescence emission ratio of BCECF-AM upon excitation at 530 nm to pH. (B) Intracellular pH of E. coli BW25113 ΔmdtM cells transformed with pMdtM or pD22A as a function of external alkaline pH. In both (A) and (B) the data points and error bars represent the mean ± SD of three independent measurements. Discussion The chief strategy employed by E. coli to maintain a stable cytoplasmic pH under conditions of alkaline challenge is that of proton

uptake mediated by cytoplasmic membrane cation/H+ antiporters [1]. Until now, only four of this type of antiporter were identified unambiguously to function in alkaline pH homeostasis in PtdIns(3,4)P2 E. coli; NhaA [32], NhaB [27], MdfA [9] and ChaA [12], and each has different value to the cell depending on the external environmental conditions [1, 5, 6]. The data presented here define another integral membrane protein, MdtM, a promiscuous multidrug resistance protein belonging to the MFS of secondary active transporters [24], as contributing to alkalitolerance in E. coli. MdtM comes into play at a distinct pH range of between 9 and 10 and provides E. coli with a sensitive mechanism by which to accommodate SRT1720 supplier proton capture under conditions of alkaline stress. Analysis of the growth phenotype of the E.