Intracellular pH regulation by Mycobacterium smegmatis and Mycobacterium bovis BCG

Mycobacteria are likely to encounter acidic pH in the environments they inh abit; however intracellular pH homeostasis has not been investigated in inh abit; however intracellular pH homeostasis has not been investigated in the se bacteria. In this study, Mycobacterium smegmatis and Mycobacterium bovis [Bacille Calmette-Guerin (BCG)] were used as examples of fast- and slow-gr owing mycobacteria, respectively, to study biochemical and physiological re sponses to acidic pH. M. smegmatis and M. bovis BCG were able to grow at pH values of 4.5 and 5.0, respectively, suggesting the ability to regulate in ternal ph. Both species of mycobacteria maintained their internal ph betwee n pH 6.1 and 7.2 when exposed to decreasing external pH and the maximum Del ta pH observed was approximately 2.1 to 2.3 units for both bacteria. The De lta pH of M. smegmatis at external pH 5.0 was dissipated by protonophores ( e.g. carbonyl cyanide m-chlorophenylhydrazone), ionophores (e.g. monensin a nd nigericin) and N,N'-dicyclohexylcarbodiimide (DCCD), an inhibitor of the proton-translocating F1F0-ATPase. These results demonstrate that permeabil ity of the cytoplasmic membrane to protons and proton extrusion by the F1F0 -ATPase plays a key role in maintaining internal pH near neutral. Correlati ons between measured internal pH and cell viability indicated that the leth al internal ph for both strains of mycobacteria was less than pH 6.0. Compo unds that decreased internal pH caused a rapid decrease in cell survival at acidic pH, but not at neutral pH. These data indicate that both strains of mycobacteria exhibit intracellular pH homeostasis and this was crucial for the survival of these bacteria at acidic pH values.