Two potassium (K+)-uptake systems, Trk and Kdp, are operative in Mycobacterium tuberculosis (Mtb), but the environmental factors
triggering their expression have not been determined.The current study has evaluated the expression of these genes in the Mtb wildtype
and a trk-gene knockout strain at various stages of logarithmic growth in relation to extracellular K+ concentrations and pH.
In both strains, mRNA levels of the K+-uptake encoding genes were relatively low compared to those of the housekeeping gene,
sigA, at the early- and mid-log phases, increasing during late-log. Increased gene expression coincided with decreased K+ uptake in
the context of a drop in extracellular pH and sustained high extracellular K+ concentrations. In an additional series of experiments,
the pH of the growth medium was manipulated by the addition of 1N HCl/NaOH. Decreasing the pH resulted in reductions in
both membrane potential and K+ uptake in the setting of significant induction of genes encoding both K+ transporters. These
observations are consistent with induction of the genes encoding the active K+ transporters of Mtb as a strategy to compensate
for loss of membrane potential-driven uptake of K+ at low extracellular pH. Induction of these genes may promote survival in the
acidic environments of the intracellular vacuole and granuloma.
Part of the work presented herein has been presented at the
South African TB Conference, Durban, South Africa, 2014