Fluoride Toxicity in Extreme Acid Stress: What's a Bacterium to do?

Abstract

Microorganisms face an environmental threat from F− anions (present at 100μM in soil), which inhibit enolase and pyrophosphatase. Many microorganisms possess fluoride exporters to reduce cellular F− concentrations. Fluoride toxicity is more troublesome at acidic pH levels, because membrane-permeant HF (pKa ∼3.4) from the acidic extracellular fluid diffuses into the cytoplasm where it dissociates at the more neutral intracellular pH, thus trapping and accumulating F− at levlels much higher than the extracellular concentration. Enteric bacteria, such as Escherichia coli, face a particularly extreme acidic stress (pH 100μM). But, different strains showed major differences in the lengths of their recovery periods before growth after acid-shock with F−, even at low F− concentrations. The ΔcrcB cells, despite having the same absolute survival rate as WT cells, took 4 hours longer to begin dividing than WT at 10μM F−, whereas the lag time for ΔcrcB cells expressing the F− exporter Pst matched or was less than that of WT at F− concentration as high as 1mM.