Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/1996
Publication Date: N/A
Interpretive Summary: Standard textbooks of microbiology and biochemistry indicate that all living cells must maintain a near neutral intracellular pH of approximately 7.0 (pH is a measure of acidity, i.e., lower pH values indicate increased acidity). Previous work showed that some bacteria isolated from the rumen of ruminant animals (cattle, sheep, goats) could grow with an intracellular pH as low as 5.4, and that this decrease in intracellular pH protected them from acid, a normal end-product of ruminal fermentation. Ruminal pH is not normally alkaline, but anaerobic bacteria (bacteria that can grow in the absence of oxygen) have been isolated from alkaline environments. We found that the bacterium, Clostridium paradoxum grew when its intracellular pH was as high as 9.5. The purpose of our collaborative research was to explore the intracellular pH limits of bacteria. This research provides valuable fundamental information on the growth physiology of bacteria.
Technical Abstract: When the extracellular pH was increased from 7.6 to 9.8, Clostridium paradoxum, a novel alkalithermophile, increased its pH gradient across the cell membrane (delta pH, pH in/pH out) by as much as 1.2 units. At higher pH values, the delta pH, Z delta pH, and delta psi eventually declined, and the intracellular pH increased significantly. Growth ceased when the extracellular pH was greater than 10.2 and the intracellular pH increased to above 9.8. The membrane potential (delta psi) increased to about 115 mV at pH 9.1, but the total proton motive force (delta p) declined from about 85 mV at pH 7.0 to 25 mV at pH 9.8. Between the extracellular pH of 8.0 and 10.3, the intracellular ATP concentration was around 1mM and decreased at lower and higher pH values concomitantly with a decrease in growth rate.