Stability of the adenosine 5'-triphosphate pool in Coxiella burnetii: influence of pH and substrate

Abstract

The ability of Coxiella burnetii to couple oxidation of metabolic substrates to adenosine 5'-triphosphate (ATP) synthesis in axenic reaction buffers was examined. Pyruvate, succinate, and glutamate were catabolized and incorporated at the highest rates of 11 substrates tested. Glutamate oxidation, however, resulted in the greatest stability of the ATP pool and highest intracellular ATP levels over a 48-h period. At pH 4.5, the optimum for metabolism by C. burnetii, glutamate oxidation resulted in maintenance of the ATP pool at a concentration of approximately 0.7 nmol of ATP per mg of dry weight over a 96-h period. In the absence of substrate, ATP declined by 96 h to less than 0.01 nmol/mg of dry weight. When cells were maintained at pH 7.0 in the presence or absence of glutamate, ATP pools were considerably more stable, presumably due to the minimal metabolic activity displayed by C. burnetii at pH 7. The stability of the ATP pool reflected viability as there was greater than an 8-log decrease in viable C. burnetii after incubation for 7 days at pH 4.5 in the absence of glutamate. Viability was retained in the presence of glutamate at pH 4.5 or 7.0 in the absence of any added substrate. The stability of the ATP pool was due to endogenous synthesis of ATP coupled to substrate oxidation as shown by depression of ATP levels in the presence of inhibitors of electron transport or oxidative phosphorylation. In addition, the adenylate energy charge increased from an initial value of 0.57 to 0.73 during glutamate oxidation with a concomitant rise in the total adenylate pool size. C. burnetii therefore appears able to regulate endogenous ATP levels in response to substrate availability and pH, thus effecting a conservation of metabolic energy in neutral or alkaline environments. Such a mechanism has been proposed to play a role in the resistance of C. burnetii to environmental conditions and subsequent activation upon entry into the phagolysosome in which this organism replicates.