FIG. 4.

Effect of CCM ion concentrations on C. burnetii metabolic activity. (A) The effect of medium ion levels on C. burnetii metabolic activity was assessed by measuring incorporation of [³⁵S]Cys-Met into C. burnetii de novo synthesized protein following a 3-h incubation in CCM with Cl⁻, K⁺, and Na⁺ concentrations similar to the host cell cytoplasm (26 mM Cl⁻, 110 mM K⁺, and 17 mM Na⁺) or extracellular (140 mM Cl⁻, 4.3 mM K⁺, and 190 mM Na⁺) environment. Incorporation of radiolabel was quantified by scintillation counting and is expressed as the increase relative to the incorporation of C. burnetii incubated in CCM (pH 7.0) with extracellular levels of Cl⁻, K⁺, and Na⁺ (negative control). Asterisks indicate statistically significant differences (P < 0.05) between media with extracellular or intracellular ion levels. The broken line represents the level of radiolabel incorporation in medium with extracellular ion levels at pH 7.0 normalized to 1. (B) The importance of the source of chloride ion on C. burnetii metabolic activity was examined by measuring [³⁵S]Cys-Met incorporation of organisms incubated for 3 h in CCM containing 125 mM KCl or NaCl or in CCM without added KCl or NaCl. Incorporation of radiolabel was quantified by scintillation counting and is expressed as the relative increase over the incorporation of C. burnetii incubated in CCM (pH 7.0) containing 125 mM NaCl (negative control). Asterisks indicate statistically significant differences (P < 0.05) compared to CCM without KCl or NaCl. The broken line represents the level of radiolabel incorporation in medium containing 125 mM NaCl at pH 7.0 normalized to 1.