Effect of Ammonia on Escherichia coli Growth and Aerobic Respiration: The Role of Cytochrome bd-II.

Bacterial terminal oxidases are essential for growth and may provide protection against environmental stressors. Bacteria often have to cope with ammonia, which, although an essential nutrient, is toxic at high concentrations. Here, we studied the influence of ammonia on the cell growth of three different Escherichia coli respiratory mutants, each possessing a single terminal oxidase: cytochrome bd-I, cytochrome bd-II, or cytochrome bo3. We also investigated the effect of ammonia on O2 consumption in cytochrome bd-II-only cells and membranes, as well as in the isolated bd-II enzyme. Using microcalorimetry, spectrophotometry and high-resolution respirometry, the following new results were obtained: (i) At pH 8.3, the addition of ammonia to both bd-I-only and bd-II-only cell cultures has virtually no effect on growth. In contrast, the growth of bo3-only cells is significantly impaired. (ii) The addition of ammonia to bd-II-only intact cells at pH 8.3 not only fails to inhibit their respiration but also accelerates O2 consumption. The same is observed with bd-II-only isolated membranes and detergent-solubilized bd-II enzyme. The maximum increase in cytochrome bd-II O2 consumption rate is approximately 150%. Physiological aspects of the findings are discussed, and molecular mechanisms for ammonia-induced acceleration of O2 consumption by cytochrome bd-II are suggested.