Potassium measurements.

LB Miller overnight cultures inoculated from single colonies were used to inoculate 2-ml cultures of glycerol minimal medium, which were grown overnight in a roller drum to optical densities of approximately 0.2. Cultures were harvested by centrifugation through a layer of a 2:1 (vol/vol) mixture of dibutyl phthalate and dioctyl phthalate following the procedure described in reference 13. The cell pellet was suspended in 0.1 ml 1 N nitric acid and digested for 1 to 3 days. The samples were then diluted with 0.95 ml diluent (2% nitric acid, 0.5% HCl) and centrifuged at 13,000 × g for 30 min. One milliliter of supernatant was removed and mixed with 2 ml more diluent. ³⁹K was measured in the samples and in standard reference controls using an Agilent 7900 inductively coupled plasma mass spectrometer (ICP-MS). The limit of quantification was determined to be 0.1 mg/liter. Potassium content was determined as nanomoles of potassium per OD₆₀₀ of the culture prior to harvesting.