Effects of culture medium on the susceptibility of M. gordonae to free chlorine. C · t values were calculated as described in the legend to Fig. 1. Experimental conditions were pH 7, a temperature of 25°C, and an initial chlorine concentration of 0.5 mg/liter. No, initial number of CFU; N, number of CFU at the time of the assays. Symbols: ⧫, water plus 10% Middlebrook 7H9-Tween medium (y = 0.01 x); ▪, 100% Middlebrook 7H9-Tween medium (y = 0.09 x). R2, correlation coefficient values.