Single and Multiple Drug Growth Costs. a. Growth cost is defined as the reduction in growth rate of cells treated with a drug relative to the growth rate of untreated cells. Solid lines, best fits to Hill functions h(x) = xn/(Kin + xn), with Ki equal to the concentration of drug i (i = S for salicylate, i = A for both antibiotics) at which growth is inhibited by 50%. KA = 1.80 (1.66, 1.93) μg/mL, n = 1.97 (1.67, 2.26) for chloramphenicol; KA = 0.41 (0.37, 0.45) μg/mL, n = 1.90 (1.61, 2.19) for tetracycline; and KS = 6.06 (5.50, 6.62) mM, n = 1 for salicylate. 95% confidence intervals from nonlinear least squares fitting in parentheses. Error bars are standard deviation of four replicates. b. In the presence of high concentrations of chloramphenicol (Cm), growth is maximal for a nonzero concentration of salicylate (upper insets: OD time series). Contours of constant growth in concentration space indicate suppression. Dashed line, contours of 45% and 55% maximum growth.