Figure 3.

Alkaline metabolic by-products from a wide range of bacterial species correlate with inhibition. (a) Different microbial strains were deposited on nutrient-free tobramycin BTB agar, on nutrient-containing LB–tobramycin BTB agar, and on antibiotic-resistant mutant lawns overlaid on LB–tobramycin agar. Images were taken immediately after deposition (for nutrient-free BTB agar), 2 h after deposition (for nutrient-containing LB BTB agar) or after overnight incubation (for antibiotic-resistant lawns on LB–tobramycin 8 μg ml⁻¹ agar). The width of inhibition (X) is shown as hatched bars. All bacterial strains produce inhibition of antibiotic-resistant mutant lawns, however, the yeast strain fails to produce inhibition. Error bars (in red) represent standard error of the mean; n = 3. (b) For different inhibiting strains, when the width of alkaline change (X_c, see electronic supplementary material, figure S18) is plotted as a function of the width of inhibition (X), a correlation is observed (R² = 0.82 from a linear fit, Pearson's correlation coefficient r = 0.9). Intensity profiles were used to measure the width of alkaline colour change (X_c), representing the distance from the edge of the disc to the edge of the alkaline colour change. For each replicate, X_c and X represent the average of four measurements made along two perpendicular axes. Error bars represent standard error of the mean; n = 2. (Online version in colour.)