FIGURE 1. Coexistence of P. putida KT2440 and Acinetobacter sp. C6 in structured (biofilm flow chamber) and unstructured (chemostat) environments with benzyl alcohol as the sole carbon source.

Fitness measures were determined over 24 h at different concentrations of benzyl alcohol and are the difference between the malthusian parameter of the initially rare species (P. putida) and that of the common species (Acinetobacter): a fitness of zero indicates equal competitive ability²⁸. Lines are second-order polynomials plus 95% confidence curves fitted to the data (a minimum of three replicate measurements were determined at a minimum of three different concentrations of benzyl alcohol in each experiment). The red line denotes the fitness of ancestral P. putida in chemostat culture, the blue line the fitness of P. putida in biofilm flow chambers, and the black line the fitness of the derived rough variant of P. putida in biofilm flow chambers. The threshold concentration of benzyl alcohol above which P. putida and Acinetobacter coexist is indicated at the point at which each curve crosses the reference line. A curve for the fitness of the derived rough variant of P. putida mutant in chemostat culture is not shown, but in this environment this genotype was unable to increase in frequency against Acinetobacter populations at benzyl alcohol concentrations lower than about 950 M (at 1 mM benzyl alcohol the fitness of derived P. putida was 0.69 (95% confidence interval 0.165–1.215)).