Motility and biofilm formation by P. putida KT2440 and its Δ ppk and Δ ppx mutant derivatives. (A) Swimming motility experiments performed in semi-solid M9 minimal medium agar plates for P. putida KT2440 (wt) and its Δppk and Δppx mutant derivatives. Two μl of a cell suspension of each strain (OD600 = 3.0) were inoculated onto the plate and photographed after 48 h of incubation at 30°C. A ΔfliM mutant (i.e., a flagellum-less strain) was used as a negative swimming control. (B) Biofilm formation. Assays were performed in multi-well plates using the crystal violet method explained in the Methods section. Comparison of biofilms formed by P. putida KT2440 (wt) and its Δppk and Δppx mutant derivatives was conducted by calculating the corresponding biofilm index values on each of the C sources tested (glucose, citrate and glycerol). Error bars represent the mean value of the biofilm index value ± SD of seven measurements from at least three independent experiments.