FIGURE 2. The role of P. aeruginosa arr in tobramycin induction of biofilm formation and biofilm antibiotic resistance.

a, Microtitre plate biofilm assay with the parental PAO1 strain, the arr mutant strain, and the arr mutant strain carrying a plasmid expressing the wild-type arr gene (pArr). Results are averages of 8 replicates s.e.m. and are representative of three independent experiments. Asterisk, P < 0.001 as compared with PAO1 cultured in the same tobramycin concentration. b, Crystal-violet-stained biofilms of PAO1 and the arr mutant strain grown for 24 h on glass coverslips spanning the air–liquid interface of standing cultures. The dark portion above the air–liquid interface biofilm in tobramycin-induced PAO1 is adherent pellicle. c, Microtitre plate biofilm antibiotic resistance assay in which biofilms of the indicated strains were challenged with tobramycin at the concentrations shown, and the survival of suspended biofilm cells determined. Results are the average of three experiments s.e.m. Asterisk, P < 0.001 as compared with PAO1 biofilms exposed to the same drug concentration. d, The indicated plasmids were introduced into the arr mutant strain. Fold induction of biofilm formation by 0.3 g ml^-1 tobramycin for each transformant was determined (average s.e.m. of 6 replicates). Asterisk, P < 0.001 as compared with cells carrying the empty vector. Cells were grown in 500 g ml^-1 carbenicillin for plasmid maintenance.