Tobramycin (Tob) treatment of P. aeruginosa and S. constellatus mixed biofilms enhances S. constellatus viability. The data represent the viability of bacteria grown as biofilms at 21 h on CF airway cells from assays of single or mixed biofilms of P. aeruginosa and S. constellatus 7155 treated with no tobramycin or with 5 μg/ml tobramycin. (A) Viability of bacteria in the biofilm fraction. (B) Viability of bacteria in the planktonic fraction. (C) Cytotoxicity of indicated mixed biofilms on CF airway cells. Cytotoxicity was normalized to total cell lysis. *, P < 0.05; **, P < 0.01; ***, P < 0.001, as determined by analysis of variance (ANOVA) with Tukey's posttest for multiple comparisons. P.a., P. aeruginosa; S.c., S. constellatus. For all panels, the bars and error bars indicate the means and SD of the results from three biological replicates.