Exoribonucleases affect biofilm formation. Effect of the deletion of the exoribonuclease on biofilm production. The different strains (wt, ∆rnb, ∆rnr and ∆pnp) were inoculated into the wells of a fresh microtiter plate and left at 37°C for 24 h. The biofilms formation was measured by determining the OD550 after staining with crystal violet. The biofilm formation values were normalized with the OD600 of the cultures measured after the 24 h inoculation. The wild-type was used as reference and all other values were obtained by the formula: normalised OD (mutant)/normalised OD (wt). Error bars represent standard deviations.