Antibiotic-inducile biofilm formation is regulated by SOS-regulated motility of Pseudomonas aeruginosa

Biofilms are multicellular communities of unicellular organisms encoded by an extracellular polymeric matrix. Bacterial biofilm formation can be induced by antimicrobial and DNA damage agents. These agents trigger an SOS response which is sensed by an activated RecA coprotease that stimulates the auto-cleavage of LexA, a repressor of SOS. We tested to see if a quinolone antibiotic, Ciprofloxacin which is a DNA replication inhibitor is able to induce this SOS-inducible biofilm formation by conducting the 96-well assay and the newly developed biofilm lipid assay. We found that it appeared repressed by the non-cleavable LexA and can thus stimulate the biofilm formation by Pseudomonas aeruginosa. This stimulation involves the SOS response because the SOS repressor LexA controls the early development of biofilm formation. Proteomic analysis of the outer membrane fraction of biofilm cells suggest that the relative abundance of flagellum-related proteins is negatively affected by LexA. The functional and morphological analysis of flagellum-based motility verifies that the flagellum-propelled swimming motility is repressed by LexA and it controls flagellation. Hence, LexA controls the early development of antibiotic-inducible biofilm formation through repression of flagellum function.