Stress inducible biofilm formation involves the SOS response
The biofilm formation in bacteria like Pseudomonas aeruginosa can be induced by antimicrobial and DNA damage agents. These agents trigger the SOS response, in which SOS sensor RecA stimulates auto-cleavage of repressor LexA. These observations led us to the hypothesis of a connection between stress-inducible biofilm formation and the RecA--LexA interplay. To test this hypothesis, three biofilm assays were conducted, namely the standard 96-well assay, confocal laser scanning microscopy, and the newly developed biofilm--on--paper assay. It was found that biofilm stimulation by the DNA replication inhibitor hydroxyurea was dependent on RecA and appeared repressed by the non--cleavable LexA of P.aeruginosa. Surprisingly, deletion of lexA led to reduction of both normal and stress-inducible biofilm formation, suggesting that the wild--type LexA contributes to biofilm formation. These results suggest SOS involvement in hydroxyurea--inducible biofilm formation. Next, the possibilities behind the SOS involvement were tested: role of release of extracellular DNA and production of outer membrane vesicles with hydroxyurea treatment. The functional analysis of outer membrane vesicles showed that, OMVs mediated macrophage cytotoxicity and contributed to biofilm formation. OMV production and the cytotoxicity increased when the OMV--producing cells were SOS--induced by ciprofloxacin treatment, but decreased when SOS was repressed by LexA. SOS appears to play a role in the antibiotic--stimulated OMV production, which contributes to cytotoxicity and biofilm formation. Then, SOS regulation of persister production was tested and non--cleavable LexA appeared to repress persister production.