Static growth of mucoid Pseudomonas aeruginosa selects for non-mucoid variants that have acquired flagellum-dependent motility

When mucoid (alginate-producing) Pseudomonas aeruginosa FRD1 is grown under low oxygen conditions in liquid culture (static), non-mucoid variants appear and eventually predominate. This conversion is not readily observed in aerobic, shaken cultures or static cultures containing the alternative electron acceptor nitrate. in this study, it is shown that the non-mucoid variants that arise under static growth conditions are almost exclusively algT mutants. It has been shown that AlgT not only positively regulates alginate biosynthesis, but also directly or indirectly negatively regulates flagellum synthesis. Indeed, during static growth, conversion to the non-mucoid phenotype is accompanied by the acquisition of flagellum-mediated motility. Surprisingly, by using a reporter gene fusion with the fliC promoter (pfliC::xylE), it was found that fliC expression begins within hours of static growth and is reversible after returning the culture to shaking conditions. The ability of the strain to produce alginate seems to be irrelevant to this phenomenon, as an AlgT(+) DeltaalgD strain showed identical results. Thus, it is suggested that the first effect of static growth is to induce motility as an adaptive measure in the presence of wildtype algT. This may afford P. aeruginosa the ability to swim towards areas of higher oxygen concentrations. Subsequent to this, algT mutations are likely to secure the motile phenotype.