Phosphate-dependent modulation of c-di-GMP levels regulates Pseudomonas fluorescens Pf0-1 biofilm formation by controlling secretion of the adhesin LapA.

Biofilm formation is commonly described as a developmental process regulated by environmental cues. In the current study we present a mechanistic model to explain regulation of Pseudomonas fluorescens biofilm formation by the environmentally relevant signal inorganic phosphate (P(i)). We show that activation of the Pho regulon, the major pathway for adaptation to phosphate limitation, results in conditional expression of a c-di-GMP phosphodiesterase referred to as RapA. Genetic analysis indicated that RapA is an inhibitor of biofilm formation and required for loss of biofilm formation in response to limiting P(i). Our results suggest that RapA lowers the level of c-di-GMP, which in turn inhibits the secretion of LapA, a large adhesion required for biofilm formation by P. fluorescens. The ability of c-di-GMP to modulate protein secretion is a novel finding and further extends the biological influence of c-di-GMP beyond that of regulating exopolysaccharide synthesis and motility. Interestingly, Pho regulon expression does not impinge on the rate of attachment to a surface but rather inhibits the transition of cells to a more stable interaction with the surface. We hypothesize that Pho regulon expression confers a surface-sensing mode on P. fluorescens and suggest this strategy may be broadly applicable to other bacteria.