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.