MapA, a second large RTX adhesin, contributes to biofilm formation by Pseudomonas fluorescens

Mechanisms by which cells attach to a surface and form a biofilm are diverse and differ greatly between organisms. The Gram-negative, Gammaproteobacterium Pseudomonas fluorescens attaches to a surface through the localization of the large type 1-secreted RTX adhesin LapA to the outer surface of the cell. LapA localization to the cell surface is controlled by the activities of a periplasmic protease, LapG and an inner-membrane spanning cyclic di-GMP responsive effector protein, LapD. A previous study identified a second, LapA-like protein encoded in the P. fluorescens Pf0-1 genome: Pfl01_1463. However, deletion of this gene had no discernible phenotype under our standard laboratory growth conditions. Here, we identified specific growth conditions wherein, Pfl01_1463, hereafter called MapA (Medium Adhesion Protein A) is a functional adhesin contributing to biofilm formation. This adhesin, like LapA, appears to be secreted through a Lap-related type 1 secretion machinery. We show MapA involvement in biofilm formation is also controlled by LapD and LapG, and that the differing roles of LapA and MapA in biofilm formation are achieved, at least in part, through the differences in the sequences of the two adhesins and their differential, cyclic di-GMP-dependent transcriptional regulation. This differential regulation appears to lead to different distributions of the expression of lapA and mapA within a biofilm. Our data indicate that the mechanisms by which a cell forms a biofilm and the components of a biofilm matrix can differ depending on growth conditions in the biofilm. ImportanceAdhesins are critical for the formation and maturation of bacterial biofilms. We identify a second adhesin in P. fluorescens, called MapA, which appears to play a role in biofilm maturation and whose regulation is distinct from the previously reported LapA adhesin, which is critical for biofilm initiation. Analysis of bacterial adhesins show that LapA-like and MapA-like adhesins are found broadly in Pseudomonads and related organisms, indicating that the utilization of different suites of adhesins may be broadly important in the Gammaproteobacteria.