A Rhizobiales-Specific Unipolar Polysaccharide Adhesin Contributes to Rhodopseudomonas palustris Biofilm Formation across Diverse Photoheterotrophic Conditions

Abstract

Bacteria predominantly exist as members of surfaced-attached communities known as biofilms. Many bacterial species initiate biofilms and adhere to each other using cell surface adhesins. This is the case for numerous ecologically diverse Alphaprotebacteria, which use polar exopolysaccharide adhesins for cell-cell adhesion and surface attachment. Here, we show that Rhodopseudomonas palustris, a metabolically versatile member of the alphaproteobacterial order Rhizobiales, contains a functional unipolar polysaccharide (UPP) biosynthesis gene cluster. Deletion of genes predicted to be critical for UPP biosynthesis and export abolished UPP production. We also found that R. palustris uses UPP to mediate biofilm formation across diverse photoheterotrophic growth conditions, wherein light and organic substrates are used to support growth. However, UPP was less important for biofilm formation during photoautotrophy, where light and CO 2 support growth, and during aerobic respiration with organic compounds. Expanding our analysis beyond R. palustris, we examined the phylogenetic distribution and genomic organization of UPP gene clusters among Rhizobiales species that inhabit diverse niches. Our analysis suggests that UPP is a conserved ancestral trait of the Rhizobiales but that it has been independently lost multiple times during the evolution of this clade, twice coinciding with adaptation to intracellular lifestyles within animal hosts.IMPORTANCE Bacteria are ubiquitously found as surface-attached communities and cellular aggregates in nature. Here, we address how bacterial adhesion is coordinated in response to diverse environments using two complementary approaches. First, we examined how Rhodopseudomonas palustris, one of the most metabolically versatile organisms ever described, varies its adhesion to surfaces in response to different environmental conditions. We identified critical genes for the production of a unipolar polysaccharide (UPP) and showed that UPP is important for adhesion when light and organic substrates are used for growth. Looking beyond R. palustris, we performed the most comprehensive survey to date on the conservation of UPP biosynthesis genes among a group of closely related bacteria that occupy diverse niches. Our findings suggest that UPP is important for free-living and plantassociated lifestyles but dispensable for animal pathogens. Additionally, we propose guidelines for classifying the adhesins produced by various Alphaprotebacteria, facilitating future functional and comparative studies.KEYWORDS Rhodopseudomonas, adhesin, biofilm, holdfast, phylogenetic analysis, unipolar polysaccharide

Layered structure and complex mechanochemistry of a strong bacterial adhesive

“…A clear advantage of this complex structure and mechanochemistry is that each component can be modified to yield a diversity of adhesive strategies adapted to various environments. Other bacteria closely related to C. crescentus generate similar nanoscopic adhesive structures 4,15,16,19,25 . Their habitats range from saltwater to freshwater to the rhizosphere, necessitating different conditions for effective adhesion.…”

“…One of the best-studied bacterial polysaccharide adhesive is the unipolar polysaccharide adhesive (UPP) produced by multiple genera of the alphaproteobacteria 4,15,16 . The prototype UPP adhesive is the holdfast produced by the oligotrophic fresh water bacterium Caulobacter crescentus ( Fig.…”

Composition of the holdfast polysaccharide fromCaulobacter crescentus

“…In the class Alphaproteobacteria, adhesins are often localized to one cell pole, giving the cell a defined geometry for attachment with respect to the surface substrate (16)(17)(18)(19)(20). Caulobacter crescentus is a dimorphic organism that has become a model for polar adhesion in this clade (13).…”

The role ofCaulobactercell surface structures in colonization of the air-liquid interface

“…Synthesis of a holdfast-like adhesin at one cell pole is a broadly conserved trait in Alphaproteobacteria. Examples of species that secrete polar adhesins or form polar rosette aggregates have been described in almost every Alphaproteobacterial order including Rhizobiales (69)(70)(71)(72)(73)(74)(75)(76)(77)(78), Caulobacterales (20,32,62,(79)(80)(81), Rhodobacterales (82-87), and Sphingomonadales (88)(89)(90).…”

“…Exceptions are Rhodospirillales and Rickettsiales, which are at the base of the Alphaproteobacterial tree (91). The ensemble of holdfast synthesis genes (29,30,69,76,79,85,92), and chemical composition of the holdfast polysaccharides (32) vary between species and families, which may reflect chemical differences in the niches particular species colonize.…”