The Matrix Revisited: Opening Night for the Pel Polysaccharide Across Eubacterial Kingdoms

Abstract

Bacteria synthesize and export adhesive macromolecules to enable biofilm formation. These macromolecules, collectively called the biofilm matrix, are structurally varied and often unique to specific bacterial species or subspecies. This heterogeneity in matrix utilization makes it difficult to facilitate direct comparison between biofilm formation mechanisms of different bacterial species. Despite this, some matrix components, in particular the polysaccharides poly-β-1,6- N-acetyl-glucosamine (PNAG) and bacterial cellulose, are utilized by many Gram-negative species for biofilm formation. However, there is a very narrow distribution of these components across Gram-positive organisms, whose biofilm matrix determinants remain largely undiscovered. We found that a genetic locus required for the production of a biofilm matrix component of P. aeruginosa, the Pel polysaccharide, is widespread in Gram-negative bacteria and that there is a variant form of this cluster present in many Gram-positive bacterial species. We demonstrated that this locus is required for biofilm formation by Bacillus cereus ATCC 10987, produces a polysaccharide that is similar to Pel, and is post-translationally regulated by cyclic-3′,5′-dimeric-guanosine monophosphate (c-di-GMP) in a manner identical to P. aeruginosa. However, while the proposed mechanism for Pel production appears remarkably similar between B. cereus and P. aeruginosa, we identified several key differences between Gram-negative and Gram-positive Pel biosynthetic components in other monoderms. In particular, 4 different architectural subtypes of the c-di-GMP-binding component PelD were identified, including 1 found only in Streptococci that has entirely lost the c-di-GMP recognition domain. These observations highlight how existing multi-component bacterial machines can be subtly tweaked to adapt to the unique physiology and regulatory mechanisms of Gram-positive organisms. Collectively, our analyses suggest that the Pel biosynthetic locus is one of the most phylogenetically widespread biofilm matrix determinants in bacteria, and that its mechanism of production and regulation is extraordinarily conserved across the majority of organisms that possess it.

Autoaggregation inStreptococcus intermediusis driven by the Pel polysaccharide

“…Previous analyses of the pel operon in different bacterial species (15)(16)(17) identified pelD as a key difference between the S. intermedius, and P. aeruginosa and B. cereus pel operons. PelD has been classified into 4 classes based on the presence or absence of the GGDEF, SDR or degenerate SDR domains.…”

Is biofilm formation intrinsic to the origin of life?

“…Equally addressing in detail the evolution of energy preserving modules with the analysis and integration of ancient energy‐gaining pathways such as anaerobic sulfur respiration (Hedderich et al, 1998 ) can give a first glimpse on these processes. On the other hand, while the presence of biofilm components such as the genetic modules for the production of the poly‐beta‐1,6‐ N ‐acetyl‐glucoseamine and cellulose exopolysaccharide and the contribution of eDNA to the extracellular matrix have been realized to occur in bacteria throughout the phylogenetic tree, its origin has not been defined (Mack et al, 1996 ; Whitfield & Howell, 2021 ; Zogaj et al, 2001 ). Equally, although a Gram‐negative acyl‐homoserine quorum sensing system has also been identified punctually in archaea (Zhang et al, 2012 ), a deep phylogenetic origin of biofilm‐regulating quorum sensing systems remains elusive (Lerat & Moran, 2004 ).…”

Combinatorial control of biofilm development by quorum-sensing and nutrient-sensing regulators inPseudomonas aeruginosa

“…We speculate that one possible mechanism might be post-transcriptional for pel as the absence of Crc decreases biofilm formation even after exchanging the PpelA promoter with an artificial promoter. Nonetheless, given that both carbon catabolite repression and Pel biosynthetic genes are widespread in bacteria (Whitfield, 2021), we expect future studies defining the molecular details underlying the link between "reverse" carbon catabolite repression and pel expression would be informative.…”