Biofilm matrix and its regulation in Pseudomonas aeruginosa

Abstract

Biofilms are communities of microorganisms embedded in extracellular polymeric substances (EPS) matrix. Bacteria in biofilms demonstrate distinct features from their free-living planktonic counterparts, such as different physiology and high resistance to immune system and antibiotics that render biofilm a source of chronic and persistent infections. A deeper understanding of biofilms will ultimately provide insights into the development of alternative treatment for biofilm infections. The opportunistic pathogen Pseudomonas aeruginosa, a model bacterium for biofilm research, is notorious for its ability to cause chronic infections by its high level of drug resistance involving the formation of biofilms. In this review, we summarize recent advances in biofilm formation, focusing on the biofilm matrix and its regulation in P. aeruginosa, aiming to provide resources for the understanding and control of bacterial biofilms.

Figure 1

Scheme of biofilm development in P. aeruginosa. Selected images showed how the matrix of Psl polysaccharide (red fluorescence) enmeshes bacterial cells (green fluorescence) within bacterial communities during biofilm development (I: initial attachment; II: irreversible attachment; III: microcolony formation; IV: biofilm maturation; V: biofilm dispersion). The figure was used with the permission of the authors [15,27] and modified herein.

Figure 2

Schematic presentation of physiological functions of c-di-GMP. In bacterial cells, c-di-GMP is generated by diguanylate cyclases (DGC) and broken down by specific phosphodiesterases (PDE). As a second messenger, low levels of c-di-GMP can promote motility by upregulating flagellar expression, assembly or interfering with flagellar motor function and are required for the expression of acute virulence genes. High levels of c-di-GMP however favor sessility and stimulate the synthesis of various matrix exopolysaccharides, such as Pel (mediated by PelD) and alginate (mediated by Alg44) [71,74,75].

Figure 3

Schematic diagram of the two-component system regulation of biofilm formation and virulence in P. aeruginosa [95,96,100,104]. Unknown environmental cues received by the input domains of the three membrane-associated sensor kinases (GacS, LadS and RetS) activate or repress the expression of genes necessary for acute or chronic infection. Free regulatory protein RsmA can bind to the promoter regions of multiple genes, thus repressing expression of biofilm associated genes such as psl locus and enhancing bacterial motility and the production of several acute virulence factors (Red lines). When the response regulator GacA is phosphorylated by the upstream sensor kinase GacS, the production of small regulatory RNAs RsmZ and RsmY are stimulated, followed by the titrating to RsmA protein, which ultimately de-represses the expression of biofilm-related genes and represses the production of virulence-related factors (Green lines). The signaling cascade going through RetS, operating in an opposite manner to that of GacS and LadS, generates more free RsmA, resulting in T3SS activation and biofilm repression (Red lines). T3SS, type 3 secretion system. EPS, exopolysacchrides. P means phosphorylated state of GacA. Dark red circle indicates RsmA protein, which binds to the genomic DNA without environmental stimulation and binds to RsmY or RsmZ when the upstream pathways are activated.

Figure 4

Diagram of AlgC-dependent enzymatic regulation of the production of exopolysaccharides in P. aeruginosa PAO1 [116]. The product of algC gene (AlgC) is a bifunctional enzyme with phosphomannomutase (PMM) and phosphoglucomutase (PMG) activities that can catalyze the conversion of mannose-6-phosphate (mannose-6-P) and glucose-6-phosphate (glucose-6-P) into mannose-1-phosphate (mannos-1-P) and glucose-1-phosphate (glucose-1-P), respectively. The PMM/PMG activity of AlgC is required for the biosynthesis pathways of four exopolysaccharides (Psl, alginate, LPS and Pel) in P. aeruginosa. Experimental data showed that overproduction of individual exopolysaccharides reduces synthesis of the other exopolysaccharides, indicating that AlgC is the checkpoint enzyme that limits the production of P. aeruginosa exopolysaccharides, influencing the biofilm formation.