Which bacterial biofilm exopolysaccharide is preferred, Psl or alginate?

In this month’s issue of the Journal of Bacteriology, Jones et al. (1) examine the relationship between the Pseudomonas aeruginosa chronically produced exopolysaccharide, alginate, and a polysaccharide that is produced during the initial stages of biofilm formation, Psl. Psl, composed of mannose, rhamnose, and glucose, was identified as one of the first exopolysaccharides in nonmucoid biofilm matrices (2). The authors show that AmrZ, a member of the ribbon-helix-helix family of DNA binding proteins, is one of the switches involved in activating alginate expression and repressing Psl expression. Furthermore, the authors demonstrated that altered expression of amrZ impacted biofilm structure indicating that the levels of AmrZ are tightly regulated. The authors conclude from these studies that AmrZ is one of the multifunctional regulators that mediate the transition from initial colonization (acute phase) to chronic biofilm formation (alginate production). This study is an important step forward in dissecting the complicated biology of this opportunistic pathogen that uses numerous mechanisms to cause and maintain infections. There are at least three exopolysaccharides produced by P. aeruginosa, and they include the following: (i) alginate, a polysaccharide composed of mannuronic and guluronic acid; (ii) a glucose-rich exopolysaccharide synthesized by enzymes encoded by the pel gene cluster (PA3058-PA3064); and (iii) a mannose-, glucose-, and rhamnose-rich exopolysaccharide produced by proteins encoded in the psl gene cluster (PA2231-PA2245) (3–5). Each of these polysaccharides is associated with some stage of P. aeruginosa biofilm development, and mounting evidence suggests that Pel, Psl, and alginate are involved in different stages of the infectious process. Alginate was the first P. aeruginosa exopolysaccharide discovered, associated with the “mucoid” phenotype of P. aeruginosa strains isolated from cystic fibrosis (CF) patients (6). Pel was initially described in terms of P. aeruginosa growing at an air-liquid interface in cultures grown in static broth (3). The third exopolysaccharide, Psl, was identified from strains that were not capable of producing alginate but were still able to form biofilm structures on solid glass surfaces (2, 4, 7, 8). Alginate is arguably the best characterized exopolysaccharide produced by P. aeruginosa, and several excellent reviews have been written on the molecular biology of its production and clinical ramifications (5, 9–13). The roles of alginate, Psl, and Pel have been examined extensively in biofilms individually (4, 7, 8, 11, 14, 15). Recently, mutants deficient in one or more polysaccharides and their effects on biofilm architecture were examined. Alginate-deficient mutants developed biofilms with a decreased proportion of viable cells and had more surface structures containing extracellular DNA. Deletion of pslA and alg8 resulted in cells that overproduced Pel. Biofilms of cells with either Psl or alginate genes deleted lost the characteristic mushroom-like structure observed in later-stage biofilms. P. aeruginosa with psl and pel genes deleted lost their ability to form biofilms altogether, indicating the importance of these exopolysaccharides in early stages of biofilm formation. These data show that alginate, Psl, Pel, and extracellular DNA interactively contribute to some aspect of P. aeruginosa biofilm architecture (16). The current state of knowledge indicates that Psl and Pel are likely involved with the initial stages of biofilm development (or acute stages of infection), whereas alginate is the “stress” response polysaccharide associated with chronic stages of infection. The ability of P. aeruginosa to switch from the acute phase of infection to the chronic stage of infection involves numerous inputs. For instance, nine DNA binding proteins have been identified to directly interact with PalgD, making it challenging to come up with a single model that incorporates all of these proteins (Fig. 1). There are at least three DNA bending proteins involved in algD expression, integration host factor (IHF), AmrZ, and Hp-1. These three proteins may or may not bind at the same time to PalgD to control its expression. It is likely that these three proteins do interact with PalgD, either individually or in some combination, to induce looping of the promoter region under certain physiological conditions that have yet to be determined. The current work by Jones et al. (1) shows that AmrZ is able to act as an activator on algD as it represses pslA. These data indicate that either an AlgUdependent gene somehow modifies AmrZ or increases the amount of AmrZ such that it becomes an activator of PalgD. There are also two sigma factors involved in controlling algD transcription, AlgU (AlgT, sigma 22) and RpoN, depending upon the mucoid strain that is examined. Consequently, it has been proposed that antagonism is a mechanism by which both are able to initiate algD transcription (17) (Fig. 1D). The best-studied sigma factor for alginate production, AlgU, probably activates algD transcription with AlgR, AmrZ, and some combination of the other two DNA bending proteins (Fig. 1B). AlgU directly transcribes algR and amrZ, as well as algD, and both AlgU and AlgR are required for alginate production in multiple mucoid strains (18, 19). It appears from the literature that there may be other strains besides muc-23 strains that are RpoN dependent, but the frequency of these strains among clinical isolates is not known. Since AlgB is an NtrC-like transcriptional regulator, it is likely that AlgB and RpoN work in concert to transcribe PalgD under nitrogenlimiting conditions or other physiological conditions distinct