Pseudomonas biofilm matrix composition and niche biology

Abstract

Biofilms are a predominant form of growth for bacteria in the environment and in the clinic. Critical for biofilm development are adherence, proliferation, and dispersion phases. Each of these stages includes reinforcement by, or modulation of, the extracellular matrix. Pseudomonas aeruginosa has been a model organism for the study of biofilm formation. Additionally, other Pseudomonas species utilize biofilm formation during plant colonization and environmental persistence. Pseudomonads produce several biofilm matrix molecules, including polysaccharides, nucleic acids, and proteins. Accessory matrix components shown to aid biofilm formation and adaptability under varying conditions are also produced by pseudomonads. Adaptation facilitated by biofilm formation allows for selection of genetic variants with unique and distinguishable colony morphology. Examples include rugose small-colony variants and wrinkly spreaders (WS), which over produce Psl/Pel or cellulose, respectively, and mucoid bacteria that over produce alginate. The well-documented emergence of these variants suggests that pseudomonads take advantage of matrix-building subpopulations conferring specific benefits for the entire population. This review will focus on various polysaccharides as well as additional Pseudomonas biofilm matrix components. Discussions will center on structure-function relationships, regulation, and the role of individual matrix molecules in niche biology.

Fig. 1

Abundant biofilm matrix molecules. Adapted representative chemical structures of (a) alginate, (b) levan, (c) cellulose, (d) Psl, (e) DNA, and (f) rhamnolipid. Brackets depict repeating units of each molecule.

Fig. 2

Prominent Pseudomonas aeruginosa colony morphology variants. Cultures of PAO1, MJK8 (RSCV), and PDO300 (mucoid) were streaked on VBMM with Congo red. All strains grow at similar rates, yet MJK8 colonies are small and more aggregative and copious overproduction of alginate is obvious from PDO300. PAO1 has well-defined smooth colonies.

Fig. 3

Polysaccharide synthesis locus (psl) in Pseudomonas spp. Relative gene organization and size are depicted for each pseudomonad and Psl component. The representative PAO1 Psl operon of approximately 18.4 kb is depicted with gene number designations for each comparison species given below individual components. Color coding represents predicted product function of each component. Information was gathered using the Pseudomonas database and prepared using the Geneious PRO software package (Stover et al., 2000; Nelson et al., 2002; Buell et al., 2003; Feil et al., 2005; Joardar et al., 2005; Paulsen et al., 2005; Lee et al., 2006; Winsor et al., 2009).

Fig. 4

Pel polysaccharide synthesis locus among Pseudomonas spp. The conservation of individual components of the pel operon, which is responsible for Pel biosynthesis, is depicted for pel-containing Pseudomonas spp. The representative PAO1 Pel operon of approximately 12.2 kb and the corresponding gene number designation for each comparison species are given below representative loci. Color-coded boxes represent gene product functions as indicated on the top of this figure (Stover et al., 2000; Friedman & Kolter, 2004a, b; Paulsen et al., 2005; Winsor et al., 2009).

Fig. 5

Psl and Pel influence aggregation, adherence, and colony morphology. Pseudomonas aeruginosa PAO1 strains with wild type, inactivated, or overexpressed status of Psl and Pel were grown in culture tubes containing Congo red to observe aggregation. Polysaccharide status is indicated below each tube.