Bacterial Extracellular Polysaccharides in Biofilm Formation and Function

Abstract

Microbes produce a biofilm matrix consisting of proteins, extracellular DNA, and polysaccharides that is integral in the formation of bacterial communities. Historical studies of polysaccharides revealed that their overproduction often alters the colony morphology and can be diagnostic in identifying certain species. The polysaccharide component of the matrix can provide many diverse benefits to the cells in the biofilm, including adhesion, protection, and structure. Aggregative polysaccharides act as molecular glue, allowing the bacterial cells to adhere to each other as well as surfaces. Adhesion facilitates the colonization of both biotic and abiotic surfaces by allowing the bacteria to resist physical stresses imposed by fluid movement that could separate the cells from a nutrient source. Polysaccharides can also provide protection from a wide range of stresses, such as desiccation, immune effectors, and predators such as phagocytic cells and amoebae. Finally, polysaccharides can provide structure to biofilms, allowing stratification of the bacterial community and establishing gradients of nutrients and waste products. This can be advantageous for the bacteria by establishing a heterogeneous population that is prepared to endure stresses created by the rapidly changing environments that many bacteria encounter. The diverse range of polysaccharide structures, properties, and roles highlight the importance of this matrix constituent to the successful adaptation of bacteria to nearly every niche. Here, we present an overview of the current knowledge regarding the diversity and benefits that polysaccharide production provides to bacterial communities within biofilms.

FIGURE 1

Adapted representative chemical structures of polysaccharides which participate in biofilm formation including (A) polysaccharide intercellular adhesin (PIA), (B) Psl, (C) alginate, capsular polysaccharide (CPS) from (Di) E. coli and (Dii) S. pneumoniae, (E) levan, (F) cellulose, and (G) colanic acid. Brackets depict repeating units. doi:10.1128/microbiolspec.MB-0011-2014.f1

FIGURE 2

Colony phenotypes conferred upon expression or overexpression of PS by representative bacteria. (A) PS intercellular adhesion producing Staphylococcus aureus. Reprinted from World Journal of Microbiology and Biotechnology (214) with permission from the publisher. (B) Pel producing Pseudomonas aeruginosa (ΔwspFΔpsl). Reprinted from Molecular Microbiology (215) with permission from the publisher. (C) Psl producing P. aeruginosa (ΔwspFΔpel). Reprinted from Molecular Microbiology (215) with permission from the publisher. (D) Alginate overproducing P. aeruginosa (mucA22). Not previously published. Credit: Daniel Wozniak. (E) Colanic acid producing Escherichia coli. Reprinted from PLoS One (216) with permission from the publisher. (F) VPS producing rugose variant of Vibrio cholerae. Reprinted from The Proceedings of the National Academy of Sciences of the USA (66) with permission from the publisher. (G) EPS producing Bacillus subtilis. Reprinted from The Proceedings of the National Academy of Sciences of the USA (217) with permission from the publisher. (H) Cellulose producing E. coli (csgD::cm). Reprinted from The Journal of Medical Microbiology (218) with permission from the publisher. doi:10.1128/microbiolspec.MB-0011-2014.f2