Its shape, adhesive properties, intricate cell-cycle regulation and asymmetric cell division are a just few of the reasons why Caulobacter crescentus is a well-studied microorganism (1). In nature, C. crescentus is typically found attached to submerged surfaces within aquatic environments via a stalk structure which protrudes from a single pole of the cell. An adhesive holdfast, located at the tip of the stalk (Fig. 1), composed of N-acetylglucosamine (sugar molecules) (Fig. 2) and other unknown substances exhibits the strongest adhesion force of any known natural material (2, 3, 4).
The majority of the C. crescentus life cycle is spent in the adherent stalked form. However, the early third of the life cycle is spent as a motile, swarmer cell (Fig. 1). It is hypothesized that the swarmer cells, which exhibit a single polar flagella, can explore new environments where nutrients are more plentiful. Once the flagella is shed, a stalk and holdfast are synthesized in the same location and promote adherence to a surface. Stalked cells undergo cell division and produce new swarmer cells, which begin the process anew (Fig. 1)(1, 5).
Figure 1. C. crescentus undergoing cell division. The stalked cell is located on the top, with the stalk and holdfast at the top of the frame. The swarmer cell is located underneath such that the flagella is protruding toward the bottom right-hand corner of the image. (This image was obtained from MicrobeWiki courtesy of Yves Brun.)
The glue: What is it and how strong is it?
In addition to other unknown elements, the holdfast is composed of oligomers of N-acetylglucosamine (Fig. 2). Treatment of the holdfast with lysozyme, an enzyme that will cleave N-acetylglucosamine oligomers, reduces the adhesive force to less than 10% (3, 4). Furthermore, C. crescentus mutants lacking the sugar molecules at the tip exhibit a significant adherence defect. It was determined that the force required to remove C. crescentus from a glass surface is over 70 Newtons per square millimeter or 5 tons per square inch (2). This force is equivalent to the downward force exerted by three cars balancing on a quarter (6).
Figure 2. N-acetylglucosamine monomer (a.k.a GlcNac, NAG).
That's pretty cool, but why should anyone care?
Unlike superglue, the adhesive substance produced by C. crescentus is non-toxic and it adheres well under water, both the fresh and salt varieties. Potential applications include, biodegradable surgical and dental adhesives and repair of surfaces exposed to water (2, 6). Furthermore, studying adherence of C. crescentus should provide insight into biofouling and biofilm formation (2).
Why so sticky?
It is thought that in its natural environment, C. crescentus attached near the surface of water must contend with the passage of waves at the air-liquid interface, which exerts a significant force (2).
(1) Brown et al. Complex regulatory pathways coordinate cell cycle progression and development in Caulobacter crescentus. Adv Microb Physiol. 2009; 54:1-101.
(2) Tsang et al Adhesion of Single Bacterial Cells in the Micronewton Range. PNAS. 2006; 103(15):5764-5768.
(3) Smith et al Identification of Genes Required For Synthesis of the Adhesive Holdfast in Caulobacter crescentus. J. Bacteriol. 2003; 185:1432-1442.
(4) Li et al. The Elastic Properties of the Caulobacter crescentus Adhesive Holdfast are dependent on Oligomers of N-Acetylglucosamine. J. Bacteriol. 2005; 187(1):257-265.
(5) Jenal. The Role of Proteolysis in the Caulobacter crescentus Cell Cycle and Development. Research in Microbiology 2009; 160:687-695.
(6) Iddo Genuth & Lucille Fresco-Cohen. Nature's Superglue The Future of Things (2006)