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Greetings SITN members!

The staff here at Science In The News have been hard at work finalizing the topics for our seminar series which will start in September. We hope to put together our best series yet with topics that will be informative, interesting and engaging for those who have attended our seminars in the past and also for new-comers. Stay tuned for next month's newsletter in which we'll announce the seminar topics!

In the meantime, we hope you enjoy this month's article which explores how scientific technology can be advanced by studying nature. These days when new gadgets and technological gizmos are clamoring for our attention every day, some scientists are discovering that nature is truly the greatest innovator. If they dare to look, scientists can find inspiration for novel products in rather surprising places….such as in bacterial secretions.

Enjoy!

Researchers at Indiana University have recently discovered a bacterial glue that is stronger than any known adhesive [1] – more than three times as strong as super glue! This super-sticky, water-resistant glue is produced by bacteria called Caulobacter crescentus . C. crescentus are notoriously tenacious microorganisms that colonize wet surfaces such as water pipes, the hulls of ships, and even medical catheters in a process called “biofouling”. While biofouling can be a costly problem for industry and medicine, materials scientists and microbiologists hope that the sticky adhesive produced by C. crescentus might be more of a boon than a bane to technologies of the future.

C. crescentus is a non-pathogenic, single-celled organism that lives in rivers, streams, and tap water pipelines. Its life cycle has two distinct stages. First, it lives as a free-swimming, non-replicating “swarmer cell” with a flagellum, or tail, that it uses to propel itself through the water. Then, when it comes in contact with a surface, it grows a long slender stalk. This stalk is tipped with a sticky patch called the holdfast, which anchors the bacterium, now in its attached “sessile” stage, firmly to the surface. The C. crescentus holdfast is a generalized attachment device that shows little specificity in terms of the surfaces to which it can adhere. [2] The sessile bacteria can form a biofilm – a dense colony of microorganisms affixed to a surface within a protein and sugar-chain matrix – and are then able to divide and give rise to new swarming cells. [3] These biofilms can be responsible for the biofouling of pipes and catheters.

A Novel Strategy for Measuring Large Forces on Small Objects

Various methods have been employed to measure the strength of C. crescentus adhesion, but none have been able to establish the force needed to remove a single cell. C. crescentus cells can withstand being washed with strong jets of water, suggesting that their attachment to surfaces is extremely strong. Pulling on cells with laser tweezers and micropipettes can be used to measure the strength of chemical bonds, but the amount of force that these techniques can exert are far too low to break the holdfast adhesion. Another method, called atomic force microscopy (AFM), in which a tiny metal tip is used to tap or scrape a surface, has been used to explore the physical properties of cells and elastic materials. AFM can generate stronger forces than laser tweezers, but the metal tip of the apparatus cannot be tightly enough coupled to cell surfaces to overcome the cell-surface adhesion (the tip detaches from the cell before the cell comes off of the surface.) [4]

In a novel approach to measuring the strength of C. crescentus adhesion, Jay Tang, Yves Brun, and colleagues at Indiana University attached a tiny suction pump to a single cell on a flexible glass rod, pulled on the cell until it detached, and calculated the force that is needed to remove the cell by measuring the deflection of the rod. [1] Having pulled on a total of fourteen individual cells, they found that the force needed to detach one C. crescentus cell was in the micronewton ( m N) range, from 0.1 to 2.26 m N. One Newton (N) is the amount of force needed to cause a 1 kg object to accelerate at the rate of one meter per second squared. (Remember Issac Newton's famous equation, F = ma: force equals mass times acceleration.) A micronewton is one millionth of a Newton . Other kinds of adhesive bacteria and fungi have been shown to produce adhesion forces of only nanonewtons (one billionth of a N) or piconewtons (one trillionth of a N), many orders of magnitude less than C. crescentus . A few micronewtons may not sound like a great deal of force, but this means that a layer of C. crescentus holdfast covering 1 cm2 could hold 680 kg (1500 lbs). [1] This is equivalent to 5 tons per square inch, or the pressure of three or four cars balanced on top of a quarter.

The researchers also showed that the breaking point of the cell-surface contact was above the holdfast-substrate contact, somewhere along the stalk. In other words, the forces they applied to the cells were enough to rip the thin stalk apart, but not enough to remove the adhesive from the surface of the rod.

Super-sticky Sugar Chains

The ability of C. crescentus to stick so firmly to surfaces is the result of special polysaccharides, or sugar chains, that dot the tip of its stalk. These polysaccharides are composed of long chains of sugars called N-actetylglucosamine (GlcNac), which are essential for the bacterial holdfast's remarkable stickiness. [5] Polysaccharide gels are formed when the long chains of sugars become entangled and cross-link (form chemical bonds between polymers) with one another to form a semi-solid mesh which becomes filled with water. C. crescentus glue, for example, is at least two-thirds water. [5] These polysaccharides are likely attached to proteins on the surface of the cell membrane. A mixture containing the GlcNac polysaccharides and proteins, as well as other chemicals forms the adhesive gel. [6] The concentration of proteins and polysaccharides affects the adhesive properties (the stickiness and stiffness) of the bacterial glue. [7]

Although the exact chemical makeup of the adhesive gel is not yet known, GlcNac polymers have been shown to be critical components of C. crescentus holdfast. Treatment with lysozyme, an enzyme that dissolves GlcNac polymers, reduces the adhesive strength of C. crescentus by more than 90%. [5] The other essential components and biophysical nature of the adhesive gel are subjects of ongoing research.

Technology Draws Inspiration From Nature

Scientists often look to nature to solve technological problems. For example, the feet of the gecko have been an inspiration for a new kind of synthetic adhesive tape that may someday be used in robotics or in surgical applications. [9] The feet of geckoes are covered in “setae”, tiny hairs that stick to surfaces through van der Waals forces (weak intermolecular bonds that result from negatively-charged electrons sloshing around inside atoms) and capillary action (the force which draws liquid up a very thin tube). [8] The additive effect of all of these tiny hairs and tiny forces allows geckos to dangle from smooth vertical surfaces by their toes.

Although its mechanism of action is very different from the gecko's feet, C. crescentus holdfast could also have important technological applications. While a single gecko seta can generate a force equivalent to 10 N/mm2 , a single C. crescentus cell can generate a force up to 68 N/mm2 . By comparison, the adhesive force of commercial superglues and dental adhesives is on the order of 20-30 N/mm2 . [1]

Since C. crescentus live in water, their adhesive works on wet surfaces, even in salty water, which would make it suitable for applications ranging from industrial-strength underwater glue to dental and surgical adhesives. Superglue-like compounds have been used for many years as an alternative to surgical stitches, because they reduce scarring and risk of infection. Bacterial polysaccharides, such as the ones found in C. crescentus , are naturally occurring and biodegradable, so the body would break down the glue over time. The combination of great strength and biocompatibility could have applications in joint-replacement, bone and cartilage repair, and eye surgery.

Further study of the nature of the holdfast compound is necessary to understand just how it can withstand such tremendous forces, and technical challenges must be overcome before bacterial superglue can be mass-produced. According to Yves Brun, one of the greatest challenges will be to “produce large quantities of the glue without it sticking to everything used to produce it.” [10]

-- Julia Sero, Harvard Medical School

Primary Articles

1. Tsang HP, Li G, Brun YV, Freund LB, and Tang JX. Adhesion of single bacterial cells in the micronewton range. 2006. Proceedings of the National Academy of Sciences USA . 103(15):5764-5768.

2. Merker RI, Smit J. Characterization of the adhesive holdfast of marine and freshwater Caulobacters. Applied and Environmental Microbiology. 1998. 54(8):2078-2085.

3. Bodenmiller D, Toh E, and Brun Y. Development of surface adhesion in Caulobacter crescentus . Journal of Bacteriology. 2004. 186(5):1438-1447.

4. Feng HH, Chan KY , Xu LC. Quantification of bacterial adhesion forces using atomic force microscopy (AFM). Journal of Microbial Methods. 2000. 40(1):89-97.

5. Li G, Smith CS, Brun YV, Tang, JX. The elastic properties of the Caulobacter crescentus adhesive holdfast are dependent on oligomers of N -acetylglucosamine. Journal of Bacteriology. 2005. 187(1):257-265.

6. Pawlicki JM, Pease LB, Pierce CM, Startz TP, Zhang Y, Smith AM. The effect of molluscan glue proteins on gel mechanics. Journal of Experimental Biology. 2004. 207(Pt 7):1127-1135.

7. Smith AM. The structure and function of adhesive gels from invertebrates. 2002. Integrative and Comparative Biology. 42(6):1164-1171.

8. Autumn K, Liang YA, Hsieh ST, Zesch W, Chan WP, Kenny TW, Fearing R, Full RJ. Adhesive force of a single gecko foot-hair. 2000. Nature. 405(6787):681-685.

9. Gein AK, Dubonos SV, Grigoieva IV, Novoselov KS , Zhukov AA, Shapoval SY. Microfabricated adhesive mimicking gecko foot-hair. 2003. Nature Materials. 2(7):461-463.

10. Press release from Indiana University : http://newsinfo.iu.edu/news/page/normal/3258.html