Bacterial adhesion on surfaces

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2012

The bacterium Caulobacter crescentus applies an adhesive when attached to surfaces through a two-stage process.

KEY CONCEPTS
• The bacterium Caulobacter crescentus attaches to surfaces through a reversible, irreversible, two-stage process.
• During the irreversible stages, a strong polysaccharide-based adhesive is synthesized by the bacterium and applied in only a matter of seconds through a just-in-time process.
• Two key parts of the bacterium involved in determining when the adhesive is produced are the flagellum and the pili.

IN APPLICATIONS SUCH AS WATER-BASED MWFs, bacterial contamination can become a major problem that affects the performance of the lubricant. Bacteria grow by literally consuming the components in the MWF as food. The result is that additives used to provide lubricity, corrosion protection and emulsification are taken away, preventing the MWF from doing its job.

An additional element is that bacteria generate acidic byproducts that can promote corrosion. The maintenance engineer does not readily see most of these steps taken by bacteria. But bacteria also form biofilms in MWF systems that can be observed. These biofilms contribute to the fluid’s deteriorating performance by plugging filters, restricting fluid flow and impeding chip removal, which promotes corrosion.

In a previous TLT article (1), Yves Brun, Clyde Culbertson Professor of Biology at Indiana University in Bloomington, Ind., discussed the reason why bacteria form biofilms. He said, “Nutrients for bacteria tend to adsorb to surfaces. This leads bacteria to grow on those surfaces where their nutrients are found. As part of this process, bacteria then develop multicellular structures known as biofilms.”

One of the research challenges is figuring out how bacteria adhere to surfaces to form biofilms. Treatment with antimicrobial agents has proved ineffective in stopping the growth of biofilms.

In the previous TLT article cited above, Brun and his co-workers discovered that the common gram-negative bacterium known as Caulobacter (C.) crescentus produces a very strong adhesive when placed on a surface. The adhesive exhibits a sticking power approaching 70 Newtons per square millimeter, which is more than double the performance of “super glue.”

The purpose of the adhesive is to ensure that the bacteria can develop a biofilm that will strongly adhere to a surface favored for establishing a colony. Determining how this process occurs could prove to be very critical so that measures can be found to prevent the formation of biofilms.

New research has now been reported on how quickly C. crescentus applies the adhesive when it approaches a surface.

TWO-STAGE PROCESS
Brun and his fellow researchers have determined that the bacterium C. crescentus attaches to surfaces through a two-stage process. Brun says, “Using its flagellum and pili, the bacterium establishes a low adherence to a particular surface but can detach at any time in the reversible phase. Once a decision is made to remain on the surface, then biosynthesis of the stronger polysaccharide-based adhesive is initiated, and the bacterium becomes situated on the surface in an irreversible manner for a long period of time.”

The permanent attachment of the bacterium to the surface occurs in only a matter of a few seconds. In his initial work, Brun indicated that the adhesive is produced and applied in only a matter of two minutes. He adds, “Further work shows that the process can take place in only 10 seconds.”

Brun equates the attachment of the bacteria to the just-in-time industrial delivery many of us expect from our suppliers. He says, “With the potency of the adhesive, bacteria cannot produce it and apply it too early because they may end up irreversibly attached to the wrong surface or the adhesive may lose its effectiveness.”

For many of us, the analogy for the adhesive is super glue. We only use it when necessary to make sure it does the intended job.

Brun indicates that the bacterium can remain on the surface for at least days and help to establish a biofilm. Two of the key parts of the bacterium involved in this process are the flagellum and the pili. 

Brun says, “The flagellum acts as a propeller to move the bacterium through its environment and to the surface. In attaching to the surface, the bacterium first becomes tethered through a process mediated by this same flagellum, but the flagellum motor is running, so the bacterium twirls on the surface. It is similar to a human-being twirling on one foot.”

At the same cell-end as the flagellum are the pili, which are thin fibers consisting of protein. They act in a similar fashion to cables to position the bacterium on the surface. Brun says, “When the pili contact the surface, it causes the flagellum motor to stop in a similar fashion to when our twirling human puts a second foot on the ground.”

The researchers found that once the flagellum motor stops, a signal is transmitted to the proteins that synthesize the polysaccharide adhesive to activate them.

Brun says, “Pilli are a very important part of the attachment process. We analyzed bacteria without pili and found that when the cells are tethered to the surface, their motor does not stop, synthesis of the polysaccharide adhesive is not stimulated and they eventually detach.”

Figure 2 shows an image of the C. crescentus with the flagellum at the bottom and the adhesive holding the bacterium at the top. The researchers observed the two-step process through the use of high-resolution video microscopy. A fluorescent stain was used that highlights the adhesive when it is applied.


Figure 2. The bacterium Caulobacter crescentus can very quickly produce a strong polysaccharide-based adhesive as part of a two-step process used to attach to surfaces. (Courtesy of Indiana University)

The researchers evaluated additional bacterial species and found that Agrobacterim tumefaciens and Asticcaculis biprosthecum display a similar two-step attachment process. It remains to be seen if other species such as Escherichia coli and Pseudomonas aeruginosa attach to surfaces in the same fashion.

The challenge facing Brun is to determine the mechanism by which the bacterium signals the biosynthesis of the adhesive and how to prevent this process from happening. In effect, stop the process that initiates the formation of a biofilm. He says, “One of the important aspects is to determine the role of the bacterial signal molecule, cyclic di-GMP in controlling the transition from reversible to irreversible attachment to the surface.”

Future work for Brun will involve examining the flagellum motor to determine what is done to prime the biosynthetic machinery needed to produce the adhesive. Additional information can be found in a recent publication (2) or by contacting Brun at ybrun@indiana.edu.

REFERENCES
1. Canter, N. (2006), “New Adhesive Fights Bacteria Growth in MWFs,” TLT, 62 (10), pp. 16-18.
2. Li, G., Brown. P., Tang, J., Xu, J., Quardokus, E., Fuqua, C. and Brun, Y. (2012), “Surface Contact Stimulates the Just-in-Time Deployment of Bacterial Adhesions,” Molecular Microbiology, 83 (1), pp. 41-51.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.