Interaction Forces

Probing Bacterial Interaction Forces with Atomic Force Microscopy

This web page provides an overview of ongoing work studying bacterial interaction forces using AFM. For full details, see the following:

Why are we interested in bacterial adhesion?

Our interest is in studying how bacterial attach to surfaces.This research has environmental and biomedical applications.

In environmental engineering, we would like to use bacteria to clean up contaminants (bioremediation). Bacteria that can degrade certain toxic chemicals have been isolated and characterized. However, many bacteria are very "sticky" because of polymers that are present on their surfaces. When these bacteria are added to soil, they do not travel with the groundwater because they immediately stick to the soil. We do not have a clear fundamental understanding of this adhesion process. AFM can be used to directly measure these interaction forces, and hopefully that will allow us to understand bacterial adhesion to soil better.

In the biomedical field, there is much interest in understanding how bacteria attach to teeth, dentures, contact lenses, implants, and other devices.

How can AFM be used to measure bacterial interaction forces?

Bacteria of interest are attached to a glass slide and the forces are measured continuously as the tip is brought into contact with the sample, and also while the tip is pulled away from the sample.

What type of interaction forces are observed?

We have studied the interaction between the AFM's silicon nitride tip and either Pseudomonas putida KT2442 or Burkholderia cepacia G4.

These interaction forces are not only very large in magnitude, but they extend over large distances.We attributed this interaction to electrosteric repulsion caused by the bacterial polymers.In addition, we saw changes in the forces as a function of pH. The extension of the forces was greatest at high pH. This was attributed to the extension of the polysaccharides

The retraction curves tell how strong the attractive forces are between the bacterium and the tip, once contact has been made.

Since we attributed these large forces and distances to electrosteric repulsion caused by the bacterial polysaccharides, we also made measurements on cells in which we had partially removed the polysaccharides. Here are the force measurements for two microbes in which the polysaccharides were altered. The top graph is Pseudomonas putida KT2442, the bottom graph is Burkholderia cepacia G4.

In summary, we have developed a protocol for measuring bacterial interaction forces with AFM. The forces measured for these two microbes are highly pH-dependent, and are explained by an electrosteric model (see Camesano and Logan, 2000 for details). Further work is being done to characterize these specific polysaccharides and to make force measurements relevant to more realistic environmental systems.

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Last modified: October 23, 2007 15:23:47