Nanoscale characterization and determination of adhesion forces of Pseudomonas aeruginosa pili by using atomic force microscopy
Type IV pili (Tfp) are filamentous organelles used by many species of bacteria to adhere to and move about a surface. Tfp are located at one pole of the bacterium, and can attach using both specific and non-specific adhesion mechanisms to anchor the bacterium to the surface. Bacteria also pull their Tfp back into the cell wall and dissasemble its pilin subunits, thereby shortening the pilus and generating a retraction force. This retraction force is used to pull the bacterium along the surface, called twitching motility. As Tfp are crucial for the formation of biofilms in many bacteria species, understanding their mechanical properties has become an important line of study. This experiment studied P. aeruginosa Tfp using AFM techinques.
Experiment and Results
Bacteria were attached to an AFM canitlever tip. The tip was then brought near a surface (made of mica) until it just touched the surface. Presumably, this allowed the bacteria, which were sitting on the side of the tip, to extend a pilus and attach to the surface. The tip was then displaced upwards, and the amount of force necessary to keep the tip at each displacement was recorded. This generated force-versus-displacement curves, giving information about the adhesion forces of the Tfp. Some of the results of these measurements are displayed below. The data showed that up to a certain displacement, there was no force felt by the tip. After that, there was an increase in the tension as the tip moved further away. Finally, the force went abruptly to zero, indicating that the pilus had either broken or become detached from the surface. The force at which the pili failed was around 100 pN. The interpretation of the force curves is depicted below.
Discussion, Relation to Soft Matter
The group tried to understand the force/displacement curves by comparing them with proposed models of the Tfp. The Tfp cannot be seen as simple springs, and so were modeled using both the worm-like chain (WLC) and the freely-jointed chain (FJC) models. In WLC, one of the crucial parameters is the persistence length of the polymer. This is length over which the polymer locally appears straight. The persistence length of Tfp has been measured to be 5 microns, which would imply that the Tfp are very stiff. The WLC model therefore failed at explaining the force spectra of this experiment. However, when the Tfp were modeled as WLC's in series with a simple spring, the model was able to fit very well to the data. The interpretation of this simple spring is unclear, though the group suggests that it might be the restoring force due to the cell wall where the pilus is attached. The group also tried to fit their data to the FJC, but were unable to make a good fit.
This paper highlights how experimental techniques to measure forces can be used to better model protein polymers. While a clear model of the Tfp has yet to emerge, this study elucidates some of the properties of the pilus adhesion mechanism and contributes to the discussion of the adhesion abilities of P. aeruginosa.