The general focus of this research area is toward understanding the factors which govern the attachment of bacteria to surfaces. This is important because when bacteria attach to surfaces it can lead to infection and disease (ranging in severity from gingivitis to cystic fibrosis). By understanding the factors which influence bacterial adhesion, surfaces can be better designed and conditions better controlled to eliminate adhesion and prevent biofilm formation.
We also speculate that bacterial appendages may be able to penetrate the electrostatic energy barrier and bring the bacterial body close enough to the surface for permanent attachment.
A correlation between the virulence of an infectious organism and its ability to swim (motility) has been suggested in the literaure. Therefore, we want to observe how the swimming behavior of a bacterium affects the initial events leading up to its attachment to a surface. To do this we use a technique called total internal reflection aqueous fluorescence (TIRAF) microscopy. One experimental goal of this project is to demonstrate the viability of TIRAF as a method for measuring the distance between swimming bacteria and a surface. The TIRAF technique allows us to measure the distance between the bacteria and the surface while it is swimming, tethered near the surface, and irreversibly attached. By knowing the distance, we can infer what forces may be responsible for keeping the bacteria closely associated with the surface prior to permanent attachment. Through a collaborative effort with Prof. Lukas Tamm in the Health Sciences Center at the University of Virginia we have captured images of motile bacteria associated with clean quartz surfaces. By quantifying the darkness of the bacteria, we can determine the distance from the surface. The closer the bacteria are to the surface, the darker their image appears. A typical image is shown in Figure 1 below.
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Examining bacterial interactions with a surface under different conditions (varying ionic strength of the solution and hydrophobic and hydrophilic surface coatings) will enable us to correlate forces with the observed attachment behavior at various distances from the surface. In preliminary studies we observed that within about 100 nanometers (nm) of the surface hydrodynamic forces caused bacteria to swim along the surface with a head-down orientation, resulting in an apparent "adhesion" between swimming cells and the surface. Closer to the surface (around 60 nm) electrostatic interactions and van der Waals forces dominated the behavior and maintained the bacteria irreversibly attached to the surface. The role of appendages such as flagella and pili is not well understood. The flagella may play a significant role in securing a cell which has become immobilized in the secondary minimum. We also speculate that these appendages may be able to penetrate the electrostatic energy barrier and bring the bacterial body close enough to the surface for permanent attachment. Our future plans include studies of a series of bacterial mutants with different surface features.