Bacterial Adhesion on Surfaces with Gradients of Bio-Inertness and Roughness

Surface chemistry and roughness play important roles in bacterial cell adhesion and biofilm formation. We recently reported that self-assembled monolayers (SAMs) presenting tri(ethylene) glycol (TEG) are highly resistant to adhesion and biofilm formation of Escherichia coli, while SAMs presenting methyl groups are non-resistant (Hou et al., Applied and Environmental Microbiology, 2007. 73: 4300-4307). To further study the response of cell adhesion and biofilm formation to surface conditions, SAM surfaces with molecularly well-defined chemical and roughness gradients were formed on gold films. The surfaces with chemical gradients were prepared with a mixture of two SAMs to obtain the gradient in bio-inertness of TEG-SAM, with the other one being either OH-SAM or CH₃-SAM_. Escherichia coli RP437 was labeled with a plasmid constitutively expressing DsRed-Express to form biofilms, which were analyzed with confocal laser scanning microscopy. The three dimensional structure of biofilms was analyzed with the COMSTAT software to quantify biomass, surface coverage, and thickness. Measurements of surface coverage indicate that the amount of biofilm formed on surfaces is linearly proportional to surface adhesiveness. In addition to chemical gradient, a gradient with the nanometer-scale topography was introduced in the gold films by creating a continuous increase in the angle of incidence of gold atoms during vapor deposition. The strong correlation between cell adhesion and surface roughness was observed. The results obtained in this study will be useful for understanding bacteria-surface interactions and for developing effective control methods.

Keywords: molecular gradient, bio-inert surface, self-assembled monolayer, biofilm, cell adhesion.