Bacterial Adhesion and Motility on Silanized Glass Surfaces
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Attachment of bacteria to surfaces is the first step in the formation of biofilms. Medical, industrial, and technological applications require effective control over biofilm formation, and control of initial bacterial attachment is a potential alternate approach to conventional techniques to promote or suppress biofilm formation. Conventional use of antibiotics and bactericidal agents as antifouling techniques has adverse environmental implications1 and potentially lead to evolution of antibiotic resistant strains of bacteria.2 Controlling the initial attachment step in biofilm formation circumvents these disadvantages associated with conventional approaches. Rational design of surfaces to control bacterial attachment, however, requires fundamental understanding of bacteria-surface interactions and adhesion mechanisms. In this work, we investigate the adhesion of bacteria on surfaces of controlled physical properties to obtain a mechanistic understanding of adhesion and near surface mobility of bacteria. First, we study the deposition behavior of Escherichia coli from flow on surfaces of controlled charge, wettability, and energy created by self-assembly of organosilanes on glass. We use high throughput bacteria tracking algorithms to analyze the trajectories for hundreds of bacteria. We characterize surface-associated motion of attached cells and find that a motility metric based on extent of motion mediated by flagella is inversely correlated with rate of bacterial deposition, whereas conventional surface characterization metrics are not well correlated. The transition from transient initial attachment to irreversible attachment is also correlated to deposition rate. Our results suggest that the techniques and methods presented here to characterize transient surface motility can potentially serve as a metric to rapidly determine the efficacy of surfaces to reduce fouling by bacteria. Next, we characterize the near-surface mobility associated with adhesion in E. coli bacteria deposited from flow at varying shear stresses on glass substrates bearing self-assembled alkylsilane and fluoroalkylsilane layers. We find that deposition of bacteria decreases with shear stress and increases with surface roughness. Bacteria also exhibit mobile adhesion on very smooth surfaces resulting in large linear displacements in the direction of flow which is independent of flagellar expression but requires absence of fimbriae on the cell surface. Speed of mobile adhesion decreases and residence time of cells increases as a function of increasing shear stress. Since surface roughness determines the transition from immobile to mobile adhesion, we suggest that strategies to reduce frictional interactions between cells and surfaces, either by engineering nanoscale-smooth surfaces or by suppressing expression of cell surface adhesins such as fimbriae, may help to reduce fouling during initial deposition. Finally, we investigate the competing effects of surface chemistry, solution ionic strength, and medium viscoelasticity on near-surface attachment and motion of E. coli. We vary solution viscoelasticity by adding xanthan gum, a model polysaccharide; solution chemistry by adding a monovalent salt NaCl; and surface chemistry by using a hydrophobic silanized glass and hydrophilic cleaned glass. We sort cells between two types of near-surface behavior: surface-associated non-swimming and near-surface swimming. We characterize the dynamics of each population of cells and find that the swimming cells show near ballistic motion; and the non-swimming cells show near diffusive behavior on short time scales and sub-diffusive behavior on long times. Of the three variables in the experiment (ionic strength, surface chemistry, and polymer concentration) the last has the most pronounced effect on dynamics and average speed of swimming cells. We show that high polymer concentrations in semi-dilute entangled regime present obstacles to bacteria locomotion and cells exhibit reversals in swimming trajectories without angular reorientation of the cell body axis. Our results suggest that characterizing the rheological properties of ambient environment is important for effective design of surfaces for applications such as medical implants and sensors in oil exploration where bacterial attachment occurs under moderate to highly viscous and Newtonian to highly non-Newtonian environments.