Determination of the Shear Force at the Balance between Bacterial Attachment and Detachment in Weak-Adherence Systems, Using a Flow Displacement Chamber

Abstract

We introduce a procedure for determining shear forces at the balance between attachment and detachment of bacteria under flow. This procedure can be applied to derive adhesion forces in weak-adherence systems, such as polymer brush coatings, which are currently at the center of attention for their control of bacterial adhesion and biofilm formation.Flow displacement systems like the parallel plate flow chamber (PPFC) are powerful tools for studying the adhesion of colloidal particles, including bacteria, to surfaces under different hydrodynamic conditions (3). Experimental observables, i.e., the number of adhering bacteria and their distribution on surfaces, are used to derive attachment and detachment characteristics. A usual way to obtain qualitative information on the strength of the bacterium-surface bond in the PPFC is to simply pass an air bubble through the chamber and analyze the number of bacteria remaining on the surface; the force exerted by the air bubble on an adhering micrometer-sized particle is around 10 Ϫ7 N (3). Therefore, this method is too insensitive to be used in systems with weak bacterium-surface interaction forces, such as polymer brush coatings.One of the major advantages of the PPFC is the ability to adjust the shear rate and shear stress at the surface. These measures are related through the equation ϭ F/A ϭ , where is the shear stress, F the force, A the area on which the force is exerted, the absolute viscosity, and the shear rate. The wall shear rate is related to the flow rate, Q (8), according to the equation ϭ 3Q/2b 2 w, with b being the half depth and w being the width of the chamber. The force on a single adhering bacterium can then be approximated as the product of wall shear stress times the bacterial surface area exposed to the shear.Sufficiently high shear stresses cause adhering bacteria to slide and roll over a surface, which may lead to detachment. In order to characterize the attachment and detachment of bacteria with respect to wall shear, notions such as "shear to prevent adhesion" and "shear to remove adhered bacteria" are used (1,3,5,6,9,10). Shear stresses in the range of 12 to 54 Pa have been reported to be necessary for the removal of different bacterial strains from regular surfaces, and usually a lower shear stress is required to prevent adhesion (17). These characteristic shear stresses, however, suffer from some ambiguity because the strength of adhesion can depend on the history of the contact between a bacterium and a substratum surface, i.e., its residence time and the shear stress applied during adhesion (11, 18). Moreover, the shear to detach adhered bacteria cannot be obtained for a wide range of adhesion forces within the laminar-flow regime.Bacterial adhesion is the first step in the development of a biofilm and represents the onset of biomaterial implant-related infection, microbially induced corrosion, and the fouling of membranes and heat exchanger surfaces in food processing systems (4). Much attention has been directed toward the development of ...

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