Impact of shear stress on the formation and structure of microbial biofilms : a review

Abstract

Biofilm attachment, development, behaviour and population characteristics are strongly influenced by many environmental factors and by biological properties. One of the most important factors affecting biofilm structure and behaviour is the velocity field of the fluid in contact with the microbial layer (hydrodynamics). Literature study is available on the influence of hydrodynamic conditions on the physical structure of the biofilms i.e. high shear forces result in thinner, denser, stronger and filamentous biofilms while low shear forces result in monolayer of biofilm cells with mound and circular shape. However, little is known about the impact of shear stress on microbial composition and detachment. Most previous investigations uses shear or velocity gradient to be analogous to the shear stresses within the fluid. This is true quantitatively and is adequate within a single set of experiments as an increase in velocity gradient is equivalent to increases in shear stress cetaris paribus , however it makes it difficult to apply or relate results across different studies. One parameter that affects wall shear stress is the frictional factor f which is dependent on the roughness of the solid surface on which the fluid flows over. Previous investigations utilized a variety of methods to generate shear such as pipe flow, movement of surfaces, flow through packed beds, rotation of concentric cylinders and aeration. Of these, pipe flow and rotation of concentric cylinders were the most common. In this study, only the investigations using pipe flow and rotation of movement of surfaces were included as these two results in similar flow characteristics. Rotation of concentric cylinders results in the formation of taylor vortex and wavy vortex flow structure. It is assumed that other flow phenomenon present may have a significant effect on biofilm growth and structure and may distort conclusions when compared alongside the simpler flow structure of the two methods considered. The reasons for not considering the other methods of shear generation are similar. The range of shear stresses achieved in the investigations was up to 0.63 for laminar flow and between 0.35 and 8.71 for turbulent flow.