Effects of shear stress on the secretion of extracellular polymeric substances in biofilms

Movement of biofilm into the suspended state as a result of erosion and sloughing is inevitable. The causes for this have been widely attributed to the presence of high shear stress, excess nutrients or a combination of both parameters. It is believed that the extracellular polymeric substances (EPS) of biofilm contribute to the mechanical stability of the biofilm; however, the effect of changing shear stress on the secretion of biofilm EPS is not well understood. This research examined the EPS of a biofilm quantitatively during the change in shear stress. Biofilm was grown in a rotating drum biofilm reactor and subjected to three different shear stresses: 0.1022 N/m² at 100 rpm, 0.1533 N/m² at 150 rpm and 0.2044 N/m² at 200 rpm. Results indicate that a sudden increase of shear stress caused a drastic increase of the EPS-polysaccharides in the biofilm. However, 20 days after each shear increase, the EPS-polysaccharides secretion dropped back to the previous values before the shear change. Physical properties of the biofilm such as porosity were also monitored and the result showed that higher shear stress produced less porous but denser biofilm. Another notable effect when the shear stress was increased was that more erosion of biofilm occurred.