The effect of treatments on the mechanical properties of Staphylococcus epidermidis biofilms under fluid shear and mechanical indentation

Abstract

Biofilms exist on most every wetted surface both in the natural environment and in industrial and medical settings. The bacterial cells are surrounded by protective extracellular polymeric substances (EPS) which provide the mechanical stability for these biofilms. Little is known about the material properties of attached biofilms, making it difficult to predict how a biofilm will behave in response to an applied force. The work presented here measured the force-deflection characteristics of biofilm by two different techniques. The first method involved time lapse imaging of a biofilm grown in a capillary flow cell reactor under a constant fluid shear stress and the second method was based on micro-indentation using an atomic force microscope. For the flow cell experiments Staphylococcus epidermidis was grown in a capillary flow cell reactor. After a day of growth the biofilms received a pretreatment fluid shear while displacements were measured. The biofilms were then treated with different agents which alter the structure of the EPS matrix and thus change the mechanical properties/response of the biofilm. The four treatments examined in these experiments were FeCl 2, chlorhexidine, DispersinB®, and urea. The same fluid shear was applied after the fifteen minute treatment soak and the deflections were recorded. These measurements revealed that i) biofilms behave viscoelastically ii) FeCl 2 and chlorhexidine made the biofilm stiffer while urea and DispersinB® reduced the viscosity of the biofilm. For the micro-indentation experiments Staphylococcus epidermidis was grown in a drip-flow reactor. After four hours of growth the biofilms received a pretreatment indentation (5 microns depth) in which force-displacements were measured. The biofilms were then treated with FeCl 2, chlorhexidine, DispersinB®, and urea. The 5 microns indentation was applied after the fifteen minute treatment soak and force displacements were again measured. The measurements revealed that again i) biofilms behave viscoelastically ii) FeCl 2 and chlorhexidine made the biofilm stiffer while urea and DispersinB® reduced the viscosity of the biofilm. Quantification of biofilm material properties and demonstration that their properties can be altered by chemical or enzymatic treatments opens the door to development of new technologies for controlling detrimental biofilm based on targeting biofilm cohesion rather than killing microorganisms.