Quantifying the viscoelastic properties of treated and untreated Pseudomonas aeruginosa and Staphylococcus epidermidis biofilms using a rheological creep analysis

Abstract

Microbial biofilms are quite difficult to kill and control, and present many problems to industry and medicine. The ability to alter the mechanical properties of biofilms could aid in the control of biofilm. The goal of this research project was to develop techniques for measuring the mechanical properties of biofilms so that the effects of chemical treatments could be assessed. Constitutive material models were developed and applied to assist in this effort to quantify the effects. Biofilms are viscoelastic in nature, therefore, rheological testing techniques were utilized for this research. Creep testing was performed on a parallel plate rheometer to determine biofilm mechanical properties. The rheometer is a mechanical device that can accurately measure and apply shear stress and strain on viscoelastic samples. The Burger material model closely approximated material behavior of most chemical treatments. This model was used for determining constitutive properties. Pseudomonas aeruginosa (FRD1) and Staphylococcus epidermidis colony biofilms were used for testing. Several treatment methods were used to investigate their effect on biofilm mechanical properties. As a source of different cations, solutions of NaCl, FeCl3, AlCl3, MgCl2, CaCl2, FeCl2 were used for testing. Multivalent cation treatments stiffened the FRD1 biofilm, but weakened the S. epidermidis. Urea treatments weakened both biofilm species. Glutaraldehyde treatments weakened the FRD1 biofilm, but had little effect on the S. epidermidis. Several treatments - EDTA, Barquat, chlorine, antibiotics (rifampin, and ciprofloxacin) - weakened biofilms of both species. The effect of the same chemical treatment between the two species of biofilm sometimes had nearly opposite effects on the biofilms mechanical properties. This research illustrated that it is possible to alter the mechanical properties of biofilm through chemical addition. Further, there are significant differences between the ways that the material properties of biofilms of different species of bacteria will be affected by a chemical treatment. Finally, it was observed that the 4-parameter Burger model for constitutive mechanical properties of biofilms fit the vast majority of the collected data, so that this model proves useful in comparing properties of biofilms grown or treated under various conditions.