Viscoelasticity of oral biofilms and antimicrobial penetration

Biofilms are widely accepted as a viscoelastic material. The mathematical analysis of viscoelasticity unveils the features of biofilms in a quantitative level. In this thesis, the viscoelasticity was interpreted via stress relaxation analysis, which primarily is composed of 3 underlying processes with characteristic time constants called the fast (E1), intermediate (E2) and slow (E3) response. Biofilm composition corresponds to the relaxation responses according to our knowledge of material properties. We assumed that water and other smaller molecules of the biofilm matrix are expected to constitute the fast response; higher molecular weight components of the extracellular matrix are assumed to cause the intermediate response; bacteria represent the slow response.
We discovered that certain components of the biofilms, contributing to the fast relaxation response (E1), have a negative impact on the antimicrobial penetration. The antimicrobial penetration increases with the decreasing importance of the fast relaxation response (E1). Meanwhile, bacteria in biofilms, contributing to the slow relaxation response (E3), have a positive impact on the antimicrobial penetration. The antimicrobial penetration increases with the increasing importance of the slow relaxation response (E3). These relations have been extensively verified in several biofilm models, e.g. in vitro/ in vivo biofilms, single/ dual/ multi species biofilms, static/ flow/ compressed biofilms and brushed/ un-brushed biofilms. Besides that, the relations were also verified with antimicrobials in different mouthrinses.
Considering the essential role that viscoelasticity plays in the quantitative determination of antimicrobial penetration into biofilms, we propose that the viscoelasticity should be viewed as a virulence factor of biofilms.