Spatial distribution of mechanical properties in Pseudomonas aeruginosa biofilms, and their potential impacts on biofilm deformation

Abstract

The mechanical properties of biofilms can be used to predict biofilm deformation, for example under fluid flow. We used magnetic tweezers to spatially map the compliance of Pseudomonas aeruginosa biofilms at the micron scale, then used modeling to assess its effects on biofilm deformation. Biofilms were grown in capillary flow cells with Reynolds numbers (Re) ranging 0.28 to 13.9, bulk dissolved oxygen (DO) concentrations from 1 mg/L to 8 mg/L, and bulk calcium ion (Ca 2+ ) concentrations of 0 and 100 mg CaCl 2 /L. Higher Re numbers resulted in more uniform biofilm morphologies. The biofilm was stiffer at the center of the flow cell than near the walls. Lower bulk DO led to more stratified biofilms. Higher Ca 2+ led to increased stiffness and more uniform mechanical properties. Using the experimental mechanical properties, fluid-structure interaction models predicted up to 64% greater deformations for heterogeneous biofilms, compared to a homogeneous biofilms with the same average properties. However, the error depended on the biofilm morphology and flow regime. Our results show significant spatial mechanical variability exists at the micron scale, and that this variability can potentially affect biofilm deformation. The average mechanical properties, provided in many studies, should be used with caution when predicting biofilm deformation.

Effects of biofilm heterogeneity on the apparent mechanical properties obtained by shear rheometry

“…Second, we used the model to explore the impact of having a continuous linear variation in the mechanical properties, rather than two discrete layers. The stratified mechanical parameters measured from our previous paper (Pavissich et al, 2021) were used as modeling inputs with a simplified geometry, as explained below.…”

“…Shear moduli and viscosities for both layers were obtained from the experiments described in Sections 2.1 and 2.2. For simulations with linearly stratified mechanical properties, we used the data from a previous study (Pavissich et al, 2021). In that study, magnetic tweezers were used to determine the spatial distribution of the elastic modulus in Pseudomonas aeruginosa biofilms.…”

“…For comparison, the layered mechanical distribution was also obtained from the same study by fitting the previous data (Pavissich et al, 2021) with a stepwise function. The fitted functions used for biofilm mechanical properties can be found in Supporting Information: Figure S4.…”

“…However, biofilms are known to have high spatial variability of mechanical properties (Boudarel et al, 2018; Li et al, 2022). For example, using microscale techniques, biofilms have been shown to be more compliant near the surface and more rigid near the base (Birjiniuk et al, 2014; Galy et al, 2012; Pavissich et al, 2021; Safari et al, 2015). The elastic compliance values in the same biofilm can spatially vary by up to three orders of magnitude (Galy et al, 2012).…”

Predicting biofilm deformation with a viscoelastic phase‐field model: Modeling and experimental studies

“…However, biofilms are complex, both in microbial ecology, morphology, and EPS chemical composition. This provides them with a great spatial variability (i.e., heterogeneity) of mechanical properties, as well as potentially large differences in mechanical properties between different biofilms (Böl, Ehret, Bolea Albero, Hellriegel, & Krull, 2012; Galy et al, 2012; Pavissich, Li, & Nerenberg, 2020; Shaw, Winston, Rupp, Klapper, & Stoodley, 2004). This is further complicated by the large variety of experimental techniques used to measure mechanical properties, and the different types of parameters obtained.…”