Biofilm mechanics: Implications in infection and survival

Abstract

It has long been recognized that biofilms are viscoelastic materials, however the importance of this attribute to the survival and persistence of these microbial communities is yet to be fully realized. Here we review work, which focuses on understanding biofilm mechanics and put this knowledge in the context of biofilm survival, particularly for biofilm-associated infections. We note that biofilm viscoelasticity may be an evolved property of these communities, and that the production of multiple extracellular polymeric slime components may be a way to ensure the development of biofilms with complex viscoelastic properties. We discuss viscoelasticity facilitating biofilm survival in the context of promoting the formation of larger and stronger biofilms when exposed to shear forces, promoting fluid-like behavior of the biofilm and subsequent biofilm expansion by viscous flow, and enabling resistance to both mechanical and chemical methods of clearance. We conclude that biofilm viscoelasticity contributes to the virulence of chronic biofilm infections.

Keywords: Biofilm; Biophysics; Extracellular matrix; Extracellular polymeric substance; Mechanics; Persistence; Tolerance; Viscoelasticity.

Fig. 1

Comparison of publication numbers between 1982–2018. Publication numbers. Keywords indicated in the legend were searched using the Scopus database. Publication numbers for each keyword search from 1982, being the earliest identified biofilm mechanical paper, to December 2018.

Fig. 2

Schematic of the different mechanical methods that can be used to analyze biofilms. Mechanical methods to analyze biofilms can be divided into indentation (application of a normal force) or shear (application of a shear force). (A) For indentation, a probe or indenter is brought into contact with the biofilm, and the mechanical properties analyzed using either compression (pushing), tension (pulling) or dynamic (cycling of compression and tension) modes. For analysis under shear, there are two main ways that shear forces can be applied to a biofilm, (B) using spinning disc rheometry (spinning) or (C) using fluid shear (flow). (B) For spinning disc rheometry, a parallel plate probe is brought into contact with the biofilm, and changes in stress or strain measured under different modes of analysis. This can be either constant, where a constant stress (creep) or constant strain (relaxation) is applied; ramp, where an increasing stress or strain (stress-strain curve) is applied; or dynamic oscillation, where the probe is oscillated at either increasing stress, strain or frequency (frequency sweep, stress sweep, strain sweep). (C) When grown under fluid shear, time-lapse microscopy can be used to visualize and measure the deflection of biofilm structures in response to changes in the fluid flow rate. Alternatively, fluorescent beads can be incorporated into the system and their movement tracked through the biofilm.

Fig. 3

Mechanical response of biofilms to a constant stress. Creep-recovery curves are powerful analyses that can be performed using both spinning disc rheology and time-lapse microscopy of flow cell biofilms, that can determine biofilm mechanical behavior. During this analysis a constant stress is applied to the biofilm (t₁) and the resulting deformation (strain) is measured. The stress is then removed (t₂) and the recovery measured. The left panel depicts typical creep-recovery curves for materials with (A) elastic, (B) viscous and (C) viscoelastic properties. The right panels depict how biofilms with these properties would respond, at different stages of the analysis. (A) Biofilms that are predominately elastic (1), in response to an applied stress will instantly deform (2) and maintain this deformed state while the stress is applied (3). However, when the stress is removed the biofilm will instantly return to the original per-deformed state or structure (4). This response is akin to elastic bands being stretched. (B) Biofilms that are predominately viscous (1), in response to an applied stress display flow behavior (2), and will continuously deform and flow while the stress is applied (3). When the stress is removed, the biofilm will maintain the deformed state, and will not return to the original structure (4). This response is akin to honey being poured onto a surface. (C) Biofilms that are viscoelastic, display both elastic (blue), viscous (red) and a combination of both, where the elastic behavior begins to transition to viscous behavior (purple), which is typically time dependent. Viscoelastic biofilms (1), in response to an applied stress, will instantly deform due to an elastic response (2). Over time this behavior will transition to a viscous response and the biofilm will begin to deform by viscous flow (3). When the stress is removed the biofilm will show an initial elastic recoil (4), however, will never return to the original pre-deformed state due to the transition to viscous recovery (5). This response is akin to silly putty. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

High shear flows induce ripple formation in biofilms. Here ripples formed in a S. mutans biofilm at 21 ​ms exposure to a high shear jet impingement of air ejected at a velocity of 42 ​m/s from a 1 ​mm diameter nozzle positioned perpendicularly 5 ​mm from the biofilm. Under these high shear conditions around the impingement site the biofilm was liquefied and flowed like water. Further detail of the experimental design can be found at [15,18].

Fig. 5

Schematic of how biofilm mechanics can influence survival. Summary of how biofilm viscoelasticity can protect the microbial community when exposed to (A–B) mechanical forces, such as shear stress, and (C–F) chemical treatment.