Diffusion in Biofilms

Abstract

Much of what makes life in a microbial biofilm different from life in a free aqueous suspension can be explained by invoking the phenomenon of diffusion. This article discusses the profound influence of the physics of the diffusion process on the chemistry and biology of the biofilm mode of growth. I have framed the discussion in the form of answers to five important questions. WHY IS DIFFUSION AN IMPORTANT PROCESS IN BIOFILMS?When microorganisms are grown in planktonic culture, diffusion is usually of little consequence. There are two reasons for this. The first reason is that planktonic cultures are generally agitated, and the resulting fluid flow transports solutes rapidly, resulting in a well-mixed system. Transport that occurs as a solute is carried by the bulk flow of a fluid (convection) is generally much faster than the transport resulting from random molecular motion (diffusion). Of course, there is no net convective flow of fluid into or out of the microbial cell. At some point close to the cell, diffusion becomes critical for moving the solute toward or away from the cell surface. The reason that diffusion does not limit this step is that the diffusion distance is small and diffusion is rapid over such short distances.Diffusion limitation arises readily in biofilm systems because fluid flow is reduced and the diffusion distance is increased in the biofilm mode of growth. The biofilm and the substratum to which it is anchored impede flow in the vicinity of the biofilm, throttling convective transport. Inside cell clusters, the locally high cell densities and the presence of extracellular polymeric substances arrest the flow of water. Diffusion is the predominant transport process within cell aggregates (7,36). Whereas the diffusion distance for a freely suspended microorganism is of the order of magnitude of the dimension of an individual cell, the diffusion distance in a biofilm becomes the dimension of multicellular clusters. This can easily represent an increase in the diffusion distance, compared to a single cell, of 2 orders of magnitude. As is explained in the next section, diffusive equilibration time scales as the square of the diffusion distance. In other words, a biofilm that is 10 cells thick will exhibit a diffusion time 100 times longer than that of a lone cell. HOW FAST DO SOLUTES DIFFUSE INTO OR OUT OF A BIOFILM?Suppose a stain is added to the medium bathing a biofilm. How long will it take this dye to permeate, by diffusion, to the interior of a cell cluster or to the bottom of the biofilm? Because I aspire in this article to avoid overwhelming the reader with mathematics, I will define a single, simple measure of diffusive penetration time. There are two versions of this measure, depending on the geometry of the system. The time required for a solute added to the fluid bathing a biofilm to attain 90% of the bulk fluid concentration at the base of flat slab (uniformly thick) biofilm is given very simply byHere, L is the biofilm thickness, and D e is the effective diffusion coefficient i...

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