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First, a look at solute transport in planktonic cultures

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). Since 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.

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Figure 4-1. In planktonic cultures, fluid flow carries solutes quickly to cells. Diffusion, necessary for solute transport into the cell, is rapid over such short distances.

Why transport is diffusion-limited in biofilms

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.

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Figure 4-2. In biofilms, fluid flow is reduced. Diffusion, the predominant transport process within cell aggregates, is vastly slowed as diffusion distance increases.

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 two orders of magnitude. As is explained in the next section, diffusive equilibration time changes 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.

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