A diffusion gradient chamber for studying microbial behavior and separating microorganisms.

The natural habitats of most microbes are dynamic and include spatial gradients of growth substrates, electron acceptors, pH, salts, and inhibitory compounds. To mimic this diffusive aspect of nature, we developed an analytical diffusion gradient chamber (DGC) that can be used to separate, enrich for, isolate, and study the behavior of microorganisms. The chamber is a polycarbonate box containing an arena (5 by 5 by 2 cm) into which is cast a semisolid growth medium. Continuously replenished solute reservoirs positioned on each side of the arena but separated from it by a porous membrane enable the formation throughout the gel of multiple, intersecting gradients of solutes in two dimensions. With glucose as the solute, a gradient which spanned a 100-fold range in concentration was established across the arena in about 4 days. The shape of the glucose gradient was accurately predicted by a mathematical model based on Fickian diffusion. The growth and migratory behavior of Escherichia coli in response to imposed gradients of attractants (aspartate, alpha-methyl aspartate, and serine) and a repellent (valine) were examined. Cells responded in predictable ways to such gradients by forming distinctive growth and migration patterns in the DGC. This was true for wild-type E. coli as well as specific chemotaxis and motility mutants. The patterns yielded information about the threshold concentration of chemoeffectors needed to elicit a response as well as their saturating concentration. It was also evident that the metabolism of attractants significantly affected the gradients and, hence, the movement of cells. Finally, it was possible to separate E. coli and Pseudomonas fluorescens in the DGC on the basis of their differential responses to gradients of various chemoeffectors.