Growth of microorganisms in two-dimensional steady-state diffusion gradients

A two-dimensional, steady-state diffusion system has been devised for growth of microorganisms in nutrient concentration gradients. Two reservoirs continually supply substrates, while unused substrates and waste products are washed into a third reservoir. Substrates are supplied in the form of a steady-state gradient within an agar sheet which contains the test organisms. Microbial populations can thus be exposed to all values and combinations of two test variables. The system was tested with Escherichia coli grown in glucose and bicarbonate gradients. Hyphomicrobium sp., Rhodomicrobium vannielii, and Thiopedia sp. in a mixed population were segregated by use of a gradient system of methylamine and sodium sulfide. Exposure of natural aquatic populations to a gradient of pond mud and acetate resulted in differential growth of Bacillus sp. and rod-shaped bacteria with terminal endospores. Thus the potential value of gradients in predicting the response of natural microbial communities to nutrients or toxicants and in enriching for specific organisms has been demonstrated qualitatively.