SUMMARY: consistently formed multiple growth bands in gel-stabilized gradient systems. The development of inoculated and uninoculated systems was followed by gel scanning and by measuring physico-chemical parameters. Factors which influenced band formation included the availability of oxygen and the presence of a glucose gradient. The relative position of bands was dependent on the glucose concentration in the source layer and the strength of the basal medium. Other factors such as agar type and concentration were relatively unimportant. The generation of steep pH gradients in the gel profoundly affected band formation. Thus, in the presence of a high buffering capacity changes in band pattern and pH profile occurred. No bands and no pH gradient were observed in gels incubated anaerobically. Addition of an alkaline overlayer, however, resulted in a pH gradient and band formation. Although pH gradients were responsible for sub-surface bands, it was not clear what caused the ‘chopping’ of growth into discrete bands. Computer simulations suggested that an asymmetric activation threshold may be responsible, and the mechanism may involve sporulation and germination.


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