IT is commonly believed that the formation of concentric zones of growth around a point of inoculation by swarming strains of is due to a rhythmic alternation of elongated, motile swarming bacteria with shorter, non-swarming ones at the leading edges of the swarm colony. The accepted physiological explanation is that swarming is a device to escape from concentrations of toxic metabolites produced by the growth of the sessile stage, and that the former reverts to the latter when that stimulus is removed (Hughes, 1957; see Grabow, 1972).

A recent study, by phase-contrast microscopy, of the motion of the swarm (Bisset, 1973) led me to the conclusion that the main agent of advance is growth rather than motility, because the movement of the swarmers is either random, or in the vicinity of the edge of the swarm frequently at right-angles to the direction of advance.

These observations gave no assistance in explaining the zonation phenomenon, because the anticipated alternation of growth phases was not, in fact, observed. Zonation did occur, but the margin was constantly in the swarming phase. The present work was undertaken for the purpose of clarifying this apparent anomaly.


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