Fragmentation in Nocardia corallina

SUMMARY: The growth cycle of Nocardia corallina involves a period of coenocytic hyphal development terminated by fragmentation. Hyphal elongation occurred by insertion of new material at the hyphal tips, not by intercalary growth. Exceptions occurred at points of branch initiation. Development of cross-walls preceded fragmentation and the new tips so produced were also capable of elongation. The fragmentation process appeared to be initiated by agents which accumulated in the medium, since used broth from actively fragmenting cultures stimulated earlier fragmentation in assay inocula. This effect was not shown by media from cultures in the coenocytic hyphal phase, nor by old resting cultures. The population density also affected fragmentation in a manner which suggested that accumulation of threshold concentrations of a diffusible metabolic product was required. A Millipore filter technique for assaying the effects of agents involved in fragmentation is described.