Morphological changes in growing colonies of Aerobacter (Klebsiella) aerogenes.

Abstract

Detailed observations are recorded of the morphological changes occurring in colonies grown on agar plates containing only one colony. This allows of protracted growth in which, in normal circumstances, the initially basically circular colony becomes increasingly irregular until eventually a distinctly floral pattern results. Superimposed on this are variations in the thickness of the colony. Changing the composition of the gel or using drug-resistant strains led to changes in morphology to varying degrees but all the forms observed are explicable in terms of diffusion-limited conditions. Once these prevail the continued expansion of any part of the growing front of cells will depend on its position relative to concentration gradients of nutrient diffusing towards the colony and toxic metabolites away from it. Various lines of evidence support this conclusion. In particular, in an apparatus in which a colony was grown on the surface of a cellulose acetate filter in contact with liquid medium, stirring the medium delayed morphological changes. Addingcharcoal to agar gels to adsorb toxic metabolites had a similar effect. Prolonged growth on charcoal-agar, however, led to a morphology bearing a remarkable resemblance to a dendritic snow crystal. A mathematical function for assessing development in terms of peripheral irregularity is also given.