Factors affecting the growth of bacterial colonies on agar plates

Abstract A study has been made of the physical chemistry of the development of colonies on the surface of a minimal agar medium by a standard strain of Aerobacter (Klebsiella) aerogenes and by drug-resistant strains derived from it. Histograms of final colony diameter, constructed from measurements on plates having different numbers of colonies, have one maximum and are approximately symmetrical. Normal Gauss curves have been verified to fit some of the histograms, most of which are of similar form, although occasionally a somewhat wide spread on the side of increasing diameter occurred. When moderate or large numbers of colonies are present the total colony area supportable by the plate is proportional to the total capacity of the plate. The distribution of sizes, however, is shown to depend partly on the variation in the lags of the single cells inoculated and upon the free space left around the different individuals on the plates. A complex interplay of individual with cooperative or competitive factors thus exists. A detailed study has been made of the growth of single colonies. The initial exponential growth of the single cell is soon replaced by a phase in which the radius of the developing colony is linearly related to the time of incubation. In this phase diffusion of nutrients, buffer or waste products is not rate determining and the colonies remain more or less disk-shaped with a fairly regular perimeter. With moderate or large numbers of colonies exhaustion of the plate stops growth at this stage, but in the protracted growth which occurs when a few colonies only are present the ‘radial law’ phase gives way eventually to a slower phase in which the area of the colony increases linearly with time. Diffusion, chiefly affecting pH rather than nutrient supply, now becomes more and more the dominant factor controlling growth and the colony perimeter becomes increasingly irregular. Morphological changes may now appear. This is dealt with briefly as a prelude to a further paper. Kinetic models have been constructed for various aspects of the research and have been tested against the results.