A mechanistic model was developed, which described the growth of a bacterial colony on an agar medium. Diffusion of substrate (nutrients/oxygen) through the colony are considered. Rate of substrate use by the organisms is assumed to follow Michaelis-Menton kinetics for substrate concentrations between prescribed limits. Above and below the prescribed concentration limits, the substrate use rate is assumed constant and zero, respectively. Supply of substrate to the colony was assumed to be non-limiting. Under these conditions, the model predicted that diffusion of substrate through the colony will eventually control colony growth. It also described a slower eponential growth rate of the colony when the organisms utilized an alternate substrate for one that became deficient throughout a portion of the colony, and a constant linear growth rate when an alternate substrate was not utilized. Consistent with published literature, a mathematical description of substrate supply through the agar indicated that, under normal conditions, glucose supply through the agar to the colony would not be expected to limit colony growth before oxygen diffusion through the colony limited growth.