The new model proposed to account for the energy requirement for growth includes both a constant maintenance energy term (m) independent of the specific growth rate and a term (m') which decreases linearly with increase in specific growth rate and becomes zero at the maximum specific growth rate. The available data for testing the model do not deviate significantly from the relations predicted. Consistent values of the maximum growth yield (YG) can be derived, irrespective of whether the cultures are energy limited or energy sufficient. Attention is drawn to the possibility that the constant maintenance energy term may be estimated from the maximum specific growth rate.

The variation, with growth rate, of the yield of organism from the substrate used as energy source is attributed to consumption of energy at a constant rate for cell maintenance. From the laws of growth, a simple relation between the maintenance requirement, the growth yield and the growth rate is derived. The relation is shown to be in good agreement with the available data. A distinction is made between ‘observed’ yield and ‘true’ yield of organisms. Values for maintenance energies and ‘true growth yields’ have been calculated from the data.

SUMMARYA model for the growth of microbial colonies on the surface of a solid nutrient medium is discussed. The model accounts for the constant rate of increase in the colony radius which is characteristic of a fungal colony growing on the surface of a nutrient medium.Experiments showed that bacterial colonies after about 12 hr of development showed a virtually constant rate of radial growth over a 12 hr period. Over longer periods (24 hr) a gradual decline in the colony radial growth rate was apparent. The initial rate of radial growth of the bacterial colony was a useful parameter of the growth rate of the organism. The effects on the initial colony radial growth rate of the following factors were determined: initial nutrient concentration depth of agar layer; maximum specific growth rate (In 2/minimum doubling time); oxygen partial pressure; humidity of gas phase; temperature. Three bacterial types, Escherichia coli, Klebsiella aerogenes and Streptococcus faecalis were studied. With E. coli growing in minimal medium in air at 1 atm. pressure when the growth was glucoselimited, oxygen became a limiting factor when the glucose concentration exceeded 0.25 % (w/v). With a glucose concentration of 1 % (w/v), the growth was strongly inhibited, probably by toxic products.When the colony growth was glucose-limited and oxygen was present in excess, the relation between initial colony radial growth rate (K,.), the initial glucose concentration (so) and the maximum specific growth rate (arn) was where k2 is a constant; st, called the 'lag concentration', is a value of the glucose concentration which must be exceeded before growth of the colony can occur. The value of si was very small or negligible except with a certain type of inhibitory condition, such as an over-optimal concentration of oxygen, which could be overcome by the organism's metabolic activity. Direct proportionality between K,. and d a m was found by varying the maximum specific growth rate by adding sulphanilamide. When a, was varied by temperature changes the linear relation between K,. and l/a, did not hold. The implications of these results and their potential applications are discussed.

The rates of utilization of energy-yielding substrates (glucose and oxygen) by Penicillium chrysogenum in glucose-limited chemostat cultures were resolved into requirements or 'rations' for growth and maintenance. The maintenance ration of glucose was almost all oxidized to carbon dioxide. Over the growth rate range 0.023-0.075 hr-l only vegetative growth occurred; although the filamentous growth form predominated, the occurrence of pellets and swollen organisms increased with growth rate. At growth rates of 0.014 hr-l and below, conidiation occurred and was maximal at a specific growth rate of 0.009 hr-l (average doubling time 78 hr). After growth in chemostat culture the organism could be maintained in a non-growing state by supplying only the maintenance ration of glucose (0.022 g. glucose/g. mycelial dry wt/hr). When growth in the chemostat was suddenly stopped by stopping the glucose feed, the mould autolyzed; autolysis was prevented by supplying the maintenance ration of glucose. When the glucose feed rate in chemostat cultures was decreased to the maintenance ration, mycelial differentiation occurred. Differentiation involved increased hyphal vacuolation, a decreased degree of oxidation of glucose, breakdown and resynthesis of nucleic acids and conidiation. The rates at which these changes occurred were inversely related to the growth rate prior to stopping growth. For maximum conidia formation there was an optimal glucose feed rate about 0.038 g. glucose/g. mycelial dry wt/hr, that is 1-7 x maintenance ration. The maintenance ration of glucose was shown to play a role in preventing autolysis and facilitating conidiation. Electron microscope studies showed that there was no change in the observed ultrastructure of cells (except degree of vacuolation) with change in specific growth rate from near the maximum to zero. The septa of the hyphae were found to be plugged.