A 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, and were studied. With growing in minimal medium in air at 1 atm. pressure when the growth was glucose-limited, 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 ( ), the initial glucose concentration ( ) and the maximum specific growth rate (α) was = (√ − √ ) √α

where is a constant; , 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 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 and √α was found by varying the maximum specific growth rate by adding sulphanilamide. When α was varied by temperature changes the linear relation between and √α did not hold. The implications of these results and their potential applications are discussed.


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