A Kinetic Study of the Mode of Growth of Surface Colonies of Bacteria and Fungi

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, Escherichia coli, Kleb-siella aerogenes and Streptococcus faecalis were studied. With E. coli 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 (K r), the initial glucose concentration (s o) and the maximum specific growth rate (αm) was K r = k 2(√s 0 − √S i) √αm where k 2 is a constant; s i, 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 s i 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 r and √αm was found by varying the maximum specific growth rate by adding sulphanilamide. When αm was varied by temperature changes the linear relation between K r and √αm did not hold. The implications of these results and their potential applications are discussed.