Growth characteristics of fungi and actinomycetes.

For studies of the development of saprophytic and pathogenic fungi in liquid culture and in sterile soils, a method for comparing growth rates under varying conditions is lacking. Growth rates of microorganisms which divide by binary fission are readily determined, since the increase in cell numbers is exponential. Thus, a plot of the logarithm of bacterial numbers or activity as a function of time is linear, the slope of the line being a measure of the growth rate. Such estimates can be made by measuring the increase in population, disappearance of substrate, formation of metabolic products, turbidimetrically, or by manometric methods. In the fungi and actinomycetes, the type of vegetative development is of an entirely different character. Proliferation of such filamentous microorganisms is frequently estimated by radial development on an agar surface (Ryan, Beadle, and Tatum, 1943) or by the linear extension of individual hyphae (Smith, 1924; Stadler, 1952). But these methods often do not give a valid comparison of growth rates of different fungi or of the same fungus under varying nutrient conditions because they fail to account for differences in the density of the mycelial mats. The weight of mycelium produced in liquid culture gives a better comparative estimate of proliferation. Since hyphal extension in liquid medium is essentially unrestricted, Emerson (1950) has suggested that the growth of Neurospora crassa in such a medium is three dimensional, and he has obtained a linear relationship between the cube root of mycelium weight and time. The present investigation was undertaken to test the validity of Emerson's findings and to use the data obtained to determine the growth characteristics of a number of filamentous microorganisms. To make the estimation more sensi-