Kinetics of vegetative growth of Thermoactinomyces vulgaris

The growth of Thermoactinomyces vulgaris on solid media was studied by microculture technique. Starting from a germinating spore the further development was followed by phase contrast microscopy. From serially taken photographs the kinetics of growth and branching of the mycelium was calculated. Also some observations on morphological behaviour were made. Regarding the total mycelium developing from one spore the growth occurred witha constant exponential growth rate, at least for the time tested (about ten doublings). The constancy of the mycelial growth rate was not attributed to constant rates of hyphal growth or branching. The hyphal growth rate increased during the development of the mycelium, so that the length produced by a hypha within a given time was smallest after germination and largest at the end of the experiment. Between the first and the 10th doubling of the mycelium the mean hyphal growth rate increased by the factor 27, in some cultures even by the factor 40. While hyphal growth changed from exponential to linear, branching was first linear and then approached exponential kinetics. Under different conditions of cultivation mycelia could have identical doubling times, but different hyphal growth rates and vice versa. Yeast extract caused a reduction of the hyphal growth rate and an enhancement of branching. Likewise the diminution of the supply of oxygen did not reduce the mycelial growth rate, but markedly decreased the hyphal growth rate, especially of the germ tube. The data obtained concerning the kinetics of growth and branching can be explained by a special form of cooperation of cytoplasmic and cell wall synthesis. With respect to the synthesis of cytoplasma a T. vulgaris mycelium behaves as one individual, which is not differentiated into replicating and non replicating areas (as recently was shown to be likely for Streptomyces hygroscopicus by Schuhmann and Bergter 1976). The mycelial growth rate thus reflects the rate of the synthesis of cytoplasm. The hyphal growth rate is moreover determined by the rate of cell wall synthesis at the tip of each hypha. The observed increase of the hyphal growth rate is explainable by an acceleration of the cell wall synthesis during the development of the mycelium. Probably the branching is regulated indirectly. According to the different manner of cytoplasmic and cell wall synthesis, within a hypha a balance between both processes is maintainable only for a limited time. As soon as the momentary apical capacity for cell wall synthesis is exhausted, the balance is attained anew by originating a new site for wall synthesis, i.e. by branching. Branching was often defective. Especially in the absence of yeast extract a lot of lateral branches arose, which ceased further growth at an average length of 2.5 micron. It is suggested that yeast extract supports the transport of the nucleoids into the lateral branches, which otherwise is often infeasible. Reduction of the supply of oxygen favoured apical branching, so that dichotomous forms resulted.