Dependence of the mycelial growth pattern on the individually regulated cell cycle in Streptomyces granaticolor

The growth behaviour of Streptomyces granaticolor ETH 7437 was studied by the microculture technique. The kinetics of growth and branching were recorded and, since elongation was found to be restricted to apical elongation sites (e-sites), the rate of elongation per site (alpha) was determined as well. The mycelia grew exponentially. Initially the growth was dependent on alpha of the germ tube, but after the start of branching, growth paralleled the exponential increase of the number of branches while alpha attained a constant average value. Further, for liquid grown mycelia showing about the same growth kinetics the cellular structure was determined after cell wall staining. Three types of cells could be distinguished: apical branchless cells (20%), non-apical branchless cells (20%) and non-apical cells with one branch each (60%). Since both the apical and the branched cells possessed an e-site, 80% of the cells must have been growing at the time of sampling. Combining detailed data obtained from both the alive and the stained mycelia a model was elaborated, which may reflect the events taking place on the cellular level during mycelial growth. The model is based on the assumption that each cell behaves as an independent unity with respect to its cell cycle. But, in contrast to the behaviour of single cell bacteria, in mycelia the two daughter cells formed upon division are neither equivalent nor uniform. Here, the sister cells differ in length, shape and possession of an e-site. Only one of the daughter cells receives the e-site of the mother cell, while the other starts its own cell cycle by generating a new e-site at the cylindrical part of its envelope. Regarding the length of sister cells the degree of heterogeneity increases with the age of the corresponding region of the mycelium, and eventually some cells lose the ability to generate an e-site, i. e. to grow. With this model the kinetic and structural peculiarities of the mycelial growth of Streptomyces granaticolor can be explained.