Electron microscopy of Streptomyces spore morphology and its role in species differentiation.

The widespread search for new antibioticproducing Streptomyces unleashed during the last two decades also brought with it the problems of their classification. In the early attempts at species differentiation excessive use was made of highly variable cultural and physiological properties as taxonomic criteria and too little attention was paid the more stable morphological characteristics. However, in some of the more recently derived Streptomyces classification systems there has been increasing evidence that certain morphological properties are finally and properly being accorded major status as criteria for species differentiation. Pridham, Hesseltine, and Benedict (1958) assigned prime importance to the morphology of Streptomyces sporophores and grouped species into primary morphological sections on this basis. In the system of Ettlinger, Corbaz, and Hultter (1958) the type of sporophore (spore chain) was also one of the characteristics given major emphasis but the morphology of the spores themselves, as observed by electron microscopy, was given equal weight. In recent years several other investigators (Flaig et al., 1952; Kiister, 1953; Flaig and Kutzner, 1954; Flaig, Kiister, and Beutelspacher, 1955; Baldacci and Grein, 1955; Baldacci, Gilardi, and Amici, 1956; Kutzner, 1956; and Preobrajenskaya et al., 1959) using the electron microscope have studied the details of Streptomyces spore structure and have shown that an apparently distinctive spore surface configuration exists in many of the species. Provided with this encouraging prospect, it seemed highly desirable to expand these studies to include mass collections of strains of individual Streptomyces species, for it is only by demonstration of constancy from strain to strain within a species that the true value of any criterion for species differentiation can be firmly established. Having available substantial numbers of strains of many species of this genus, which in the past were used to demonstrate that H2S production merits major status in Streptomyces systematics (Tresner and Danga, 1958), it was logical to make use of this assemblage of organisms again in the further evaluation of spore ornamentation as a criterion.