A Membranous Component of the Cytoplasm in Streptomyces coelicolor

Most studies of bacterial fine structure in thin sections have shown the cytoplasm to consist mainly of minute granules (1). Occasionally, small regions with a membranous structure have been observed (2, 3, 5, 10-12). In the present work, improved fixation of the actinomycete Streptomyces coelicolor by the method of Kellenberger, Ryter, and S6chaud (5), has revealed an unusually complex organisation of the fine structure of a bacterium. The most remarkable feature is an extensive membranous component in the cytoplasm. Much of the cytoplasm of the hyphae of Strep-tomyces coelicolor is occupied by membranous material (Fig. 1), which is often continuous with the plasma membrane (Figs. 2 and 3). The individual membranes are "double," consisting of two dense layers, each 2 to 3 m# thick, separated by a lighter region, the whole complex having a thickness of about 9 m/z. In our preparations the plasma membrane also appears to be double (Fig. 3), as described by Tokuyasu and Yamada (12) in Bacillus ~ubtilis, and in contrast to the single structure observed by Kellenberger and Ryter (4) in Escher-ichia coli, and it has the same dimensions as the membranes in the cytoplasm. In some regions the membranes in the cytoplasm appear to have a random orientation with respect to each other (Fig. 1, M), while in others they are associated in an ordered arrangement to form discrete bodies with oval or circular outlines. In some of these bodies the membranes are closely packed (Fig. 4), while in others they are separated by larger spaces containing-cytoplasmic granules (Fig. 5). Thus the membranes have some features in common with those of the endoplasmic reticulum of mam-malian cells (9). Some of the membranous bodies are similar to the cytoplasmic inclusions observed MaalCe (10) in Bacillus subtilis and by Kellen-berger, Ryter, and S6chaud (5) in Escherichia coli, which these authors term "chondrioids"; they also bear some resemblance to the "mitochondria" observed in Mycobacterium tuberculosis by Shinohara, Fukushi, Suzuki, and Sato (11). Sometimes the membranes of the discrete bodies are seen to be continuous with those in the less ordered regions (Fig. 5), and in other areas the arrangement of the membranes shows varying degrees of regularity (Fig. 1). It seems, therefore, that we are dealing with a single system of membranes, and many of the different appearances observed may, in fact, be due to cutting the membranous system in different planes. Membranous bodies are often found …