The extensive literature on the iron bacteria and the present status of our knowledge of this interesting group of microorganisms recently have been reviewed critically and exhaustively by Pringsheim (1949a). It is clear from this review that every aspect of the iron bacteriaecology, isolation, culture, morphology, nutrition, biochemistry, and taxonomy-requires extensive investigation if there is to be a sound knowledge of these microorganisms. The present investigations were begun as part of a systematic attempt to fill in some of the. existing gaps. The studies which are reported here are concerned with one of the commonest of the iron bacteria, Sphaerotilu natans. This orgaimsm was described and named first by Kttsing (1833) who noted its presence in large numbers in polluted waters. It was rediscovered later by Cohn (1875) under similar conditions, but because the filaments appeared to be dichotomously branched, although falsely so, Cohn thought that he had found a new organism and accordingly named it Cladothrix dichotoma. Both names have appeared in the subsequent literature and were considered by some investigators (Linde, 1913) to apply to one and the same organism and by others (Zikes, 1915) to organisms that are different. But Pringsheim (1949b) has shown that S. natans exhibits dichotomous false branching whenever it is grown in media of low organic matter content. It seems best, therefore, to consider C. dichotoma as being the cladothrix form of S. natans. In this paper, therefore, we shall use the name S. natans exclusively. Zopf (1882) claimed that S. natans is highly pleomorphic and described the occurrence of micrococci, vibrios, spirilla, and spirochetes along with the usual rod shaped cells. But this was based on observations of impure cultures and has not been supported by subsequent studies with pure strains. Biisgen (1894) was the

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STOKES. 1973. The fine structure of Sphaerotilus nafatrs.Can. J. Microbiol. 19: 309-313. Sphaerotilus natans developed sheathed filaments in stationary liquid cultures and motile swarm cells in shaken ones. Electron microscopy of negatively stained preparations and thin sections showed that the sheath consists of fibrils. When the filaments wcre grown in broth with glucose added, the sheath was much thicker and the cells were packed with granules of poly-!3-hydroxybutyrate.Swarm cells possess a subpolar tuft of 10 to 30 flagella and a polar organelle which is usually inserted in a lateral position and believed to be ribbon-shaped. The polar organelle consists of an inner layer joined by spokes to an accentuated plasma membrane. The flagellar hook terminates in a basal disk, consisting of two rings, which is connected by a central rod to a second basal disk. HOENIGER, J. F. M., H.-D. TAUSCHEL et J. L. STOKES. 1973. The fine structure of Splrnerotilus natans.Can. J. Microbiol. 19: 309-313. Sphaerotilus nafans developpe des filaments feuilletes dans des cultures liquides stationnaires, et des cellules "swarm" mobiles dans des cultures agitks. La microscopic Blectronique de preparations colorees negativement et de sections minces montre que ces feuillets consistent en des fibrils. Lorsque les filaments se developpent en bouillon additionne de glucose, le feuillet est beaucoup plus Bpais et les cellules sont remplies de granules de poly-!3-hydroxybutyrate.Les cellules "swarm" possedent une touffe sous-polaire de 10 A 30 flagelles et une organelle polaire qui est habituellement pla& en position laterale et est probablement en forme de ruban. L'organelle polaire consiste en une couche interne jointe par des pics A une membrane plasmique accentuee. Le crochet flagellaire se termine par un disque basal, consistant en deux anneaux, lesquels sont connectes par une tige centrale i un deuxieme disque basal.[Traduit par le journal]