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The Chlamydobacteriaceae are a family of filamentous bacteria varying extremely with environmental conditions. The tacit assumption that species can be distinguished by mere microscopic inspection has led to the creation of numerous genera, species, and varieties of doubtful validity. In order to detect reversible modifications and hereditary differences cultures were undertaken. Threads ofSphaerotilus natans, Cladothrix dichotoma andLeptothrix ochracea were washed in sterile soil extract and transferred to agar plates with a low concentration of meat extract. By repeated plating several strains of each form were obtained in pure culture. All of these were identical, despite their origin from the various ‘species’ mentioned. By modifying the cultural conditions each strain could be made to change into the three original forms, as well as into certain others generally believed to belong to distinct genera. SinceSphaerotilus natans Kütz. is the oldest name it must be retained for all these forms. Schwers’sMegalothrix discophora , which was later referred to other genera and has been grown in culture by various authors, was not obtained from the cultures ofSphaerotilus natans . It represents a second related species of Sphaerotilus which may be namedS. discophorus . Further species ofSphaerotilus have been described under the name ofLeptothrix , but of the remaining socalled genera of the Chlamydobacteriaceae Crenothrix alone seems to be well founded. Conclusions are also reached on the colour of iron-containing deposits. If these appear brown, it is due to admixture of manganese oxides. Otherwise the colour is only slightly yellowish under the microscope and orange-ochre in larger accumulations. The envelopes produced by various organisms may contain much iron without its presence being revealed by their colour, although it can be recognized by the very refractive and brittle character of the envelopes and by micro-chemical reactions.

The filamentous bacteria Sphaerotilus, Leptothrix, Cladothrix , and their relation to iron and manganese