considered as such. The problem was aggravated when Nathansohn, Beijerinck and Jacobsen published their studies on small, colorless, Pseudomonas-like bacteria capable of oxidizing hydrogen sulfide, sulfur and thiosulfate, and evidently dependent upon this oxidation process for their development. Morphologically these organisms have little in common with the Beggiatoaceae; they were designated by Beijerinck as species of Thiobacillus and have since been rightly considered as members of the order Pseudomonndales (see p. 35). Nevertheless, these organisms are physiologically in no way different from the Beggiatoaceae, so that if physiology only is considered, a good case could be made out for their incorporation in the Thiobacteriales.
Furthermore, Molisch (Die Purpurbakterien, Jena, 1907, 95 pp.) described in some detail a number of bacterial species which, in view of their characteristic pigment system, appeared closely related to the Rhodobacteriaceae , but which develop only in organic media and are, therefore, not sulfur bacteria in the sense of Winogradsky or Migula. In stressing the importance of pigmentation, Molisch combined the red sulfur bacteria and the newly-discovered purple bacteria into an order Rhodobacteria with the two families Thiorhodaceae and Athiorhodaceae. It is this grouping that has been followed in the present edition of the Manual.
Among the non-sulfur purple bacteria, or Athiorhodaceae, is included an organism which, on the basis of its morphology and manner of growth, does not conform to the criteria of the order Pseudormonadales . This is Rhodomicrobium vannielii Duchow and Douglas (Jour. Bact., 58, 1949, 409). Physiologically it is a typical non-sulfur purple bacterium in that it is capable of development in strictly anaerobic media supplied with an appropriate oxidizable substrate only when the cultures are illuminated and carries out a photosynthetic metabolism without oxygen evolution. Multiplication is not, however, by transverse fission but by bud formation at the end of a thin filament growing out of a pole of the mother cell followed by the formation of a cross wall in the connecting filament. This mode of development is similar to that encountered in the non-photosynthetic bacterium Hyphomicrobium vulgare.
It should also be emphasized here that some of the sulfur purple bacteria (Thiopedia, for example) and all of the green sulfur bacteria appear at present to be permanently immotile.
Only a very small number of typical sulfur bacteria have been studied in pure cultures. As a result the descriptions of genera and species rest mainly on observations made with collections from natural sources or crude cultures. Most investigators have implicitly accepted differences in cell size or in colonial appearance as a sufficient justification for establishing independent species. Evidently this procedure presupposes a considerable degree of constancy of such characteristics in the organisms in question. It is true that Winogradsky's investigations have provided a reasonable basis for this belief, but later studies with pure cultures of certain purple bacteria have established beyond a doubt that environmental conditions, such as composition of the medium and temperature, may exert a profound influence on the general morphology of these organisms. By this it is not intended to infer that the previously proposed genera and species of sulfur bacteria should be abandoned, but it does follow that a cautious evaluation of the distinguishing features is necessary. In the absence of carefully conducted investigations on morphological constancy and variability of most of the previously recognized species of sulfur bacteria with pure cultures grown under a variety of external conditions, the best approach appears to be a tentative arrangement of these organisms based upon those characteristics which are readily ascertainable. Experience with this group over the past twenty-five years has shown that, while Winogradsky's fundamental work must remain the foundation of present taxonomic efforts, it is advisable to simplify the much more elaborate classification developed by Buchanan which was followed in previous editions of this Manual.
Certain genera of sulfur purple bacteria, created by Winogradsky, will very probably be consolidated when detailed information concerning the morphology of the organisms is available. Until such time it seems, however, best to retain most of them, even though the
