ORDER I. PSEUDOMONADALES 37 



considered as such. The problem was aggravated when Nathansohn, Beijerinck and Jacob- 

 sen published their studies on small, colorless, Psendomonas-like bacteria capable of oxidiz- 

 ing 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 Psendomonndales (see p. 35). Never- 

 theless, these organisms are physiologically in no way different from the Beggiatoaceae, so 

 that if phj'siology only is considered, a good case could be made out for their incorporation 

 in the Thiohacterinles. 



Furthermore, Molisch (Die Purpurbakterien, Jena, 1907, 95 pp.) described in some de- 

 tail a number of bacterial species which, in view of their characteristic pigment system, 

 appeared closely related to the Rhodobacieriaceae , but which develop only in organic media 

 and are, therefore, not sulfur bacteria in the sense of Winogradsk\' 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 Athioihodaceae, is included an organism which, 

 on the basis of its morphology and manner of growth, does not conform to the criteria of 

 the order Pseudornonadalcs . 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 metabo- 

 lisii 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 Htjphomicrobium 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 pernianentlj- immotile. 



Only a very small number of t^'pical 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 ac- 

 cepted differences in cell size or in colonial appearance as a sufficient justification for es- 

 tablishing independent species. Evidently this procedure presupposes a considerable degree 

 of constancy of such characteristics in the organisms in question. It is true that Wino- 

 gradskj^'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 environ- 

 mental conditions, such as composition of the medium and temperature, may exert a pro- 

 found 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 aban- 

 doned, but it does follow that a cautious evaluation of the distinguishing features is neces- 

 sary. 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 ascer- 

 tainable. Experience with this group over the past twenty-five years has shown that, while 

 Winogradskj^'s fundamental work must remain the foundation of present taxonomic ef- 

 forts, it is advisable to simplifj' the much more elaborate classification developed by Bu- 

 chanan 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 toj-etain most of them, even though the 



