SURVEY OF PHOTOS YNTHE TIC BACTERIA 463 



to form irregular, elongated cells (rods and filaments). Occasionally 

 they appear as tightly wound coils, but the conditions under which 

 these are formed are not clearly defined. 



All CJilorobium strains thus far tested are strict anaerobes, and 

 appear to be obligate sulfur bacteria. 



The two hitherto recognized species, Chi. limicola and Chi. 

 thiosidfatophilum , can be most readily distinguished by their differ- 

 ential behavior towards thiosulfate, which the former does not oxidize 

 to sulfate, while the latter can do so (10). 



Pfennig (personal communication) has recently isolated strains of 

 nonmotile green bacteria that are uniformly rod- shaped. Except for the 

 absence of flagella, they morphologically resemble Chloropseudo- 

 monas. Physiologically they also differ from the latter by their in- 

 ability to oxidize organic substrates. They have not yet been studied in 

 sufficient detail to determine whether it would be advisable to estab- 

 lish a separate taxon for these organisms, 



Chloropseiidomonas . 



The cells of representatives of this genus are small, straight rods, 

 nonsporeforming. Gram-negative, and motile by means of polar flagella. 



They are anaerobes that can grow autotrophically in media con- 

 taining H2S, or heterotrophically at the expense of ethanol, acetate, 

 pyruvate, sugars, and some amino acids. This feature, together with 

 the cell shape and motility, distinguishes these green bacteria from 

 Chlorobmm species. 



Only a single species, Cps. ethylicum, has so far been described 

 (8,9). 



The purple sulfur bacteria: Thiorhodaceae. 



Like the green bacteria, the purple sulfur bacteria found in natural 

 habitats represent an assortment of morphological types, which differ 

 in the shape, size, color, and aggregation patterns of the individual 

 cells. But they are generally much larger than the green bacteria, and 

 can therefore be differentiated more satisfactorily on the basis of 

 morphology alone. 



Winogradsky (11) proposed a subdivision of the group into thirteen 

 genera, under the names Chromatium, Thiocapsa, Thiocystis, Thio- 

 dictyon, Thiopedia, Thiopolycoccus, Thiosarcina, Thiothece, Amoebo- 

 bacter, La)nprocystis , Lampropedia, Thio spirillum, and Rhabdo monas . 

 The description of these genera was based entirely on observations of 

 material collected from natural sources and of slide cultures prepared 

 therefrom; pure cultures were not obtained. 



The subsequent introduction of simple, defined media for the culti- 

 vation of Thiorhodaceae in the laboratory permitted the isolation of a 

 number of pure cultures, representing morphologically distinguishable 

 types (12), But studies with such cultures in media of different com- 



