38 ORDER I. PSEUDOMONADALES 



distinguishing characteristics are not always very clear. For the benefit of those who are 

 familiar with previous methods of classification, it will be indicated where deviations have 

 been adopted. 



The non-sulfur purple bacteria {Athiorhodacene Molisch; Rhodobacterioideae Buchanan) 

 have been subjected to a comparative morphological and physiological study comprising 

 more than 150 strains, among which all previously proposed genera and species are repre- 

 sented (van Niel, Bact. Rev., 8, 1944, 1-118). It has been found that the characteristics 

 upon which Molisch based the seven genera of this group are inadequate, and a new classi- 

 fication with only two distinguishable genera has been proposed. This system will be fol- 

 lowed here. 



Nadson (Bull. Jard. Imper. Bot., St. Petersburg, 13, 1912, 64) described a new type of 

 small, green bacteria not containing sulfur globules in the presence of hydrogen sulfide but 

 excreting elemental sulfur. They arephotosynthetic and are capable of growing in anaerobic 

 culture when illuminated. The green pigment differs from the green plant chlorophylls and 

 from the bacteriochlorophyll of the purple bacteria but has the characteristics of a chloro- 

 phyllous compound. These are grouped in the family Chlorobacteriaceae. 



FAMILY I. THIORHODACEAE MOLISCH, 1907. 

 (Die Purpurbakterien, Jena, 1907, 27.) 



Thi.o.rho.da'ce.ae. Gr. noun thium sulfur; Gr. noun rhodum the rose; -aceae ending to 

 denote a family; M.L. fem.pl.n. Thiorhodaceae (probably intended to mean) the family of 

 sulfur red bacteria. 



Unicellular organisms, often developing as cell aggregates or families of variable size 

 and shape. Single cells have the form of spheres, ovoids, short rods, vibrios, spirals, long 

 rods or, occasionally, chains. Thej^ occur in nature in environments containing sulfides and 

 require light for their development; infra-red irradiation of a wave-length extending to 

 about 900 millimicrons is effective. They produce a pigment system composed of green bac- 

 teriochlorophyll and yellow and red carotenoids. As a result they appear as bluish violet, 

 pale purple, brownish to deep red cell masses. Single cells, unless they are of considerable 

 size, usually appear to be unpigmented. These are anaerobic or microaerophilic organisms 

 with a photosynthetic metabolism in which carbon dioxide is reduced with the aid of spe- 

 cial hydrogen donors without the liberation of molecular oxygen. Where these organisms 

 are found in nature, hydrogen sulfide acts as a hydrogen donor, and sulfur, the first inter- 

 mediate oxidation product, accumulates as sulfur droplets in the cells. Probably all mem- 

 bers of the group can utilize a number of organic substances in place of hydrogen sulfide as 

 hydrogen donors for photosynthesis. Thus they are potentially mixotrophic. 



Characterization of the genera in this group has, since Winogradsky's studies (Beitrage 

 zur Morphologie und Physiologie der Schwefelbacterien, Leipzig, 1888), been based upon 

 the mode of development of the cell aggregates. Pure-culture studies (Bavendamm, Die 

 farblosen und roten Bakterien, I. Schwefelbakterien, Pflanzenforschung, Heft 2, 1924, 74 

 pp.; van Niel, Arch. f. MikrobioL, 3, 1931, 1-112; Manten, Antonie van Leeuwenhoek, 8, 

 1942, 164 pp.) have shown, however, that not only the sequence of events in the formation 

 of the aggregates but also the appearance and form of the latter, even including the size 

 and shape of the component cells, are influenced to a considerable extent by environmental 

 conditions. This obviously casts doubt upon the usefulness of the previously used diag- 

 nostic criteria for genera and species. On the other hand, the scope of pure-culture studies 

 has not yet attained sufficient breadth to warrant the use of a different approach. As a 

 provisional measure, Winogradsky's genera are therefore maintained. Even the larger 

 taxonomic units must be regarded as being of tentative value only. 



