464 APPENDIX 



position revealed that the morphology of each individual isolate varied 

 considerably with the environmental conditions, so that an adequate 

 description and classification of the strains could not be accomplished 

 without taking this morphological variability into account. This was 

 further complicated by the fact that the cultures of any single strain 

 often suggested that they consisted of mixtures of cell types more or 

 less resembling several of Winogradsky's genera. The most serious 

 drawback was that none of the truly distinctive forms found in natural 

 habitats could be cultivated in the laboratory under controlled condi- 

 tions. This precluded an evaluation of Winogradsky's classification on 

 the basis of comparative studies with pure cultures. 



Fortunately, this situation has been drastically changed as a result 

 of the notable advances scored by the recent work of Schlegel and Pfen- 

 nig (13,14) and of Pfennig (15, and unpublished results communicated 

 to the author). It is now clear that the previously used media are unfit 

 for the cultivation of most species of purple sulfur bacteria and permit 

 only a few of the small representatives to grow; these are obviously 

 the types that were obtained in the earlier work. By successive modi- 

 fications of the culture solution, involving the addition of sewage sludge, 

 subsequently replaced by ascorbic acid and vitamin B12, the use of low 

 H2S concentrations, repeated "feeding" and adjustment of the pH to 

 various levels, and by introducing special regimes of incubation in 

 light and darkness at specified temperatures, it has now been possible 

 to prepare elective cultures of a number of the large and distinctive 

 species of Thiorhodaceae and to isolate these in pure culture. The 

 morphological uniformity of such cultures is impressive and supports 

 the view that Winogradsky's concepts of speciation in this group are 

 likely to be upheld to a large extent by future comparative studies. It 

 is hoped that this important program will be diligently pursued and 

 adequately supported; it may be confidently expected that the availabil- 

 ity of the many kinds of purple bacteria that exist in nature will be 

 of great benefit to those who are engaged in studies of the photosyn- 

 thetic bacteria. 



A brief description of the two best known genera follows. 



Chro matin m. 



The only pure cultures of purple suKur bacteria that have so far 

 been used extensively are small species of Chromatium. This genus 

 can be defined as composed of Thiorhodaceae whose cells are ovoid 

 to vibrioid in shape, and motile by means of a single polar flagellum. 

 When grown in media containing H2S, the cells are stuffed with sulfur 

 globules; these disappear after the sulfide has been used up. 



It is probable that the currently used cultures, including Roelofsen's 

 "strain D," should be assigned to the species group that includes Chr. 

 minus, Chr. gracilis, and Chr. miiiNtissimnm. These species have 

 been established on the basis of differences in the size of the cells as 



