104 



A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



than that concerned with mere species. Occasionally, however, a particular 

 property has been encountered whicli is qualitatively so striking that it would 

 appear suitable as the special distinguishing character of a family or order. This 

 happened, for example, when Migula (1897) introduced the order Thiobacteria 

 for those microbes which Winogradsky had called "sulfur bacteria." It was the 

 first time that the bacteria were divided into two separate orders; and Migula 

 justified the procedure by emphasizing that both the cellular organization and 

 the physiology of the sulfur bacteria were clearly distinct from those of the 

 "true" bacteria, or Eubacteria.^ Morphologically the former are conspicuous on 

 account of their relatively large size and their content of sulfur globules ; physio- 

 logically they represent the prototype of the autotrophic bacteria ; they can grow 

 in strictly mineral media, and are dependent on an external supply of sulfide 

 which is oxidized to sulfate. 



It was also the first time that a physiological property was used for the 

 establishment of a large systematic group of the bacteria. Coupled as it was 

 in this case with some morphological peculiarities, this may have appeared de- 

 fensible. But later developments have shown how much confusion was created 

 by this ostensibly simple expedient. 



Elsewhere I have sketched these developments in some detail (van Niel, 

 1944) ; suffice it here to recapitulate the major aspects. The Thiobacteria, in 

 1900, comprised two subgroups, viz., the colorless, filamentous organisms which, 

 except for lack of pigmentation, closely resemble the bluegreen algae of the 

 family Oscillatoriaceae (see, e.g., Pringsheim, 1949), and the red-colored, so- 

 called purple sulfur bacteria which are much more "bacteria-like," though gen- 

 erally much larger. Within a decade, however, two more groups of organisms 

 were discovered with characteristics that made their incorporation into one or 

 the other of Migula 's orders largely a matter of personal preference. Tliese 

 were the small, colorless Thiohacillus species, physiologically typical sulfur bac- 

 teria, but morphologically in no way distinguishable from many eubacterial 

 types, and the small purple bacteria that are physiologically not sulfur bacteria, 

 though their pigment system, composed of chlorophyllous and carotenoid com- 

 ponents, closely resembles that of the purple sulfur bacteria. 



The properties of these four groups obviously show "interrelationships" 

 which can best be presented in the form of a diagram, as follows : 



Thiobacillus species 



(similar ptiysiology) 



(morpliologically "true bacteria") 



Colorless, filamentous 

 sulfur bacteria 



Nonsulfur purple bacteria 



(intracellular 



sulfur globules) 



(similar pigment 

 systems) 



Sulfur purple bacteria 



2. In an earlier publication (van Niel, 1944) I erroneously stated: "One looks in 

 vain, however, for an exposition of the reasons which had induced Migula to create the 

 new orders" (p. 71). A vague attempt at rationalizing this measure can be found in 

 the brief section on the sulfur bacteria at the end of Vol. 1 of Migula's System. 



