HETEROTROPHIC BACTERIA REQUIRING COMBINED NITROGEN 151 



groups of soil organisms, such as the Thiobacillus group of sulfur bac- 

 teria, the nitrite, nitrate forming and other autotrophic bacteria, 

 as well as various heterotrophic bacteria, such as many of the aerobic 

 cellulose decomposing bacteria, certain urea bacteria, etc., belong to this 

 group of organisms. 



Except for the physiological groups just mentioned, the study of the 

 heterotrophic, non-spore-forming bacteria, which do or do not develop 

 on the common synthetic and nutrient media, has been neglected. This 

 has been largely a result of the lack of proper methods of study. 

 Organisms have been looked for which take part in the various known 

 processes, largely in the nitrogen transformation in the soil. If an 

 organism did not take part in the processes of nitrification or sulfur- 

 oxidation, nitrogen-fixation or cellulose decomposition, and if it did 

 not produce ammonia rapidly from proteins, it was assumed to be un- 

 important in the soil. 



We must assume from the meager information available that the 

 non-spore-forming bacteria take part in the slow but constant decom- 

 position of the soil organic matter. Winogradsky considers them as the 

 native (autochtonous) population of the soil. When fresh organic matter 

 is added to the soil, the fungi and the spore-forming bacteria at once 

 become active, until the organic matter is reduced to a certain consist- 

 ency (so-called "humus"). These organisms then become inactive 

 again until a fresh supply of energy is introduced into the soil. 

 The "humus" left is constantly acted upon by the non-spore-forming 

 bacteria (and actinomyces), which mineralize it, liberating the nitro- 

 gen and other mineral elements into available forms. 



Classification. The heterotrophic, non-spore-forming aerobic bac- 

 teria usually produce punctiform colonies on agar and gelatin, are 

 chromogenic or non-chromogenic, motile or non-motile, some liquefy 

 gelatin rapidly, while others only slowly or not at all. The non-spore- 

 forming bacteria, which form the most abundant group of soil organisms, 

 as shown both by microscopic and cultural methods, can be divided into 

 (1) the forms that liquefy gelatin rapidly and (2) the forms that liquefy 

 gelatin slowly. 



The most important representatives of the group of rapid liquefying 

 organisms is the Bad. fluorescens or Pseud, fluorescens. The whole 

 group is often spoken of as the fluorescens group, although many of the 

 organisms never produce any fluorescens. 



The representatives of the second group comprising the bacteria 

 which form pin-point colonies on the agar and gelatin plate and which 



