HETEROTROPHIC BACTERIA REQUIRING COMBINED NITROGEN 153 



The term "Micrococcus" was applied 21 to designate the genus of 

 irregular mass-forming cocci. Sixteen species were separated on the 

 basis of gelatin liquefaction, nitrate reduction and ability to utilize 

 certain ammonium salts as the only source of nitrogen. 



The fact must also be mentioned that a few spore-forming organisms 

 are found to form punctiform colonies. They are not related to the 

 other spore-forming bacteria in their general physiology and stand also 

 apart from the "slow growers." Except the Bad. fluorescens and 

 Bad. caudatum, the other non-spore-forming bacteria cannot be readily 

 recognized by their colonies on gelatin or agar. The rapid liquefaction 

 of the gelatin by the former and the orange color of the latter allow a 

 ready recognition of these two types. Bad. aerogenes and Bad. coli 

 isolated from the soil can be readily distinguished from Bad. coli of 

 fecal origin by the fact that the former use citrates as a source of carbon 

 and the latter do not; 22 also by the production of a red iron rust growth 

 on Harder's medium. 23 



Occurrence of non-spore-forming bacteria in the soil. The non-spore- 

 forming bacteria are numerically the largest group of soil microorganisms. 

 It still remains to be demonstrated, however, what bulk they occupy in 

 the soil population and what relative importance may be ascribed to 

 them in transformations in the soil. Seventy-five per cent of the total 

 number of colonies developing on the plate are non-spore-forming bac- 

 teria and cocci, the other 25 per cent include the spore-forming organ- 

 isms and actinomyces. The seventy-five per cent of the colonies is made 

 up largely of the slow growing organisms; between 2 to 60 millions of 

 these organisms, as determined by the plate method, are found per 1 

 gram of soil. The non-spore-forming bacteria are believed to be very 

 active in the soil (Conn, Winogradsky) , particularly in view of their 

 great variability as affected by the soil treatment. Conn found that 

 a certain soil contained 350,000 rapid-liquefying colonies, 11,000,000 

 punctiform colonies and 4,700,000 spore-formers and actinomyces, be- 

 fore aeration; after aeration for one day these numbers changed to 



21 Hucker, G. J. Studies on the coccaceae. I. Previous taxonomic studies 

 concerning the genera of the coccaceae. N. Y. Agr. Exp. Sta. Tech. Bui. 99. 

 1924; II. A study of the general characters of the micrococci. Ibid. 100. 1924; 

 III. The nitrogen metabolism of the micrococci. Ibid. 101. 1924; IV. The 

 classification of the genus micrococcus Cohn. Ibid. 102, 1924. 



22 Koser, S. A. Utilization of the salts of organic acids by the colon-aerogenes 

 group. Jour. Bact. 8: 493-520. 1923;9:59-77. 1924. 



23 Murray, T. J., and Skinner, C. E. Differentiation of B. aerogenes and B. coli 

 of fecal origin. Proc. Soc. Exp. Biol. Med. 23: 104-106. 1925. 



