152 PRINCIPLES OF SOIL MICROBIOLOGY 



liquefy gelatin only very slowly, are characterized by poor growth in all 

 liquid media and by the formation of punctiform colonies on gelatin. 

 These organisms are usually less than one micron in length and less 

 than half a micron in diameter, being short rods or cocci; growth on 

 agar streaks is fair, soft, smooth, glistening, slimy to watery. Due to 

 the fact that they grow only poorly in liquid media, they do not lend 

 themselves readily to physiological studies. They also appear much 

 the same morphologically. The silica gel plate with soil extract as the 

 nutritive ingredient presents a new and promising method for their 

 study. 



Conn 19 divided these organisms into five groups on the basis of their 

 growth upon a synthetic medium (1.0 gram NH 4 H 2 P0 4 , 0.2 gram 

 KC1, 0.2 gram MgS0 4 , 10.0 sugar in 1000 cc. of water, adjusted to 

 pH 7.0): 



1. Organisms forming small short rods, usually under 0.5 mi- 

 cron in diameter, non-motile or having one or possibly two polar fla- 

 gella; no tendency to change in morphology but very variable in their 

 physiology. 



2. Organisms that appear for a day or two after inoculation on a 

 new medium as small short rods, less than 0.5^ in diameter, then shorten 

 and appear like micrococci. All liquefy gelatin, slowly however, and 

 can be separated on the basis of physiological characteristics. 



3. Small short rods, with a tendency to produce long filaments, usually 

 unbranched, but frequently branched. 



4. Organisms consisting, in young cultures, mostly of branching forms, 

 apparently produced by the germination of small spherical arthrospores. 

 The branching forms disappear in a few days, leaving the coccoid forms. 



5. Organisms occurring normally as cocci, but with a tendency to 

 produce rods and filaments after a few days of growth on ordinary 

 media. This group is more abundant in manure than in soil. 



The first two groups are most numerous in the soil. According to 

 Winogradsky 20 the cocci predominate, as shown by the direct micro- 

 scopic method (including probably also the short rods). These organ- 

 isms were found to grow in groups (masses, zooglea) imbedded in the 

 colloids which cover the soil particles. 



19 Conn, H. J. Soil flora studies. VI. The punctiform-colony-forming bac- 

 teria in the soil. N. Y Agr. ^xp. Sta. Tech. Bui. 115. 1925. 



20 Winogradsky, 1925 (p. 7). 



