70 Occurrence of Microorganisms in Soil 



shorten and begin to look like micrococci. All liquefy gelatin slowly. 

 Bacterium globiforme can be considered a representative of this 

 group; it is most abundant in the soil; its presence was believed to 

 be a good index of the availabiHty of the soil nitrogen. 



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

 usually unbranched, but frequently branched. 



4. Organisms consisting mostly of branching forms, especially in 

 young cultures; they are apparently produced by the germination 

 of small spherical artlnospores. The branching types 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. 



Topping suggested classification of the non-spore-forming, non- 

 acid-fast, rod-shaped soil bacteria into four groups: 



1. Gram-positive, motile bacteria, capable of producing branching 

 variants. 



2a. Gram-positive, nonmotile, rod-shaped bacteria. 



2b. Gram-positive, nonmotile, mycelium-shaped bacteria. 



3. Gram-negative bacteria (motile and nonmotile). 



Some of the forms included under 2b are identical with Nocardia 

 corallinus, which Jensen placed between Corynebacterium and No- 

 cardia. The forms included in 2a are related to the Nocardia but 

 are classified with C. liquefaciens. 



Thermophilic Bacteria. Miquel was the first to isolate, in 1879, 

 bacteria capable of developing at 72°C. These organisms were 

 found in river mud, sewage, animal excreta, dust, and soil. It was 

 soon established that various bacteria capable of growing at 50-70°C, 

 but not at room temperature, are found in the soil. Organisms ca- 

 pable of growing at temperatures up to 79.5 °C were also found in 

 stable manure. The self-heating and burning of hay, cotton, peat, 

 and manure are caused by bacteria which Schloesing designated in 

 1892 as "thermogenic bacteria." Some of these organisms may be 

 rather thermotolerant than strictly thermophilic. Their distribution 

 depends on the nature of the soil: the sands of the Sahara Desert 

 contain such organisms, but forest soils do not. The nature and 

 amount of manure and fertilizer applied to the soil have a marked 

 effect: heavily manured garden soils may contain 1-10 per cent 

 thermophilic forms, as measured by the plate method; field soils 

 contain only 0.25 per cent or less of these bacteria; uncultivated soils 

 may be entirely free from them. 



