88 PHARMACEUTICAL BACTERIOLOGY 



D. Culturing Soil Bacteria. Soil is a mixture of dead and decayed 

 organic matter, sand and living organisms and their spores. Near the 

 surface the soil contains large numbers of bacteria, from 10,000 to 10,000,- 

 ooo per gram, and more. In fact the fertility of the soil is practically 

 proportional to the number of bacteria present. Most species of soil 

 bacteria are harmless to man though the bacilli of tetanus (lockjaw), 

 of typhoid fever, of malignant edema, -of anthrax, and of pus formation 

 may be present. The tetanus germ is quite common in garden soils and 

 the anthrax germ is apt to occur in cattle pens, pastures and other places 

 frequented by cattle. Other soil bacteria are decidedly useful as will be 

 more fully explained elsewhere. 



PIG. 37. Graduated fermentation tube. These tubes are required for gas determi- 

 nation with colon bacillus and other gas-forming micro-organisms. 



Some soil bacteria (the nitriners) do not grow on the usual media while 

 others thrive exceedingly well in such media. Anaerobic forms must be 

 cultured in the absence of air or oxygen. 



The root nodule bacteria of the leguminosae can be grown readily on 

 gelatin or agar. The tubercles or nodules must be thoroughly cleansed and 

 repeatedly washed in boiled distilled water, then rinsed for ten seconds in a 

 i-iooo corrosive sublimate solution, and finally thoroughly rinsed (three 

 minutes) in boiled distilled water. Crush several of the sterilized nodules 

 in a sterile watch crystal, by means of a sterile glass rod and from this 

 make the dilution plate cultures and set aside at room temperature. Colo- 

 nies of small motile bacteria (Rhizobium mutabile) will appear in about four 

 days. 



To test the soil bacterially, select thoroughly mixed samples and plate 

 out as already suggested, using every precaution to prevent the introduc- 

 tion of extraneous germs. Cultures can also be made from internal plant 

 tissues by following, in general, the directions given under root nodule 



