BACTERIOLOGICAL TECHNIC. 6 1 



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 catde pens, pastures and other places frequented by catde. Other soil 

 bacteria are decidedly useful as will be more fully explained elsewhere. 



Some soil bacteria (the nitrifiers) 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 leguminosas 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. Colonies of 

 small motile bacteria (Rhizobium mutahile) 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 introduction of 

 extraneous germs. Cultures can also be made from internal plant tissues 

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

 excepting that after the washing and rinsing, the root, instead of being 

 crushed, is cut or broken across and the inoculation material is taken from 

 the inner tissue by means of a platinum needle or scalpel. 



E. Bacteria of the Air. — ^Air currents carry the germ-laden dust and dirt 

 particles. The number and kind of air bacteria depends upon environment, 

 climadc conditions, moisture, sunKght, etc. The air currents are the main 

 factors in germ dissemination. Spores and dry (though not dead) bacilli 

 may be carried many miles. Air microbes are derived from the soil surface 

 and from the objects surrounded by the air. Bacteria are exhaled with the 

 breath and are carried and distributed from and by animals, plants and 

 clothing. The air carries pus germs, tubercle bacilli, anthrax bacilli and 

 their spores, besides other pathogenic microorganisms, including also yeast 

 cells and the spores of higher fungi. 



Air microbes may be studied by exposing a Petri dish containing steril- 

 ized agar or gelatin, for two minutes or longer. The number of colonies 

 that will appear will depend upon the locality, season, air moisture, etc. 

 To determine the number of microbes in a given volume of air the Sedgwick- 

 Tucker aerobioscope is used, though similarly constructed apparatus may 

 be made by any fairly skillful student. The aerobioscope consists of a glass 

 cylinder as shown in the illustration. The open ends are plugged with cotton. 



