ELECTIVE CULTURE METHOD 55 



development of proteolytic bacteria; a medium containing starch 

 or cellulose will favor the growth of organisms capable of utilizing 

 starch or cellulose. The incubation of the plates under aerobic 

 conditions will favor the development of aerobic organisms; 

 under anaerobic conditions the development of anaerobic bacteria 

 is favored. At high temperatures (50 to 65° C) thermophilic 

 organisms develop. An acid medium favors the development of 

 acid-tolerant microbes. The value of the method as an agency for 

 measuring the total numbers of microbes in the soil is further lim- 

 ited by the fact that not all of the cells of even those organisms for 

 which the medium is suited develop and form colonies. Fre- 

 quently only 3 to 10 cells out of 100 will grow into colonies. Many 

 fungi and protozoa fail to grow even upon the most carefully 

 prepared media. Many bacteria and fungi require special media 

 and conditions of incubation which are adapted to a highly specific 

 type of development. 



Elective Culture Method. — To meet the peculiar nutrition 

 of the numerous soil organisms, the third method, namely, the 

 solution or elective culture method, has been introduced. It con- 

 sists in adding a definite quantity of soil to definite volumes of 

 sterile water, thus diluting the soil to a desired degree, then adding 

 measured quantities of the various dilutions to media favorable 

 for the development of the particular organisms. The cultures 

 are then incubated under favorable conditions, and observations 

 are made to determine at what dilutions growth of the specific 

 microbes has taken place. One can thus measure the approxi- 

 mate number of the specific microbes in a given quantity of soil. 



To determine, for example, the number of aerobic cellulose- 

 decomposing bacteria in a certain soil, the following method has 

 been used. A series of flasks or tubes containing a liquid or solid 

 medium favorable for the development of cellulose-decomposing 

 bacteria is prepared and sterilized. A medium containing cellulose 

 in the form of filter paper or cotton and the other nutrients as 

 inorganic salts in dilute solution was found favorable for this 

 purpose. The soil is diluted with sterile water so that 1 cc. 

 quantities of the dilution contain the following portions of a gram 

 of soil in suspension: 1 100, 1/400, 1/1,000, 1/5,000, 1/10,000, 

 1/25,000. One cubic centimeter portions of the suspensions are 

 added to the flasks or tubes of the medium, and the flasks are 

 incubated at 25 or 28° C. After a few days, the cultures are 



