138 AGBICULTUEAL AND INDUSTEIAL BACTERIOLOGY 



the extent that a particular disinfectant measures up in its 

 characteristics to those of the ideal disinfectant, it is valu- 

 able for general use. The important characteristics of dis- 

 infectants are as follows : 



1. High Germicidal Power. — The ability of a particular 

 disinfectant to kill microorganisms is usually compared 

 with that of phenol. In making such comparisons it is cus- 

 tomary to use the Bacterium iyphosum, the cause of typhoid 

 fever, as the test organism. A series of test tubes are pre- 

 pared containing varying dilutions respectively of phenol 

 and of disinfectant to be tested. Equal numbers of Bac- 

 terium typhosum are then introduced into each tube. At in- 

 jtervals of two and one-half minutes samples are taken from 

 each tube and transferred to a nutrient medium suitable 

 for growth. The transfers are continued for fifteen minutes. 

 The strength of phenol which is required to kill the organ- 

 isms, that is, so that no growth is secured when a loopful is 

 transferred to sterile broth after two and one-half minutes 

 exposure, is determined, likewise the strength of the 

 disinfectant to be tested which will produce the same 

 results. The ratio between the dilutions of the disinfec- 

 tant to be tested and the phenol is determined and the 

 nurrj-ber recorded. The ratio between the concentration 

 of the phenol and of the test disinfectant required to 

 kill in fifteen minutes is also determined. These two 

 ratios are averaged. For example, if phenol kills in two 

 and one-half minutes in a strength of one to one hun- 

 dred, and the disinfectant to be tested in one to five hun- 

 dred, the first ratio would be five. If in fifteen minutes 

 phenol kills in one to two hundred and the disinfectant 

 to be tested in one to twelve hundred the ratio would be 

 six. The average of the two would be five and one-half. 

 Most commercial disinfectants, particularly the coal tar 

 products, are sold upon the basis of their plienol co- 

 efficient. 



