242 MANUAL OF BACTERIOLOGY. 



The many conditions affecting the action of germicidal agents 

 probably account for the greater or less discrepancy in the 

 results obtained by various investigators in testing the value of 

 disinfectants. Moreover, it has been shown that the failure of 

 the bacteria to grow after exposure to a germicide cannot be 

 regarded as a certain criterion of the death of the organism 

 as is usually done; for enough of the germicide may be trans- 

 ferred in the inoculation to inhibit growth without actually 

 killing the bacteria. Enough of the germicide may even ad- 

 here to bodies of the bacteria to inhibit growth and yet not kill 

 the bacteria, as shown by ultimate growth or by growth after 

 neutralizing the chemical agent. In all tests of the germicidal 

 action of chemical agents it is necessary to exclude this source 

 of error. The bacteria should be thoroughly washed after 

 exposure to the germicide and if possible treated with some 

 neutralizing agent which is not itself a germicide and which 

 does not form a germicidal compound with the germicide which 

 is being tested. The culture tubes or plates made to test the 

 viability of the organisms after exposure must be observed for 

 a much longer time than is done with ordinary cultures; for 

 development may be delayed for many days and yet take place 

 abundantly at last. 



The manner in which disinfectants destroy bacteria differs 

 in different cases. In some cases the injurious effect is due 

 to interference with the nutrition, in some cases it is due to 

 oxidation; but much more frequently it is due to coagulation of 

 the protoplasm of the cell. Heat, metalic salts, phenol, all 

 act in this way. On the other hand, not every agent which 

 coagulates albumin acts as a powerful germicide. Alcohol 

 and tanic acid are weak .disinfectants, and yet they coagulate 

 albumin very strongly. Lysins found in blood-serum act by 

 breaking up and disintegrating the bacteria. 



The methods employed for testing the germicidal value' 

 of any disinfectant arc as follows: 



