DISINFECTANTS 81 



Chick and Martin (1908) made some experiments on 

 the interference of proteins with disinfection. They 

 found that phenol was not much affected by 10% blood 

 serum. In the average of 4 experiments, the concen- 

 tration of phenol had to be increased only from 0.806 to 

 0.924% to accomplish sterilization in 15 minutes. Ex- 

 pressed in terms of death time, this would mean 

 15(0.924/0.806)^ = 34 minutes, an increase of 125%, 

 which is relatively little. Mercuric chloride (0.05%) was 

 considerably more weakened by blood serum, as shown 

 in the following table : 



Cone, of blood serum (per cent) : 5 10 20 30 



Death time (in minutes) : 7.2 10 14.2 39 62 



Per cent increase in death time : 39 97 340 760 

 Animal charcoal interfered especially with emulsified dis- 

 infectants. Dried feces acted partly chemically, partly 

 by adsorption. 



Many disinfectants have been tried in the presence 

 of organic matter, serum, blood, milk, urine, feces, etc., 

 but usually only the phenol coefficients are recorded and 

 this gives very little information on interference by 

 foreign matter because the phenol coefficients include the 

 effect of organic matter upon phenol as well as upon the 

 disinfectant under test. 



Our own data with skimmed milk diluted 1:10 show 

 for phenol, formaldehyde and hydrogen peroxide a pro- 

 longation of the death times by about 20 to 35% while, 

 with mercuric chloride, the prolongation was about 

 1000% and with mercurochrome, 10,000%. Washed bac- 

 teria were used in these experiments in order to exclude 

 all foreign matter from the controls. With the customary 

 method of placing 0.5 cc. of the bacterial culture into 5 

 cc. of disinfectant, a considerable amount of organic 

 matter is introduced, namely, 1500 ppm. of peptone and 

 meat extract. This is sufficient to interfere with strong 

 disinfectants capable of killing bacteria in very low con- 

 centrations. The difference of procedure explains the 



