PHENOL COEFFICIENT OF DISINFECTANTS 211 



N. B. Have the assistant carefully keep track of the exact 

 time of all operations, to the second. 



In actual practice determinations are made oftener than 

 every five minutes, two and one-half minutes being the 

 standard interval. This requires the most careful attention 

 of both operator and assistant. 



6. Using the 1 c.c. pipette, add 0.1 c.c. of the culture to 

 one tube of each dilution of the disinfectant and mix quickly 

 with a sharp rotary motion of the tube. 



7. At the end of one minute from the time of each 

 separate operation, make a loop transfer from the tube of 

 each dilution of the disinfectant inoculated with the cul- 

 ture into the corresponding tube of broth in the water 

 bath. 



Note. The assistant takes the tubes from the water bath and 

 hands them to the operator, then, after the operation of transferring, 

 returns the inoculated broth tube to the water bath, sterilizes the 

 needle and places it in the most handy position for the operator. j 



8. This operation is then repeated; working as quickly 

 as possible, add 0.1 c.c.- of the culture to the remaining 

 tubes of the different dilutions of the disinfectant. 



9. When, in each case, the culture has been exposed for 

 exactly five minutes, ten minutes and fifteen minutes respec- 

 tively to the action of the disinfectant, a loop transfer is to 

 be made to the corresponding tube of broth. 



10. When all transfers are made, place the broth cul- 

 tures at 37 C. Examine after forty-eight hours for growth 

 and record growth as + or . 



Note. The phenol coefficient of a disinfectant is the ratio of the 

 strength of the unknown disinfectant which will kill a filtered 24- hr. 

 broth culture of B. typhosus in a certain length of time, to the strength of 

 phenol which will accomplish the destruction in the same length of time, 

 the dilution of phenol taken as 1 . 



For example: The dilution of an unknown disinfectant required to 

 kill B. typhosus in 7| minutes was 1 : 550, and the dilution of phenol 

 necessary to kill B. typhosus in the same time was 1 : 100. 550-;- 100 

 = 5.5, the phenol coefficient of the unknown disinfectant. This means 



