248 BACTERIOLOGICAL METHODS 



come into direct contact with any part of the test-tube con- 

 taining the germicidal solution. If any of the organisms adhere 

 to the sides or to threads of cotton in the mouth of the tube they 

 might be inoculated into the bouillon without being exposed to 

 the germicide. As soon as the last one of the five drops is added 

 to the germicidal solution, they are mixed thoroughly for 15 sec. 

 or longer by holding the test-tube in the left hand and shaking it 

 with the fingers of the right hand; a formation of air bubbles 

 shaped like a long funnel extending from the bottom of the tube 

 toward the surface of the liquid shows that the mixing is effi- 

 ciently done, thus bringing every bacterial cell into direct contact 

 with the germicidal solution. 



V. The subcultures are taken each minute for 5 min. by means 

 of a 23 platinum wire loop of 4 mm. inside diam. 



Tests of two dilutions can be carried on at the same time by 

 one person, as a half-minute is more than is ordinarily necessary 

 for taking out a subculture and planting into the tube of culture 

 medium. Tests of five dilutions of a sample and three of the 

 standard can therefore be made in about 25 min. 



To obtain a loopful of the mixture, the test-tube should be 

 tilted so as to prevent getting the foam formed on the surface of 

 the mixture during shaking instead of getting a liquid portion of 

 the mixture. The wire loop should be plunged almost to the 

 bottom of the tube before withdrawing. 



These loopfuls are inoculated into test-tubes containing about 

 6 cc. of the standard culture medium above described. They 

 are placed in the incubator at 38° C. for 48 hr. or for such a 

 time as it is found that no further development of bacteria takes 

 place. 



VI. Conclusions as to the value of a disinfectant from a test 

 conducted as described are drawn by comparing the dilutions of 

 the sample and the standard, which are equally efficient. While 

 in general one dilution of each is used for comparison, it is often 

 necessary to take the mean of a number, since it is not uncommon 



