210 GENERAL MICROBIOLOGY 



EXERCISE 29. DETERMINATION OF THE PHENOL 

 COEFFICIENT OF SOME COMMON DISINFECTANTS 



(Two students working together are required in this exercise.) 



Apparatus. Copper water bath; test-tube rack for 

 above bath, thirty-two test tubes of uniform size containing 

 exactly 5 c.c. of sterile nutrient broth (use a graduated 

 burette or a similar apparatus for filling tubes) ; eight clean 

 dry test tubes of uniform size; several (4 or 5) platinum 

 loops, of 4 mm. inner diameter; sterile 1 c.c. pipettes with 

 fine point; three clean 5 c.c. pipettes; phenol, 5%; mer- 

 curic chloride, 1 : 500; small funnel; filter paper to fit 

 funnel; sterije test tube; watch with second hand. 



Culture. B. typhosus, twenty-four hour broth culture 

 grown at 37 C. 



Method. 1. Place the filter paper in the funnel, wrap in 

 paper and sterilize in the hot air. 



2. Filter the twenty-four-hour broth culture of B. 

 typhosus into the sterile test tube. This is for the purpose 

 of removing clumps of bacteria and any foreign matter. 

 Funnel and filter paper are to be treated immediately after 

 use with 1 : 1000 HgCl 2 . 



3. Regulate the water bath at 20 C. and keep at this 

 temperature. 



4. Mark the thirty-two test tubes, each containing 

 exactly 5 c.c. of nutrient broth, with the name of the disin- 

 fectant, the dilution, and the time exposed, according to the 

 following table. Then place the tubes, in order, in the rack 

 in the water bath. 



6. Mark each set of clean, dry test tubes carefully with 

 the name of the disinfectant and the dilution to be added 

 (see table on p. 212), and place in each, 5 c.c. of the dilu- 

 tion of the disinfectant as indicated on the labels. Keep in 

 a test-tube rack at 20 C. Work with one disinfectant at a 

 time. 



