528 BACTEBIOLCXIY. 



The student should test the antiseptic action of several chem- 

 icals, according to the following directions, on one of the bacteria 

 employed in the disinfection experiments. 



Antiseptic action of merav/ric chloride. — A bacterial suspension is 

 made by adding a few drops of a fresh bouillon culture of the germ 

 to about 200 c.c of sterile bouillon. The 1: 500 mercuric chloride solu- 

 tion is also used. 4 large sterile tubes are numbered consecutively, 

 and equipped as follows: 



No. 1—1 c.ci of the HgCb + 9 c.c. of the suspension = 1:5,000. 

 No. 2—0.5 c.c. of the HgClj + 9.5 c.c. of the suspension = 1: 10,000. 

 No. 3-0.25 c.c. of the HgCla + 9.75 c.c. of the suspension = 1: 20,000. 

 No. 4-0.1 c.c. of the HgClj + 9.9 c.c. of the suspension = 1:50,000. 



The tubes are then placed in the incubator and examined at the 

 end of 24 hours. Prom the cultures that show no growth, or at most 

 a very faint cloudiness, a few drops should be transplanted to sterile 

 bouillon tubes. The two sets should be returned to the incubator and 

 examined on the following day. Hanging-drop preparations of the 

 two sets should be made. Involution forms may be expected. The re- 

 sults should be tabulated. 



Antiseptic action of carbolic acid. — An aqueous 1 per cent, solution of 

 phenol is placed in a stoppered flask or bottle and steamed for a few 

 minutes. The same bacterial suspension is used as above. 5 sterile 

 tubes are numbered and equipped as follows: 



No. 1 — 2 c.c. of the phenol + 8 c.c. of the suspension =: 1: 500. 

 No. 2 — 1 c.c. of the phenol + 9 c.c. of the suspension = 1: 1,000. 

 No. 3 — 0.5 c.c. of the phenol + 9.5 c.c. of the suspension = 1: 2,000. 

 No. 4 — 0.25 c.c. of the phenol + 9.75 c.c. of the suspension = 1: 4,000. 

 No. 5—0.1 c.c. of the phenol -\- 9.9 c.c. of the suspension = 1: 10,000. 

 The tubes are then placed in the incubator and examined as in 

 the preceding experiment. 



Antiseptic action of formaldehyde, — 2.5 c.c. of the commercial 40 per 

 cent, solution of formaldehyde are diluted to 100 c.c. with sterile 

 water ( = 1 per cent.). A series of 5 dilutions is then made employing 

 the same quantities as in the above experiments. 



Antiseptic action of sodium benzoate. — An aqueous 1 per cent, solu- 

 tion of this salt is sterilized by exposure to steam. The 5 dilutions 

 are prepared and^ tested in exactly the same manner as in the case of , 

 carbolic acid. 



