INHIBITION COEFFICIENT 311 



Formaldehyde ; 



Carbolic acid ; 



investigated with regard to B. anthracis, but the tech- 

 nique is practically similar for all other chemical 

 disinfectants. 



Inhibition Coefficient. 



Apparatus Required: 



Case of sterile pipettes, 10 c.c. (in tenths). 



Case of sterile pipettes, i c.c. (in tenths). 



Sterile tubes or capsules for dilutions. 



Tubes of nutrient bouillon each containing a measured 10 c.c. 

 of medium. 



Twenty-four-hour-old agar culture of a recently isolated B. 

 anthracis 



Germicides: 



1. Five per cent, aqueous solution of carbolic acid. 



2. One per cent, aqueous solution of perchloride of mercury. 



3. One-tenth per cent, aqueous solution of formaldehyde. 



METHOD. 



1. Number six bouillon tubes consecutively i to 6. 

 Inoculate each from the stock cultivation of B. anthra- 

 cis and at once add varying quantities 1 of the carbolic 

 acid solution, viz. : 



To tube i add 2 . o c.c. ( = i : 100) 

 To tube 2 add i . o c.c. ( = i : 200) 

 To tube 3 add 0.6 c.c. ( = i 1300) 

 To tube 4 add 0.5 c.c. ( = i : 400) 

 To tube 5 add 0.4 c.c. ( = i : 500) 

 To tube 6 add o . 2 c.c. ( = i : i ,000) 



2. Prepare a similar series of tube cultivations 

 numbered consecutively 7 to 12 and add varying 

 quantities of the mercuric perchloride solution, viz. : 



To tube 7 add o.i .( = 

 To tube 8 add 0.05 ( = 

 To tube 9 add o . 03 ( = 

 To tube 10 add 0.025 ( = 

 To tube ii add 0.02 ( = 

 To tube 1 2 add o . o i ( = 



:i,ooo) 

 : 2,000) 

 .3,000) 

 14,000) 

 .5,000) 

 : 10,000) 



1 The quantities here given are not absolutely correct. If exactitude is 

 essential the student must calculate the amount required by the aid of the 

 Percentage Formula, Appendix, page 496. 



