}02 BIOLOGY AND TECHNIQUE 



As only the 2i/2-imnute and 15-minute intervals are used in 

 determining this result it seems unnecessary to make plants at .the 

 intervening periods except in special cases where more detailed 

 information is desired. 



The procedure may be modified by adding some organic sub- 

 stance such as killed bacteria to the diluted antiseptic. For many 

 substances, e.g., bichloride of mercury, the antiseptic value in 

 presence of organic matter is much lower than in watery solution. 

 Anderson and McClintic insist that great care in making the dilu- 

 tions and rigid adherence to a uniform technique are necessary to 

 obtain consistent results in such tests. 



DETERMINATION OF ANTISEPTIC VALUES. The antiseptic or in- 

 hibitive strength of a chemical substance, sometimes spoken of as 

 the "coefficient of inhibition," is determined by adding to definite 

 quantities of a given culture medium, graded percentages of the 

 chemical substance which is being investigated, and planting in these 

 mixtures equal quantities of the bacteria in question. The medium 

 used for the tests may be nutrient broth or melted gelatin or agar. 

 If broth is used, growth is estimated by turbidity of the medium 

 and by morphological examination ; if the agar or gelatin is em- 

 ployed, plates may be poured and actual growth observed. 



Thus, in the case of carbolic acid, a 5 or 10 per cent solution is 

 prepared and added to tubes of the medium, as follows: 



Tube 1 contains 5% carbolic 2 c.c. -f- broth 8 c.c. 1:1,000 carbolic acid. 



" 2 " 5 " 1 c.c. -f broth 9 c.c. = 1:200 " " 



" 3 5 " .5 c.c. -f broth 9.5 c.c. 1:400 " " 



" 4 " 5 " 7 c.c. 4- broth 9.8 c.c. = 1:1,000 " 



" 5 " 5 " .1 c.c. -f broth 9.9 c.c. = 1:1,500 " " 



To each of these tubes a definite quantity of the bacteria is added 

 either by means of a standard loopful of a fresh agar culture, or 

 better by a measured volume of an even emulsion in sterile salt 

 solution. The inoculated tubes are then incubated at a temperature 

 corresponding to the optimum growth temperature for the micro- 

 organism in question. The tubes are examined for growth from 

 day to day. From tubes containing higher dilutions, in which no 

 growth is visible, transplants are made to determine the presence 

 of living bacteria and to distinguish between inhibition or antisepsis 

 and bacterial death or disinfection. 



