DIPHTHERIA. 75 



agar than on plain agar. On such plates they appear as small, coarsely 

 granular colonies with a central dark area. In size the colonies 

 resemble the streptococcus. On blood-serum the colonies are larger 

 1 1 12 to 1/8 inch in diameter. In bouillon it tends to form a surface 

 growth. It is at the surface that the toxin function is most marked, 

 hence in growing diphtheria for toxin formation we use Fernbach 

 flasks which expose a large surface to the air. It is a marked acid 

 producer bouillon with a + i reaction becoming +2.5 to +3 in 36 

 hours. The nitrate from a tw r o- or three-weeks-old broth culture is 

 highly toxic, and is usually referred to as diphtheria toxin. It is used 

 in injecting horses to produce antitoxin. Ehrlich uses as a standard to 

 measure the toxicity of toxin the minimal lethal dose (M. L. D.) This 

 is the amount of toxin which will kill a 350 gram guinea-pig in just four 

 days. Some toxins have been produced whose M. L. D. was 1/500 

 c.c., so that one c.c. of such toxin would kill 500 guinea-pigs.)^ Theoreti- 

 cally, the measure of an antitoxin unit is the capacity of neutralizing 

 200 units of a pure toxin.^( (On exposure to light, etc., toxin loses its 

 toxic power and is termed toxoid.) Inasmuch, however, as toxone 

 and toxoid are also present, we may practically consider an antitoxin, 

 or immunizing unit (i.e., Immunitatseinheit), as about capable of 

 making innocuous 100 M. L. D. 



In obtaining material from a throat, be sure that an antiseptic 

 gargle has not been used just prior to taking the throat swab. The 

 part of the swab which touched the membrane or suspicious spot 

 should come in contact with the serum slant. This is best accom- 

 plished by revolving the swab. An immediate diagnosis is possible in 

 probably 35% of cases by making a smear from a piece of membrane. 

 In doing this Neisser's stain is the most satisfactory. 



If there is any doubt about the nature of an organism in a throat 

 culture, always stain: (i) with Loffler's alkaline methylene blue for two 

 minutes; (2) with Gram's method, being careful not to carry the 

 decolorization too far, and (3) by Neisser's method. With Loffler's 

 you obtain a picture which, after a little experience, is characteristic ; 

 at times the polar bodies show as intense blue spots in the lighter blue 

 bacillus. One is liable to confuse cocci lying side by side for diphtheria 

 bacilli with segmental or banded staining. This difficulty is not ap- 



