THE COLON-TYPHOID GROUP 133 



its strength having first been compared with a standard fur- 

 nished by the United States Government. Unless kept in 

 the dark and at low temperature, it loses strength rapidly. 



Antitoxic Unit. An immunity unit, according to Ehr- 

 lich, is the amount of antitoxic serum which will neutral- 

 ize 100 times the minimum lethal dose of toxin, when serum 

 and toxin mixed and injected into a 25o-gram guinea-pig 

 does not cause death in four days. Thus, if the serum will 

 protect in doses of ^V c.c., then each cubic centimeter has 

 50 units' power, and 20 c.c. will contain 1000 units, or will be 

 sufficient to neutralize an amount of toxin that would be 

 fatal for 25,000 kilos (12,500 pounds) of guinea-pigs, or 

 100,000 pigs weighing 250 grams each. The serum is con- 

 centrated by precipitation and separation from the blood- 

 serum of the pseudo-globulins containing the antitoxic prin- 

 ciple, so that 10 c.c. contain more units than formerly. The 

 doses given now are much larger than when first introduced. 

 As much as 100,000 units have been employed in a single 

 case. (Sera containing 1000 units to i c.c. are now being 

 marketed.) 



Streptococcus in Diphtheria. Streptococci have been 

 found quite constant in diphtheria, but they resemble the 

 Streptococcus pyogenes, and have no specific action. 



CHAPTER XX 

 THE COLON-TYPHOID GROUP 



IN this group are placed a variety of organisms similar in 

 form and growth and having many biologic properties in 

 common, but differing in pathogenesis. The more impor- 

 tant members of this group are: Bacillus coli, B. typhosus, 

 B. enteritidis, B. dysenteric. Another closely related organism 

 is the B. suipestijer (hog cholera). The form is usually a 

 plump rod with rounded ends. Gram-negative. No spores. 



