HYPERSENSITIVENESS 505 



phosi)liat(\ aininonium citrate, sodium carbonate, sodium chloride, 

 maiinesium sulphate, and ferric ammonium citrate. The propor- 

 tion and concentration of the constituents are such that the mix- 

 ture is neutral and at the same time well buffered. Long states 

 that growth in this medium yields 100 to 150 grams of moist 

 tubercle bacilli per liter. 



Synthetic iMediuji of the Bi-reau oe Animal Industry. — An- 

 other synthetic medium which Seibert (1934) says yields a heavier 

 groAvth of tubercle 1)acilli than Long's medium has been developed 

 by Dorset in the Bureau of Animal Industry. According to 

 Seibert it differs from Long's medium in that it contains ap- 

 proximately two and one-half times more asparagine and twice 

 as much glycerol as the latter. It contains also 10 per cent dex- 

 trose but no carbonate or ammonium ion except that which might 

 1)C derived from tlie aspai-agine. This is the medium which Seibert 

 (1934) and Long (1934) recommend for the production of a 

 standard tuberculin which is now called PPD (Purified Protein 

 Derivative of tuberculin). 



The second objection lias been overcome by Long and Seibert 

 (1926) and Seibert (1932, 1934), although Long states that the 

 greater part of the chemical work was carried out by Seibert. 

 They grew the tubercle bacillus in Long's protein-free synthetic 

 medium and studied the chemical changes that occurred. From 

 these and other studies the following important results were ob- 

 tained : 



The Active Substance, Its Recovery and Properties. — 1. That 

 the tuberculin activity of filtrates develops simultaneously with 

 the appearance of protein in the medium. The protein is derived 

 apparently by autolysis or extraction of the tubercle bacillus. 



2. If the tuberculin is subjected to dialysis, there remains in 

 the dialyzing sac only the active substance together with protein 

 and the polysaccharide peculiar to the tubercle bacillus. 



3. The protein and active substance are completely precipi- 

 tated out of the filtrate by full saturation with ammonium sul- 

 phate or by tricliloracetic acid. 



4. The ammonium .sulphate precipitate contains both eoagulable 

 and noncoagulable protein and in addition proteose. The activity 

 is destroyed by pepsin which splits the protein, but the activity 



