584 PATHOGENIC MICROORGANISMS 



transparency from the other bacteria apt to appear in feces. Agar 

 plates, therefore, are important in the isolation of these organisms. 



Coagulated blood serum is fluidified by the cholera vibrio. On 

 potato, growth is profuse and appears as a brownish coarse layer. In 

 milk, growth is rapid and without coagulation. In broth, general 

 clouding and the formation of a pellicle result. The rapidity and luxuri- 

 ance of growth of the cholera spirillum upon alkaline pepton solutions 

 render such solutions peculiarly useful as enriching media in isolating 

 this microorganism from the stools of patients. In pepton solution, 

 too, the cholera spirillum gives rise to abundant indol, demonstrated 

 in the so-called "cholera-red" reaction. This reaction has. a distinct 

 diagnostic value, but is by no means specific. 1 In the case of the cholera 

 vibrio the mere addition of strong sulphuric acid suffices to bring out 

 the color reaction. This is due to the fact that, unlike some other indol- 

 producing bacteria, the cholera organism is able to reduce the nitrates 

 present in the medium to nitrites, thus itself furnishing the nitrite 

 necessary for the color reaction. The medium which is most suitable 

 for this test is that proposed by Dunham, 2 consisting of a solution 

 of 1 per cent of pure pepton and .5 per cent NaCl in water. 



Dieudonne 3 has recommended a selective medium upon which 

 cholera spirilla will grow well, but upon which the colon bacillus will 

 grow either very sparsely or not at all. Its preparation is very simple. 

 To 70 parts of ordinary 3 per cent agar, neutralized to litmus, there are 

 added 30 parts of a sterile mixture of defibrinated beef blood and normal 

 sodium hydrate. 



The latter is sterilized by steam before being added to the agar. 

 This pure alkali agar is poured out in plates and allowed to dry several 

 days at 37 or 5 minutes at 60. The material to be examined is smeared 

 upon the surface of these plates with a glass rod. 



The principle of this medium is that cholera will grow in the presence 

 of an amount of alkali which inhibits other fecal bacteria. The medium 

 has been studied by Krumwiede, Pratt, and Grund, 4 who have recom- 

 mended a modification. They find the following combination sat- 

 isfactory and an improvement upon Dieudonn^'s medium because 

 transparent and more easily prepared. They prepare the following 

 mixtures : 



1 See indol reaction, p. 167. 2 Dunham, Zeit. f. Hyg., ii, 1887. 



3 Dieudonne, A., Cent. Bakt., 1., orig., 1909. 

 4 Krumwiede, Pratt, and Grund, Jour, of Inf. Dis., x, 1912, 



