192 OUTLINES OF BACTEEIOLOGY 



Another characteristic which it is important to bear in mind when 

 cultivating these organisms is their need of an abundant supply of 

 oxygen. In Winogradsky's experiments most of the culture flasks 

 had flat bottoms 12 cms. in diameter, and the height of the culture 

 fluid was not allowed to reach more than 1 cm. high, in order to 

 secure an adequate supply of oxygen. Several contrivances have 

 lately been described which accomplish the same purpose in a more 

 elaborate but more efiective way. Of these, mention may be made 

 of two, viz. one in which the free access of air is secured by placing 

 the culture fluid in a rotatory apparatus, and the other in which this 

 fluid is placed in a flask filled with some very porous material like 

 slag. 



§5. THE NITEATE-BACTEEIA. 



As already mentioned, the first of the nitrate-bacteria was discovered 

 by Winogradsky in 1891. We have already stated the circumstances 

 which led to the search for these bacteria. A sample of soil from 

 Quito (S. America) yielded the first of them, and nitrate-bacteria have 

 since been isolated from Russia, Germany, Great Britain, and other 

 European countries. As was the case with the nitrite-bacteria, all 

 these proved to be so much alike that all of them have been included 

 under one species. 



To cultivate these bacteria the following nutrient medium is 

 generally employed : 



Sodium nitrite, l-O gram 



Potassium phosphate, Oo 



Magnesium sulphate, 0'3 



Soda (free from water), 1-0 



Sodium chloride, 0-5 



Ferrous sulphate,- 0'4 



Distilled water, 1 litre 



If a solid medium be required, the medium is made up as follows : 

 Sodium nitrite, 2 grams 



Soda (free from water), 1 gram 



Potassium phosphate, tiny amount 



Agar, 15 grams 



Tap water, 1 litre 



It will be noticed that these prescriptions, like those for the 

 cultivation of the nitrite-bacteria, are devoid of organic substances. 



