THE CULTURE OF THE SULPHUR BACTERIA 57 



placed in sterilized fresh water, or a solution made up as 

 follows : — • 



A small quantity of ammonium sulphate was added, and 

 the three gases O2, H2S, and CO2 introduced. 



In the second week, the threads of Thiothrix had begun 

 to liberate segments from their free ends. The culture was 

 periodically washed with sterile water. It was maintained 

 by Keil that by this method his cultures were free from im- 

 purities because the subsequent addition to them of O-l — 0-2 

 per cent, of sterile peptone did not result in a development 

 of saprophytic bacteria. 



[h) Beggiatoa. — Pure cultures of Beggiatoa were obtained, 

 but with greater difficulty, due to the threads being free. 

 The purity of the cultures was tested in the same way. Keil's 

 conclusions were : — 



1. The best source of nitrogen is ammonium sulphate. 



2. Organic matter, although not harmful, cannot be used 



as a source of N. (Contrast the conclusions of Wino- 

 gradsky and Molisch.) 



3. Nitrate cannot be used as a source of nitrogen. 



4. Oxygen, carbon dioxide, and sulphuretted hydrogen 



are necessary for development, but their amounts 

 must be within the limits given above. 



5. A carbonate of one of the alkaline earths is necessary. 



Either the calcium or magnesium salt may be used. 



6. The organism is indifferent to the salts of the alkalis. 



7. A chloride furthers growth, and the culture fluid must 



contain phosphorus. 



8. Growth increases with rise of temperatures up to 30° C. 



The threads rapidly degenerate above 35° C. Beggi- 

 atoa is somewhat more resistant than Thiothrix. 

 Thermal death point : Thiothrix, 37° — 38° C. 

 Beggiatoa, 45° C. 



9. The colourless sulphur bacteria are indifferent to light. 



