56 SULPHUR BACTERIA 



forms. For pure cultures obtained from the raw cultures 

 mentioned above, he used the following medium : — 



River water or sea water . . looo c.c. 



Peptone . . . . .5 grams. 



Dextrin or glycerine . . • 5 ,, 



Agar 18 grams or gelatine lOO grams. 



Colonies were obtained by plating from this medium. He 

 also obtained colonies by plating a drop of the inoculated 

 melted nutrient medium under a coverslip on a glass slide and 

 ringing the coverslip to exclude atmospheric oxygen. 



Molisch's results are noteworthy in that they showed that the 

 purple sulphurless bacteria, which are genetically allied to the 

 purple sulphur bacteria, grow abundantly when supplied with 

 organic matter, and in that they showed that a greater intensity 

 could be secured in the growth of the coloured sulphur organism 

 Chromaiium in a medium containing organic matter but no H2S. 



A very important advance was made by Keil when he 

 succeeded in preparing pure cultures of the colourless sulphur 

 bacteria, Beggiatoa and Thiothrix. This investigator assumed 

 that the following factors were necessary : sulphuretted 

 hydrogen, oxygen, carbon dioxide, and some source of nitro- 

 gen supply, and then proceeded to show that the influences 

 which favourably affected growth were favourable only 

 within certain limits. The upper limit of oxygen pressure 

 was 20 mm. Hg, the lower limit 10 mm. Hg, and the 

 optimum 15 mm. Hg. Using a fixed oxygen supply of 15 

 mm. Hg pressure he determined experimentally that the 

 optimum concentration of sulphuretted hydrogen was exerted 

 at a pressure of 0-8 mm. Hg (about 5 c.c. HoS in a vessel of 

 4500 c.c. capacity). The lower limit of carbon dioxide was 

 0-5 mm. Hg, approximately the amount normally present in 

 the atmosphere. The upper limit was 360 mm. Hg, and the 

 optimum 25 mm. Hg. 



Keil then made use of these three substances in appro- 

 priate amounts to obtain pure cultures. 



Pure Cultures. — [a] Thiothrix. — A clump of material made 

 up almost wholly of Thiothrix threads was washed and 



