220 SULPHUR BACTERIA 



3. It is frequently observed that spirilla at rest are straight, 

 and assume the helicoid form only after the organism has 

 begun to move. 



It is not possible at present to state whether the vital 

 action which brings the cilium into the helicoid form is preceded 

 by a circular movement of the end of the body which ex 

 hypothesi is the seat of movement. 



The Thionic Acid Bacteria. 

 Introduction : Methods of Culture. 



The thionic acid bacteria, like the sulphur bacteria in 

 general, oxidize sulphur compounds, but they differ from the 

 sulphur bacteria in that they do not store sulphur in their cells. 

 Strictly speaking, therefore, they are not sulphur bacteria, 

 but it is appropriate to consider them briefly here. 



These organisms oxidize thiosulphates to tetrathionic and 

 sulphuric acids. They were first isolated by Nathansohn 

 from a crude culture of organic debris and calcium sulphide 

 in sea-water, prepared for the investigation of sulphur bacteria. 

 Nathansohn isolated instead small actively motile thionic 

 acid bacteria, which were readily found in the white sulphur 

 surface scum. 



He recommended the following medium for their culture : — 



When this mixture was inoculated with mud containing 

 these organisms a white scum appeared on the surface after 

 a few days. This consisted of oily amorphous sulphur in the 

 substance of which various bacteria were found. Among 

 them were the thionic acid bacteria, which were isolated 

 without difficulty. According to Nathansohn the reaction 

 which takes place is as follows : — 



3Na2S203 +50 = Na2S406 + 2Na2S04. 



