THE TH IONIC ACID BACTERIA 221 



Their development is possible only when the CO2 of the 

 atmosphere is freely admitted ; or, failing this, a free supply 

 of carbonates. Nathansohn considered that they were not 

 able to utilize the carbon of organic matter. 



These organisms were also isolated by Beijerinck (5) from 

 material taken from the sea on the Dutch coast, and cultivated 

 in fresh water. The medium used by him was made up as 

 follows : — 



NaaSgOg . 5H2O, 0-5 per cent. 



NaHCOg, o-i 



K2HPO4, 0-02 „ 



NH4CI, o-oi ,, 



MgClg, o-oi ,, 



A culture made up of these ingredients, and inoculated with 

 the appropriate material, became covered with a film of sulphur 

 in which were swarms of bacteria, including the thionic acid 

 bacteria. These were isolated and grown in pure culture. 

 The chemical reaction was stated to be as follows : — 



NaaSgOg + O = NagSOj + S. 



The sulphur was not deposited inside the cell, but in the 

 surrounding medium. In a similar fashion, though with 

 greater difficulty, it was found possible to oxidize tetra- 

 thionic acid with these organisms, according to the following 

 reaction : — 



Na2S406 + NaaCOa + = 2Na2S04 + CO2 + 2S. 



Pure cultures were obtained by adding the appropriate 

 amount of agar-agar to the fluid culture, and plating in the usual 

 manner. The colonies which form on the plates are distin- 

 guished by the large amount of sulphur which collects on their 

 surfaces. 



The thionic acid bacteria have been placed by Beijerinck 

 in one genus, which he names Thiohacillus* One of them, 

 Thiobacilliis thioparus, is made up of small thin rods, 0-3 — 0-5/Lt 

 in length, and is very motile. It is non-sporing. Another 

 somewhat similar form has been named Thiobacilliis denit- 

 rificans. 



* See page 226. 



