INTEODUCTION TO SULPHUR-BACTEKIA 145 



vegetation of the previous summer has had time to reach a high 

 stage of decomposition. At these times they may be seen on 

 almost every decomposing ditch or pool. The presence of decom- 

 posing vegetable and animal remains in still waters explains why 

 they are often found in sea-water pools on the coasts. Wino- 

 gradsky gives the following method for cultivating these bacteria : 

 Small portions of the water-plant, Butomus umbellatus, together with 

 some mud from the place from which the plant was collected, are 

 placed in a deep vessel, and to these is added some 3-5 litres of 

 water. Next about 2 grams of gypsum (CaSO^) are added, and 

 the whole, uncovered, is put aside at the temperature of the room. 

 After 5-7 days, the development of sulphuretted hydrogen will be 

 noticed, as a result of the reduction of the gypsum by some bacteria 

 present in the water. These prepare the medium for the sulphur- 

 bacteria. After 3-5 weeks the medium will be found to contain an 

 abundance of sulphur-bacteria. 



§ 2. CLASSIFICATION OF THE SULPHUR-BACTEEIA. 



All bacteria, inside the cells of which sulphur may be found, are 

 grouped together under the term Sulphur-bacteria. The group is not 

 a natural one, for the organisms composing it have no characteristics in 

 common, other than the power of assimilating sulphuretted hydrogen, 

 and effecting its oxidation in the manner mentioned above. This 

 grouping is, however, very convenient in many ways. 



The bacteria are divided into the following genera : 



1. Beggiatoa. Colourless, motile thread-bacteria. 



2. Thiothrix. Colourless, motionless thread-bacteria. 



3. Thiophysa. Colourless bacteria with no formation of threads. 



4. Purpur-bacteria. Coloured bacteria. 



I. Beggiatoa. The sulphur threads classed under Beggiatoa are 

 cylindrical, and actively motile. They may attain even a centimetre 

 in length. Inside the threads, at a certain stage, are seen round, 

 strongly refractive sulphur bodies. No transverse walls can be seen at 

 this stage (Fig. 82ffi), but they are developed later after the sulphur has 

 disappeared from the threads (Fig. 82c). When the supply of sul- 

 phuretted hydrogen runs short the threads break up, as represented in 

 Fig. 82d, which is the first sign of their approaching death. Spore 

 formation is as yet unknown. 



K 



