162 MICRO-ORGANISMS AND FERMENTATION. 



11. Iron and Sulphur Bacteria : Nitrifying Bacteria. 



The bacteria described in this section are of particular 

 interest, because they possess the property of oxidising in- 

 organic substances. 



In microscopical examinations of water we often meet the 

 characteristic forms of Crenothrix Kiihniana (Fig. 31), or 

 spring pest, described by Cohn and Zopf. 



This ferment occurs in all water containing organic matter, 

 and sometimes it multiplies to such an extent that it may 

 render the water unfit for use. Thus, according to Zopf, 

 great calamities have been caused by this fungus in the water 

 supplies of Berlin, Lille, and certain Russian towns. In 

 consequence of its power of storing iron compounds within its 

 walls, it forms red or brown flakes in water. Its forms are 

 very beautiful ; it occurs in the form of motionless cocci or 

 gonidia (a-f), which by division and formation of viscous 

 matter form zoogloaa (g) ; these cocci frequently grow to 

 articulated filaments, which are provided with distinct 

 sheaths (h, i-r) ; they then increase in thickness towards 

 their free end, and when they reach a certain age, they 

 divide within the sheath into smaller fractions, which become 

 round and issue either as rods, macro- or micrococci. 



Leptothrix ochracea is a widely distributed iron bacterium 

 with colourless cylindrical cells connected in threads and 

 surrounded by a sheath, which at first is thin and colour- 

 less, but afterwards, by accretion of hydrated oxide of iron, 

 assumes a yellow or brown colour. Oval and motionless 

 gonidia develop in the threads. The empty sheaths may 

 form large yellowish-brown deposits in water containing iron. 



Cladoihrix (Sphcerotilus] dichotoma is also of frequent 

 occurrence. Its cells are surrounded by a similar thin sheath. 

 By displacement of single rods in a filament, false branching 

 takes place. The rods are finally set free, and are then pro- 

 vided with cilia, with which they swim about until they 

 settle down, and grow into new threads. These iron bacteria 

 are commonly met with in water containing the soluble basic 

 ferrous carbonate. According to Winogradsky this salt is 

 oxidised by the bacteria and ferric oxide is deposited on the 



