152 OUTLINES OF BACTERIOLOGY 



samples of iron-water however, the deposit is laid on diatoms or allied 

 organisms, though such samples are not common. In a very small 

 percentage of iron-water samples there is no trace of organic life. 



Crenothrix polyspora (Cohn). The best known of the iron-bacteria 

 is Crenothrix polyspora, which was first described by Cohn in 1870, 

 attention being then directed to it by the fact that the drinking 

 water in the neighbourhood of Breslau had suddenly assumed a deep 

 red colour. In 1878 the Berlin drinking water was similarly affected, 

 the course of the water being for a time completely choked up by 

 the deposition. Since this time, the same phenomenon has been 

 witnessed in various parts of Germany, France, Russia, and Great 

 Britain. In this country the best known visitation of this nature is 

 that which befell Cheltenham in 1896, when the water in the reser- 

 voirs of this town suddenly assumed a deep red appearance. The 

 water was red for about three months, with apparently no danger to 

 the public health. In all these cases the redness was ascertained to 

 be caused by a deposition of ferric hydroxide on the membranes of 

 myriads of individuals belonging to this organism. 



Crenothrix polyspora is thread-like in structure, each thread being 

 made up of a single row of cells (Fig. 88), and invested by a delicate 

 sheath. The sheath is formed by the splitting up of the cell-membrane 

 into two layers, of which one becomes the membrane of the cell 

 itself, whilst the other contributes to the sheath-membrane. The 

 latter is therefore the result of contributions from each of the cells 

 enclosed by it. As seen in Fig. 88, the threads are not perfectly 

 cylindrical, as the- lower part has a narrower diameter than the 

 upper portion. Also the cells of the lower part are longer but 

 narrower than those higher up the thread. Attachment is by the 

 narrow end. The diameter of the thread varies from 1'5/x to 5/x 

 at the base, and from 4 /x to 9 ^ at the top. In very young threads, 

 the sheath-membrane is very delicate, and in the youngest of all, 

 absent altogether. With regard to the method of reproduction, 

 according to Zopf, the upper part of a thread divides up to form a 

 comparatively large number of cocci. These vary in size according 

 to the rapidity of division : the smaller are called micrococci, and 

 the larger macrococci (Fig 88). They vary from 1 /x to 6 /* in diameter. 

 When set free from the thread, the walls of the cocci often swell, 

 with the result that many of them become fastened together, forming 

 a Zoogloea (Fig. 88/). The thread is gradually emptied of its 

 cocci, after which it collapses and disappears. If the conditions are 

 favourable, the cocci do not form a zoogloea, but instead each 



