SECTION IX. 

 OXIDISING FERMENTATIONS. 



CHAPTER XXXIV. 



THE IRON BACTERIA. 



197. Morphology of the Genera Crenothrix and Cladothrix. 



THE group of Schizomycetes known as thread bacteria can be divided into two 

 sub-groups. To the one of these belong the two genera (described in the follow- 

 ing chapter) which store up globules of free sulphur in their cells. The 

 appearance of these organisms is so characteristic that a skilled eye can detect 

 them with ease. On the other hand, the five genera of the second sub-group 

 lack this peculiarity. In three of these latter, viz., Streptothrix, Leptothrix, and 

 Cladothrix, the fission of the cell takes place in one direction only ; con- 

 sequently, rods are formed. In the other two, however, viz., Crenothrix and 

 Phragmidiothrix, the extremity of the thread is broken up, by division in all 

 three directions of space, into coccus-like cells. No perfectly satisfactory morpho- 

 logical description, or definite separation of these five genera into species, has 

 yet been drawn up on this basis. This defect to which reference has also been 

 made by C. SAUVAGEAU and M. RADAIS (I.) therefore restricts us to the con- 

 sideration of individual examples. A typical one is afforded by Crenothrix 

 polyspora. This thread bacterium was first described by F. COHN (XI.) in 1870, 

 and was subsequently also named Crenothrix Kiihniana. It is illustrated in 

 Fig. 65, which shows that the threads are sessile, each of them consisting of a 

 single row of short cells held together by a common tubular integument, called 

 a sheath. This latter is formed by the splitting of the cell membrane into two 

 layers, the outer one of each cell becoming merged into the adjacent membranes 

 above and below, and thus forming a uniform tube in which the cells can move 

 up and down. The threads are not cylindrical, but increase in diameter from 

 about half-way along their length towards the free end, so that the cross-sectional 

 diameter varies from 1.5 to 5 p. Moreover, as may be seen from the figure, the 

 various segments of the thread are of unequal length. As the cells multiply 

 rapidly, the upper members are forced out of the sheath, but are, for the most 

 part, already subdivided by the development of partition walls in all three 

 directions of space into small rounded cells, which thereupon make their 

 escape. The new cells vary in size according to the rapidity of this process of 

 subdivision, and are correspondingly distinguished by Zopf as micrococci (r), 

 and macrococci (q) (Fig. 65). Cohn proposed the terms microgonidia and 

 macrogonidia, because their method of formation bears a slight apparent re- 

 semblance to the endospores (known as gonidia) of certain Eumycetes, with 

 which we shall become acquainted in the second volume. The dimensions of these 

 cocci vary between i p and 6 /*. Their cell-walls swell up readily, and unite to 

 form zoogloea (f) up to i cm. in diameter. Under favourable conditions the 

 cocci then grow, by repeated subdivision and sheath-formation, into the thread 



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