BACTERIA. 33 



frequently employed. -This maybe prepared by adding to , the nutrient solu- 

 tions (broth) either gelatine, or when the Bacteria are to be cultivated at 

 blood-heat serum of sheep's or calf's blood, agar-agar or carragen ; serum 

 alone may in itself serve as a nutrient medium. The so-called "plate-cul- 

 tures " are frequently employed, i.e. the germs are isolated by shaking them 

 with the melted liquid nutrient gelatine, which is then spread on a glass plate 

 and allowed to coagulate ; when later on the individual germs grow into 

 colonies, these remain separate in the solid substratum and it is easy to pursue 

 'heir further development. Similar plate-cultures may also be cultivated in 

 test-tubes and on microscopic slides. The slides and glass plates must be 

 placed in " moist chambers" free from Bacteria, By sowing a few cells (if pos- 

 sible one) by means of a fine platinum wire, pure cultures for further investi- 

 gation may be obtained. 



In order to prove the relationship between pathogenic Bacteria and certain 

 diseases, the experimental production of pathogenic Bacteria by the inoculation 

 of Bacteria from pure cultures into healthy animals, is very important. 



It has not so far been possible to establish a classification of the 

 Bacteria, as the life-history of many species has not yet been 

 sufficiently investigated. 1 The opinions of botanists are at 

 variance, in many cases, with regard to the forms of growth of a 

 particular kind. Some species are pleomorphic (many-formed) 

 while others possess only one form. 



The following Bacteria are Saprophytes : 



Cladothrix dichotoma is common in stagnant and running water 

 which is impregnated with organic matter ; the cell-chains have 

 false branching. According to Zopf, Leptothrix ochracea is one of 

 the forms of this species which, in water containing ferrous iron 

 (e.g. as FeC0 3 ), regularly embeds ferric-oxide in its sheath by means 

 of the activity of the protoplasm. Leptothrix odiracea and other 

 Iron-bacteria, according to Winogradsky (1888), do not continue 

 their growth in water free from protoxide of iron ; while they 

 multiply enormously in water which contains this salt of iron. 

 The large masses of ochre-coloured slime, found in meadows, bogs, 

 and lakes, are probably due to the activity of the Iron-bacteria. 



Those forms which, according to Zopf's views, represent the- 

 forms of development of Cladothrix dichotoma are placed together 

 in Fig. 30. A represents a group of plants, seventy times 

 magnified, attached to a Vancheria. The largest one is branched 

 like a tree, with branches of ordinary form; a specimen with, 

 spirally twisted branches is seen to the right of the figure, at 

 the lower part some small Leptothrix-like forms. .B shows the- 



1 On the classification of Bacteria see " System der Bakterien," by Dr. 

 W. Migula. Jena, 1897. 



W. B. D 



