BACTERIA. 81 



With regard to the chemical composition of bacteria, a 

 number of analyses have been published. Before the analyses 

 were made, the growth was thoroughly washed to remove, as 

 far as possible, every trace of the culture medium. They 

 show a content of about 85 per cent, of water, 8 to 14 per cent, 

 of albuminoids, 1 to 4 per cent, of fat and waxy bodies, and 

 about 1 to 2 per cent, of ash (sulphur, phosphorus, chlorine, 

 potash, lime, magnesia, iron, manganese, and silica). Their 

 composition is, however, obviously influenced to a considerable 

 extent by the nutriment. The slime formed by many bacteria 

 is either a compound of a carbo-hydrate and an albuminoid, 

 or a carbo-hydrate alone, as in the case of " frog-spawn }> 

 (Leuconostoc) and similar species. Bacteria contain a number 

 of enzymes of a more or less pronounced albuminoid character, 

 and this is also the nature of the various poisonous substances 

 which occur in several species. 



For nutrition, bacteria require carbonaceous and nitro- 

 genous compounds, as well as the inorganic substances found 

 in the ash. The majority of bacteria do not appear to possess 

 the power of building up their organic constituents from 

 inorganic material ; they are dependent upon those organic 

 compounds that have already been built up in animals and 

 plants. The nitrifying bacteria form an exception in that 

 they can directly absorb carbon dioxide from the air, and 

 the bacteria which occur on the nodules of the leguminosse 

 in like manner absorb nitrogen from the air and assimilate it. 



It has been customary to distinguish between Saprophytes 

 or organisms of putrescence and parasites which feed only upon 

 living animals and plants. A corresponding classification of 

 the bacteria has been attempted in a biological sense. Alfred 

 Fischer divides them into prototrophic, metatrophic, and para- 

 trophic (from the Greek, trophd, nourishment). 



By prototrophic bacteria are meant those, like the nitri- 

 fying, the iron and sulphur bacteria, which can take up these 

 substances in an inorganic form. The vast majority of bacteria 

 are metatrophic ; they utilise organic compounds of the most 

 varied kind, while they promote putrescence or fermentation. 

 Lastly, the paratrophic are parasites ; they do not occur in . 

 nature in a free state, but can only grow upon other forms of 



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