FOOD STUFF* 57 



the carbon compound provides the necessary energy for the full utilisation 

 of inferior nitrogenous .substances (u and 12). 



The organic carbon compounds are of very different degrees of import- 

 ance as food-stuffs, their value depending mainly but not exclusively upon 

 their heat of combustion. Grape sugar and sugars in general are the best, 

 then glycerine and other polyacid alcohols, such as mannite and dulcite, 

 and finally compounds like tartaric and succinic acid, benzoic acid, and the 

 monacid alcohols and their derivatives such as fatty acids and amines*. 

 Compounds, in which an oxygen atom is linked by t\vo valencies to the 

 carbon atom, as in urea CO(NH 2 ) 2 , or oxalic acid (COOH) 2 , are useless as 

 sources of carbon, as are also the cyanogen compounds. It would appear, 

 therefore, that the carbon is in its most utilisable condition when it is linked 

 with hydrogen in the form CH 2 . being less valuable as CH, still less so as 

 CHOH, and not at all in the CO or CN radicles. Exceptions to this rule, 

 however, must not be forgotten. 



Speaking generally, the best data for the discrimination of a bacterial 

 species from the standpoint of nutritional physiology are those afforded by 

 its behaviour towards nitrogenous compounds, the demands made for carbon 

 being less narrow and well defined. 



The paratrophic bacteria are, as our table shows, able to grow only in 

 those solutions that contain peptone. They approach in this respect most 

 closely to the metatrophic forms, but their requirements are often much more 

 special. The gonococcus, for instance, grows only upon albuminous media, 

 such as coagulated blood serum. Upon this too the diphtheria bacillus 

 thrives best. The tubercle bacillus, which we are obliged for the present to 

 regard as a strictly parasitic species, is able to grow upon inferior substrata, 

 even on those of the ammonia bacteria (see Chap. XVI). 



For the culture of bacteria there are commonly used, besides bouillon 

 and other fluids, the so-called solid media prepared with gelatine or agar. 

 The commonest recipe is an infusion of meat (i Ib. meat to I litre water), 

 with about 1-2 per cent, peptone and sugar, to which 10 per cent, gelatine 

 or 1-2 per cent, agar is added, the whole being boiled and filtered hot. 

 This gives a clear gelatinous mass, in which the nutritive substances are 

 equal ly distributed throughout the non-nutrient gelatine or agar. The 

 introduction of such culture media (29) has been an important factor in the 

 rise of bacteriological science, for it is only by the use of easily melted 

 and easily congealed substrata that we are able to isolate bacteria without 

 difficulty, and obtain them in pure cultures. 



The nutritive fluids mentioned in the table on p. 55 may of course be 

 mixed with gelatine or agar, and the advantages of several methods united. 



* For details, a re-examination of which is desirable, see Nageli ; appendix No. 28. 



