56 PHYSIOLOGY OF NUTRITION 



their nitrogen requirements: the power of utilising the nitrogen in different 

 compounds is largely dependent upon the form in which the carbon is 

 supplied. Thus, the ammonia bacteria can utilize the nitrogen of saltpetre 

 if sugar is present, but they are unable to do so if the carbon is presented 

 to them in the form of glycerine. No bacteria can dispense with nitrogen 

 entirely, and the feeble growth of B. pyocyaneus in sugar solution alone (13) 

 was probably due to the presence of impurities, or to the absorption of 

 ammonia from the air of the laboratory. 



The first attempts to characterise bacteria according to their behaviour 

 towards different nitrogen compounds were made by Nageli, who also 

 examined yeasts and mould fungi in the same way. The subject has been 

 still more recently studied by Beyerinck (28). 



In medical bacteriology, fluid culture-media are now very rarely 

 employed, having been driven out by the stereotyped recipes for peptonised 

 gelatine and agar. That this has been disadvantageous there can be no 

 doubt. The case of B. typhi and B. coli, the one an amido-bacterium, the 

 other an ammonia bacterium, shows the value of fluid media of vaiying 

 composition for the differentiation of species. Not less important are the 

 conclusions which may be drawn from such investigations as to the 

 occurrence of pathogenic bacteria in nature. This subject will be more 

 fully discussed in later chapters. 



The form in which nitrogen is offered affects greatly the influence 

 which the reaction of the culture medium has upon the growth of the 

 bacteria. The more assimilable the nitrogen compounds, the less sensitive 

 are the bacteria to the deleterious effect of free acid. B, pyocyanetts, as the 

 table shows, is, when it gets its nitrogen in the form of asparagine (5-6), 

 unaffected by the acidity of the medium which palpably retards growth 

 when ammonium chloride is the only source of nitrogen. The extremely 

 sensitive V. cholerae is totally overpowered by free acids (4 and 6), 

 while B. subtilis, which grows well on weakly acid hay-infusion, is less 

 marked in its behaviour, and B. coli seems to be quite uninfluenced by the 

 reaction. 



The formation of pigment too, in B. pyocyaneus, as in other chromogenic 

 bacteria, is largely dependent upon the reaction of the medium and the 

 nature of the nitrogenous bodies it contains. The organic compounds of 

 nitrogen, although they all of them contain carbon also, are not in themselves 

 sufficient for vigorous growth, as a comparison of Nos. i, 2, 3 and 5 of the 

 table shows. The carbon of ammonium tartrate (8), for instance, is 

 absolutely useless to B. subtilis, and even B. coli and B. pyocyaneus are 

 half starved when they have no other carbon-containing compound to draw 

 upon. The value of a special source of carbon, so clearly shown in the 

 table, is twofold : the excess of organic carbon is used in the building up 

 of proteids; and, more important still, the breaking down and oxidation of 



