CHEMICAL COMPOSITION OF BACTERIA, ETC. 59 



of phosphorus, especially in the acid-fast bacteria. The 

 ash of most bacteria contains 10 to 45 per cent, of phos- 

 phorus, that of the acid-fast bacteria 43 to 74 per cent, 

 and of yeasts 47 to 59-4 per cent. The ash of yeasts 

 also has a particularly high potassium content, averaging 

 38 per cent. 



Proteins. — Estimation of the protein content of micro- 

 organisms is usually based on the total nitrogen content 

 as determined by the Kjeldahl method, the nitrogen 

 value being multiplied by the factor 6-25. There are 

 several fallacies in this method. First of all the Kjeldahl 

 method estimates only about 85 per cent, of the total 

 nitrogen present, since the nitrogen of certain types of 

 compound (nitro-, nitroso-, azo- or azoxy-compounds 

 and such ring compounds as pyrimidines and purines) is 

 not capable of estimation by this method. Secondly, 

 it is assumed that all the nitrogen is present as protein, 

 which is not the case. Nor is it true that all proteins 

 contain 16 per cent, of nitrogen, the value on which the 

 conversion factor 6-25 is based ; 16 per cent, is only an 

 average value. 



The values recorded for nitrogen are 2 to 14 23er cent, 

 for bacteria, 5 to 12 per cent, for yeasts and 2-3 to 8-3 

 per cent, for moulds, corresponding to approximate 

 protein contents of 12-5 to 87-5 per cent., 32 to 75 per cent, 

 and 14 to 52 per cent, respectively. The carbon content 

 of each of the three groups of organism lies between 45 

 and 55 per cent., and assuming that all the nitrogen is 

 present as protein, and deducting the corresponding 

 amount of carbon, it can be seen that the bacteria are 

 relatively rich in proteins, whilst the moulds are richer 

 in the non-protein carbon compounds. 



Most of the protein of micro-organisms is, of course, 

 found in the protojDlasm of the cells. This protoplasm 

 is not homogeneous, however, as most cells contain 

 granules of nuclear material ; in fact, some bacterial 

 protoplasm seems to be entirely composed of nuclein, 



