CHEMICAL COMPOSITION OF BACTERIA, ETC. 63 



per cent, for young actively growing cultures to 15 per 

 cent, for old cultures. Certain yeasts, however, particu- 

 larly Torula lipofeni and Endomyces vernalis can produce 

 up to 60 per cent, of fat under favourable conditions ; 

 they have been used as a source of fats when more usual 

 supx3lies have failed due to w^ar conditions, for instance. 

 The fungi have lipoid contents from 4 to 41-5 per cent. 



As with other substances the lipoid content varies 

 with the conditions of growi^h of the organism. Media 

 rich in glycerol or sugar yield organisms with a higher 

 fat content than do media containing but little sugar. 

 Aeration usually increases the fat content. 



The Fats. — The fats appear to serve the purpose of 

 reserve material in many species, although the high fat 

 content of old yeast cells and of partially poisoned cells 

 of other types has led to the view that fat production may 

 be a sign of degeneration. 



Waxes and Higher Alcohols. — In view of the excej)- 

 tionally high lipoid content of the tubercle bacillus this 

 organism has naturally been the most studied as regards 

 such products. As will be seen later (Chapter XX), 

 this and other acid-fast organisms contain a variety 

 of waxes and alcohols. The diphtheria bacillus also 

 yields waxes, which have not yet been investigated 

 chemically. 



Sterols. — It has been claimed that the unsaponifiable 

 matter of the fat of some bacteria contains mixtures of 

 sterols in small amounts. They are quite common in 

 the yeasts, forming up to 20 per cent, of yeast fat. Yeast 

 is a commercial source of the sterol, ergosterol, used in 

 the manufacture of calciferol and synthetic vitamin-D. 

 Certain fungi also contain sterols, ergosterol being found 

 free and as the ester with palmitic acid. 



Phosphatides. — The phosphatides precipitated from 

 the " ether extract " by acetone are widely distributed 

 in micro-organisms, and, in fact, are probably present 

 to more or less extent in all of them. The non-acid fast 



