CHEMICAL COMPOSITION OF BACTERIA, ETC. 57 



their individual investigation. Since the organisms are 

 so very small it is essential to grow them in large quantities 

 in order that appreciable amounts of their constituents 

 may be obtained. 



The usual methods are (1) to wash off the growth from 

 solid media, or (2) to separate the cells from a liquid 

 medium by means of a centrifuge or by filtration. Most 

 species of bacteria or yeasts grow well on the surface of 

 appropriate nutrient media rendered solid by the addition 

 of agar, from which the cells may be scraped or washed. 

 The mycelia of the moulds or lower fungi, however, fre- 

 quently grow into such solid media and resist removal. 

 Centrifugalisation is the simplest and quickest means of 

 separating the growth of yeasts or bacteria from liquid 

 cultures, and the deposit of cells can easily be washed 

 free from the soluble constituents of the medium. 

 Bacteria, particularly, are not easy to recover by filtra- 

 tion, since they are far too small to be retained by 

 ordinary filter papers, and if in any quantity soon clog 

 the pores of a porcelain filter candle. The moulds are 

 normally easy to obtain by filtration since they form a 

 compact mass of mycelium. 



The cells of the micro-organism, having been obtained 

 free from extraneous substances derived from the m^edium, 

 can be submitted to analysis in bulk to determine the 

 Avater content and the amount and nature of the mineral 

 constituents. Usually the organisms are submitted to a 

 fractionation in order to isolate the various types of 

 substance present. For instance, the bacterial gums may 

 be dissolved out in water ; fats and waxes may be 

 extracted mth such solvents as alcohol, ether, chloroform 

 or acetone ; nucleic acids are extracted with weakly 

 alkaline buffer solutions ; the ba.sic proteins can be 

 dissolved in acid buffer solutions . 



The residts of sucli analyses of micro organisms vary 

 very considerably, from organism to orga.nism, with the 

 conditions of growth and age of the organism and mth 



