82 BACTERIOLOGICAL CHEMISTRY 



The denitrifying organisms use nitrate as a source of 

 nitrogen. At the next stage are those organisms which 

 cannot fix atmospheric nitrogen but can thrive on 

 ammonium salts ; of course, they also require organic 

 compounds as energy source and to supply raw materials 

 for synthesis. An extremely wide range of substances 

 may serve as the sole carbon source for many micro- 

 organisms. Of these carbohydrates and similar com- 

 pounds are most readily assimilated, whilst hydroxy- 

 acids, fatty acids and monohydric alcohols are pro- 

 gressively less easily utilised. Amino -acids can often 

 serve as both carbon and nitrogen source. Amines 

 are not very satisfactory as carbon sources. Nearly all 

 saprophytic organisms belong to this group ; as examples 

 may be mentioned bacteria which can decompose formic 

 acid and methyl alcohol, the genus Chromohacterium and 

 the genus Escherichia. It is of interest to note, in this 

 connection, that often organisms will not grow in syn- 

 thetic media if they are sown in only very small numbers, 

 but if a large inoculum is used, growth proceeds vigor- 

 ously. This may be due to the introduction of essential 

 growth factors, bacterial vitamins, which are absent 

 from the medium but present in sufficient quantity in 

 large inocula to allow growth to commence ; once the 

 organism has started it can synthesise sufficient of the 

 growth factor to allow of continued growth. An alter- 

 native explanation is that in sjmthetic media there is 

 not, initially, a sufficiently high concentration of carbon 

 dioxide to permit growth. It has been shown by several 

 workers that carbon dioxide is an essential prerequisite for 

 the growth of many organisms, of which Esch. coli is one ; 

 again, large inocula carry over sufficient carbon dioxide 

 to allow growth to start. The truth of this explanation 

 is borne out by the fact that Esch. coli grows quite 

 regularly from small inocula in synthetic media under 

 aerobic conditions where carbon dioxide is produced by 

 respiration ; but under anaerobic conditions, where 



