NUTRITION OF HETEROTROPHIC BACTERIA 81 



of the synthetic power of the autotrophs and depend in 

 more or less degree on preformed organic material for their 

 existence. The degree of dependence varies considerably ; 

 organisms like Escli. coli can thrive on very simple 

 synthetic media containing a single carbon source, like 

 lactate or glucose, and a single nitrogen source, such as 

 an ammonium salt, together with the appropriate mineral 

 salts. Synthetic media are those which contain only 

 constituents of known composition and no proteins, broth 

 or similar components. Further along the scale are the 

 organisms like the diphtheria bacillus which will grow 

 on synthetic media, but which require a more or less 

 extended number of amino -acids. Some of these amino - 

 acids, for example, tryptophane and cystine, appear to 

 be essential, whilst others can be replaced by alternatives. 

 More exacting still are those organisms like the gono- 

 coccus and the influenza bacillus which demand the 

 so-called " enriched " media, containing blood or some 

 tissue fluid or extract, for their growth. Almost cer- 

 tainly these enriched media support growi^h because of 

 the gro^vth factors (see p. 98) which they contain. Finally, 

 there are the viruses which have so far lost their synthetic 

 powers that they can only live and grow in the presence 

 of living tissue, on which, it seems possible, the}^ depend 

 for their supply of ready-made cell constituents, or at 

 least for materials which are well on the way to being 

 the finished product. The less exacting organisms might 

 be compared with country people who bake their own 

 bread, and the exacting bacteria with town dwellers who 

 have lost the art of making bread and who must bu}^ it 

 ready made. 



The simplest heterotrophs, from the nutritional point 

 of view, are those which depend on an organic carbon 

 source but which can still use inorganic nitrogen, either 

 as gaseous nitrogen or as nitrate or as ammonia. The 

 nitrogen-fixing organisms may be free-living, like the 

 Azotobacter or symbiotic with plants like the RJiizohium. 



