ESSENTIAL NUTRIENTS 63 



simpler compounds as foods ; and under other conditions bacterial strains may- 

 lose synthetic powers and become more heterotrophic. It is impossible to say 

 of a given bacterium whether it has descended from an autotrophic or a hetero- 

 trophic ancestor. 



Among groups of bacteria that are closely related on other grounds, there are 

 grades of nutritional relationships that may, perhaps justifiably, be regarded as 

 stages in evolution. But with bacteria as a whole, we must beware of regarding 

 the nutritional series exclusively as the trunk of an evolutionary tree, when it is 

 perhaps more likely that each nutritional group represents the end results of 

 evolutionary branches whose common origin is now lost to us. 



Moreover, as van Niel (1943) suggests in a review of the biochemistry of the 

 autotrophs, the distinction between heterotrophs and autotrophs is not only 

 difficult, but sometimes illusory. As an example he quotes two bacteria utiUzing 

 molecular hydrogen, which, in mixed culture, had the nutritional requirements 

 of an autotroph, but which, in fact, were both heterotrophs requiring thiamin. 

 One bacterium supphed the other with pyrimidine for the synthesis of the vitamin, 

 and was in its turn rewarded by the supply of thiazole for the same purpose. It 

 is possible that the present-day autotrophs are ultimately as dependent on hetero- 

 trophs as heterotrophs are on autotrophs, though the dependence in most cases 

 is not as immediate and apparent as in this obvious example of interdependence. 



Essential Nutrients and Growth-stimulating Substances. — The bacteria with 

 which we are concerned in this book are mainly heterotrophs in Stages 3 and 4. 

 The elucidation of the growth requirements of many pathogenic species of bacteria 

 is still incomplete, but in recent years the nutrition of a number has been studied 

 in detail. The media usually employed for the cultivation of bacteria have been 

 derived empirically, and for the most part contain complex organic matter of 

 animal origin, like peptones, meat extracts, serum or egg albumin. With increas- 

 ing biochemical knowledge of the constitution of organic materials, it has been 

 possible to simplify many of these media to the point where all the constituents 

 are chemically defined, and in certain cases to build up the media with ingredients 

 that have been synthesized in the laboratory. Both " synthetic " and " defined " 

 ingredients should whenever possible be tested, for both may contain chemically 

 undetectable traces of organic substances which may be accessory growth-factors ; 

 but since it is unlikely that both preparations contain the same impurities the 

 probability that the observed result is due to a contaminant is thus reduced. 



With a battery of amino-acids, carbohydrates, and vitamins, a simple basal 

 solution of inorganic salts may be enriched until the mixture supports the growth 

 of bacteria. Alternatively, solutions of peptone, or watery extracts of yeast, 

 may be fractionated, and the fractions found necessary for growth chemically 

 identified. The similarity of metabolic processes in all Uving cells, whether fungal, 

 bacterial, plant or animal, often makes possible a short cut in the identification 

 of nutrients in culture media, especially with vitamin-like substances. A particular 

 fraction, before the active principle is identified, may exhibit the properties of a 

 known animal or yeast vitamin, and tests with the known vitamin rapidly elucidate 

 the composition of the medium under analysis. But even though the minimum 

 growth-requirements of a bacterium have been established by these methods, 

 we have by no means necessarily defined the essential nutrients of the organisms. 

 The inorganic requirements may apparently be satisfied by the ions potassium, 

 sodium, calcium, magnesium, iron, phosphate, sulphate, carbonate and chlorides, 



