12 INTRODUG TION 



Studies to determine the forms of carbon, hydrogen, oxygen, and nitrogen 

 which could most easily be utilized by bacteria for their development were 

 originally carried on by Naegeli between 1868 and 1880, and were published by 

 him^ in the latter year. Naegeli's report covered the use of a large variety of sub- 

 stances including carbohydrates, alcohols, amino acids, organic nitrogen com- 

 pounds, and inorganic nitrogen salts. 



The first reference to the use of peptone for the cultivation of microorganisms 

 is that made by Naegeli in the report referred to above, when in 1879 he compared 

 peptone and ammonium tartrate. Because of its content of amino acids and other 

 nitrogenous compounds which are readily utilized by bacteria, peptone soon be- 

 came one of the most important constituents of culture media, as it still remains. 

 In the light of our present knowledge, proteins are believed to be complex com- 

 pounds of amino acids joined together mainly by means of the peptide linkage. 

 When subjected to hydrolysis proteins yield metaproteins, proteoses, peptones, 

 polypeptides, and finally the chemically simpler amino acids and their analogs. 

 The intermediate products should be considered as classes of compounds, rather 

 than individual substances, for there exist no sharp lines of demarcation between 

 the various classes — one group shades by imperceptible degrees into the next. All 

 bacteriological peptones, thus, are mixtures of various products of protein hy- 

 drolysis. Not all the products of protein decomposition are equally utilizable by 

 all bacteria. In their relations to proteins, bacteria may be divided into two 

 classes; those which decompose naturally occurring proteins, and those which re- 

 quire simpler nitrogenous compounds such as peptones and amino acids. 



The relation of amino acids to bacterial metabolism, and the ability of bacteria 

 to use these compounds, have been studied by many workers. DuvaP.^, for ex- 

 ample, reports that cysteine and leucine are essential in the cultivation of Myco- 

 bacterium leprae. Kendall, Walker, and Day* and Long^ report that the growth 

 of M. tuberculosis is dependent upon the presence of amino acids. Many other 

 workers have studied the relation of amino acids to the growth of other organisms, 

 as for example, Hall, Campbell, and Hiles^ to the meningococcus and strepto- 

 coccus; Cole and Lloyd''' and Cole and Onslow^ to the gonococcus; and Jacoby 

 and FrankenthaP to the influenza bacillus. Indispensable as amino acids are to 

 the growth of many organisms, certain of them in sufficient concentration may 

 exert an inhibitory effect upon bacterial development. 



From the data thus far summarized, it is apparent that the problem of bacterial 

 metabolism is indeed complicated, and that the phase concerned with bacterial 

 growth and nutrition is of the utmost practical importance. It is not improbable 

 that bacteriological discoveries of the greatest etiological value await merely the 

 evolution of suitable culture media and methods of utilizing them, just as in the 

 past important discoveries were long delayed because of a lack of similar require- 

 ments. Bacteriologists are therefore continuing to expend much energy on the 

 elucidation of the minutiae of bacterial metabolism, and are continuing to seek 

 methods of applying, in a practical way, the results of their studies. 



While the importance of nitrogenous substances for bacterial growth was 

 recognized early in the development of bacteriological technique, it was also real- 

 ized, as has been indicated, that bacteria could not always obtain their nitrogen 

 requirements directly from protein. It is highly desirable, in fact essential, to 

 supply nitrogen in readily assimilable form, or in other words, to incorporate in 

 media proteins which have already been partially broken down into their simpler 

 and more readily utilizable components. Many laboratory methods, such as 

 hydrolysis with alkali^^, acid^^'^-'^^, enzymatic digestion^^'^'^^'^^'^^-^^, partial di- 

 gestion of plasma^^, et al., have been described for the preparation of protein 

 hydrolysates. 



The use of protein hydrolysates, particularly gelatin and casein, has stimu- 



