REACTION OF THE NUTRIENT MEDIA. 35 



no differential diagnosis is possible, and no variety can be 

 considered regularly described which is not tested in its 

 relation to all these nutrient media (with the exception of 

 glycerin-agar). 



More rarely the following nutrient media are employed : 

 Potato water, veal bouillon, fluid and coagulated blood serum, 

 serum-agar, ascites-agar, blood-smeared agar, meat, pieces of 

 bread, potato-pap, rice-pap, cooked or raw eggs. (See Tech- 

 nical Appendix. ) 



Uncooked, sterile organs of animals are actually poorer 

 nutrient media for most bacteria than when cooked (Liv- 

 ingood, C. B. xxni, 980 and 1002). Studies upon nutrient 

 media containing liver, kidney, thymus, adrenal extract, 

 etc., have been carried out, but without giving anything 

 of practical importance. Literature : (Wroblewski, C. B. 

 xx, 528). 



2. REACTION OF THE NUTRIENT MEDIA. 



As stated above, the great majority of bacteria, especially 

 the pathogenic, prefer a neutral or faintly alkaline nu- 

 trient medium, and formerly the advice was always given 

 to neutralize the nutrient medium with soda solution, em- 

 ploying sensitive litmus paper as the indicator — i. e., to 

 add alkali until red litmus paper was turned faintly blue. 



Every chemist knows that no accurate terminal reaction 

 for the titration of nutrient media containing phosphates 

 is obtained with litmus ; that, further, various litmus 

 papers influence the result ; and, finally, that the titration 

 is practically impossible with gaslight. As early as 1891, 

 N. K. Schultz had therefore advocated phenolphthalein 

 as an indicator in the titration of agar. He recommended 

 that 8-10 c.c. less of normal sodium hydroxid be added 

 than is required for complete neutralization with the indi- 

 cator. Such a medium is found to be suited to many bac- 

 teria, yet there are others which demand a complete neu- 

 tralization (C. B. x, 52). 



Without having noticed this proposal, I came upon the 

 same idea in 1892, during my investigations upon bread- 

 acids. Often since then, and exclusively since the autumn 

 of 1894, in my institute there has been employed as neutral 



