370 NATHAN BERMAN AND LEO F. RETTGER 



The quantitative determination of ammonia production in 

 nutrient broth has been extensively made use of in estimating 

 the comparative abiUty of various groups of bacteria to attack 

 protein. Among the staunch supporters of this method are 

 Kendall and his co-workers. Caution is necessary in draw- 

 ing conclusions from the results of this test, for an increase in the 

 ammonia content of a medium does not necessarily indicate 

 that the protein has been attacked. There are other sources of 

 ammonia in nutrient bouillon besides albumoses and peptones, 

 as for example amino acids and extractive substances. 



The biuret test was employed in these experiments, since 

 experience in the earlier work had proven its merit. It is of 

 value in demonstrating the disappearance by digestion or other- 

 wise of protein in a fluid medium. Peckham (1897) used this 

 test to show that B. coli is unable to utilize completely the 

 protein present in ordinary nutrient media. Vernon (1904) was 

 the first, however, to demonstrate the value of the biuret method 

 as a quantitative measure of peptone digestion, in his study of 

 the action of erepsin upon commercial peptone. 



The formol titration method of Sorensen (1908) was employed 

 in conjunction with the biuret test. This method has already 

 been applied to bacteriological problems. The titration figure 

 is a measure of the amount of primary amino acids present in a 

 medium. Rice (1915) claims to have found the Sorensen more 

 satisfactory than the Van Slyke method for the determination 

 of free amino nitrogen. 



The biuret test was employed in essentially the same manner 

 as described by Vernon (1904) and as given in a previous paper 

 from this laboratory (1916). The Sorensen method was con- 

 ducted as follows. Five cubic centimeters of the medium to be 

 tested were mixed with 50 cc. of tap water and neutralized with 

 n/20 NaOH or HCl. Two cubic centimeters of neutral for- 

 maldehyde were then added and the solution again neutralized, 

 with phenolphthalein as the indicator.' 



In the tables the formol titration values are recorded as the 

 number of cubic centimeters of n/20 NaOH required for 100 cc. 

 of the test medium. In the column under 'Reaction' a similar 

 plan has been followed. 



