786 ELEMENTS OF WATER BACTERIOLOGY 



plates, and another green plate with twice this amount 

 the distribution of colonies is usually satisfactory. 

 (By ' two successive Endo plates ' is meant streaking 

 two plates without burning off the platinum spatulum.) 

 Sometimes even this amount will give sterile or 

 nearly sterile plates, but usually one of the plates will 

 be neither overcrowded nor sterile. 



" The use of 1.5 per cent agar allows some diffusion 

 of the color, but as most of the lactose-fermenting 

 types are inhibited, this is not as a rule troublesome. 

 Three per cent agar has been tried, but the dye, in 

 the concentration used for the 1.5 per cent agar, does 

 not act under these conditions. Any change in the 

 constituents of the medium will necessitate a readjust- 

 ment of the concentration of the dye. 



" The sample dye employed is BrilHant Green Extra 

 Crystals No. 366, from Bayer, 117 Hudson Street, 

 New York City. The concentrations of the dye may 

 have to be varied with different samples. 



" Sixty-six stools known to contain typhoid bacilli 

 (normal stools to which had been added small amounts 

 of carrier stools), were examined in parallel on Endo 

 and brilliant green agar with the following results: 



Endo 30 positive 36 negative 45% positive 



Green Agar. . . 57 positive 9 negative 86% positive." 



The use of the inhibitive anilin dyes like crystal 

 violet and malachite green has the disadvantage of also 

 inhibiting to some extent the development of the weaker 



