THE PREPARATION OF CULTURE MEDIA 137 



Each bottle is mixed and poured into six plates only (a thick layer of agar 

 gives the most characteristic colonies). Plates are left uncovered until agar 

 has "jellied"; porous tops are used; dry plates are essential to avoid diffusion. 



Standardization: The agar must have proper "balance." The reaction is 

 important; sediment reduces the activity of the dye and light colored media 

 are better than darker ones. Different lots of agar with the same dye solution 

 act ununiformly; a new batch or a new solution must be tested. 



Any variation in the composition of the media necessitates a readjustment 

 of dye concentration; this statement cannot be over-emphasized. 



Brilliant green, in appropriate dilutions, not only inhibits all Gram- 

 positive and many Gram-negative bacteria, but exhibits differential 

 action on the colon-typhoid group. Paratyphoid and the B. lactis 

 aerogenes are untouched, typhoid is restrained only at low dilutions, 

 while dysentery and the other colon group are extremely susceptible. 

 The typhoid colony on this medium is characteristic. Looking through 

 the plate against a dark surface, in oblique light the colony has a snow- 

 flake appearance; the edge delicately serrate. With artificial light and 

 a hand lens, the texture is that of a coarse woolen fabric. Acid produc- 

 tion from the trace of glucose may tinge the colony. The colony is large. 



Malachite-Green Bouillon (Peabody and Pratt). 1 To 100 c.c. of beef in- 

 fusion broth add 10 c.c. of one per cent solution of malachite green, Hochst 

 120, made with sterile water. This is tubed. 



This medium is used as an enriching fluid. One drop of the suspected 

 material (emulsified stool) is added to each tube and after incubation 

 for eighteen to twenty-four hours inoculations may be made upon plates. 



Peabody and Pratt found a reaction of .5 per cent acidity to phenol- 

 phthalein most favorable. 



Bile Medium 2 . (Recommended -for blood cultures by Buxton and 

 Coleman.) The medium is prepared as follows: 



Ox-bile 900 c.c. 



Glycerin 100 c.c. 



Pepton 20 grams 



Put into small flasks containing quantities of about 100 c.c. each and sterilized 

 by fractional sterilization. 



Jackson's Lactose-Bile Medium. 3 This medium is of great use in 

 isolating B. typhosus and B. coli from water, and serves as a valuable 

 enriching medium in isolating them from other sources. Jackson and 



1 Peabody and Pratt, Boston Med. and Surg. Jour., clviii, 7, 1908. 



2 Conradi, Deut. med. Woch., 32, 1906. 



Jackson, "Biol. Studies of Pupils of W. T. Sedgwick," 1906, Univ. Chicago Press. 



