MEDIA FOR SEPARATING BACTERIAL GROUPS 51 



tVie typhoid bacillus, — being often heaped up in the centre, — and the 

 contour of the colony is often double. 



Fawcus's Picric Acid and Brilliant Green Medium. —This is a 

 modification of Conradi's medium which has been used with great 

 success at the Royal Army Medical College in the investigation of 

 typhoid carriers. It is made as follows: To 900 c.c. tap water add 

 5 grms. sodium taurocholate (which is commercially prepared from ox 

 bile), 30 grms. powdered agar, 30 grms. Witte's peptone, 5 grms. sodium 

 chloride ; steam for three hours, clear with wl.ite of egg, filter through 

 cotton wool, and bring to a, reaction of +15 with normal lactic acid or 

 caustic soda, and sterilise. Dissolve 10 grms. lactose in 100 c.c. sterile 

 distilled water, and add to melted agar. Mix and filter through Chardin 

 paper, sterilise Carefully, and store in 100 c.c. flasks. For use, add to 

 each 100 c.c. flask 2 c.c. of a 1-1000 watery solution of brilliant green 

 and 2 c.c. of a 1 per cent, watery solution of picric acid. Pour into 

 large Petri dishes, and leave these to stand inverted at 37° C. till the 

 surface hardens. Inoculate as usual. Colonies of b. typhosus of twenty- 

 four hours' growth are of about 1 mm. in diameter, transparent and re- 

 fracting ; those of b. coli, on the other hand, have a deep green centre, 

 though later typhoid colonies may also present a pale green centre. 



Petruschky's Litmus Whey. — The preparation of this medium, which 

 is somewhat difficult, is as follows : Fresh milk is slightly warmed, and 

 sufficient very dilute hydrochloric acid is added to cause precipitation of 

 the casein, which is now filtered off. Dilute sodium carbonate solution 

 is added up to, but riot beyond, the point of neutralisation, and the fluid 

 steamed for one to two hours, by which procedure any casein which has 

 been converted into acid albumin by the hydrochloric acid is precipitated. 

 This is filtered off, and a clear, colourless, perfectly neutral fluid should 

 result. Its chief constituent, of course, will be lactose. To this, sufficient 

 Kubel-Tiemann solution of litmus is added, the medium is put into 

 tubes and then sterilised. (This is the original method, but it is 

 better, after the casein has been precipitated, to make the medium 

 slightly alkaline with the sodium carbonate and bring to the boiling- 

 point ; then filter, neutralise, add the litmus, and sterilise.) After 

 growth has taken place, the amount of acid formed can be estimated by 

 dropping in standardised soda solution till the tint of an uninoculated 

 tube is reached. 



Any one of these media in the hands of a worker accustomed 

 to its use will yield good results. MacConkey's medium is that 

 most used by British workers, and it has the merit of being 

 easily prepared. 'As the result of a considerable experience we 

 have found it most useful and reliable. Next to it we would 

 place Fawcus's modification of Conradi's brilliant green method. 



Browning's Brilliant Green Method. — In this method advantage is 

 taken of the fact that brilliant green has a greater inhibitory effect on 

 b. coli generally than on b. typhosus and the paratyphoid group of 

 bacilli. The amount of the dye necessary to bring about the desired 

 result is not a fixed quantity in each case, as it depends on the number 

 of organisms in the feces and also on the organic matter. A number of 

 dilutions of the dye are therefore used. Tubes of peptone water (Witte's 

 peptone 2 per cent, and sodium chloride '5 per cent.), each containing 



