MEDIA FOR SEPARATING BACTERIAL GROUPS 51 



pathogenic organisms and also some that are pathogenic have their free 

 growth inhibited in bile-salt media. Thus, if any growth takes place 

 on this medium when inoculated with, say, water, the probability is that 

 the bacteria have been derived from faices, but of course further procedures 

 for their identification must be undertaken. 



When growth of a bacterium producing acid and gas occurs in neutral- 

 red fluid media the latter turns a rose colour, and gas appears in the 

 Durham's tube. Sometimes a fluorescent appearance is also observed, 

 the significance of which will be discussed in the chapter on b. coli. 

 With the neutral-red solid media the colonies of any organism giving 

 rise to acid will be of a rose-red colour. 



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 precipita- 

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

 solution is added up to, but not 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 

 liy dropping in standardised soda solution till the tint of an uninoculated 

 tube is readied. 



Eisner's Medium. This is another of the media introduced in the 

 study of the comparative reactions of the typhoid bacillus and the b. coli. 

 The preparation is as follows : 500 grammes potato arc grated up ill a 

 litre of water, allowed to stand over night, then strained, and added to 

 an equal quantity of ordinary 15 per cent, peptone gelatin which has not 

 been neutralised. Normal sodium hydrate solution is added till the 

 reaction is feebly acid to litmus, the whole boiled together, filtered, and 

 sterilised. Just before use potassium iodide is added so as to constitute 

 1 per cent, of the medium. Moore has used a similar agar preparation. 

 Here 500 grammes potato are scraped up in one litre of water, allowed to 

 stand for three hours, strained, and put aside over night. The clear 

 fluid is poured oil', made up to one litre, rendered slightly alkaline, 20 

 grammes agar are added, and the whole is treated as in making ordinary 

 agar. The medium is distributed in test-tubes 10 c.c. to each and 

 immediately before use, to each is added *5 c.c. of a solution of 10 

 gramnif> j.ntassium iodide to 50 c.c. water. 



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 

 i-;i>ily prepared. As the result of a considerable experience 

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

 would place r'a\vi-us's modification of Conradi's brilliant green 

 method. 



