MEDIA FOR SEPARATING BACTERIAL GROUPS 51 



first group and b. typhosus to the second, and to these groups also belong 

 most ordinary organisms growing in faeces, practically none of which are 

 found in the third and fourth classes. Further, a number of ordinary 

 non-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 fseces, 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 precipitation 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 oiiginal 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. 



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 grms. potato are grated up in 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 percent, of the medium. Moore has used a similar agar preparation. 

 Here 500 grms. 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 off, made up to one litre, rendered slightly alkaline, 20 

 grms. 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 

 grms. potassium 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 

 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. 



