MEDIA FOE SEPAEATING BACTEEIAL GEOUPS 49 



Media used for separating the Members of Bacterial Groups. 



A great number of media have been devised for use in 

 differentiating the members of the coli-typhoid and other 

 bacterial groups. The general feature of these media is that 

 they contain certain substances, often sugars, which tend to 

 bring out the special characters of the organism under in- 

 vestigation. Sometimes also substances are present which 

 inhibit the growth of bacteria other than those belonging to 

 the group. The following are the media which here deserve 

 most attention : — 



MacConkey's Bile-Salt Media. — These media were introduced for the 

 purpose of differentiating the intestinal bacteria, and have been exten- 

 sively used for the study of the b. coli, b. typhosus, b. dysenterise, etc. 

 The characteristic ingredients are bile salts and various sugars. The 

 stock solution is the following : Commercial sodium taurocholate, '5 

 gramme ; Witte's peptone, 2 grms. ; tap water, 100 c.c. (if distilled 

 water be used, '03. per cent, of calcium chloride should be added). The 

 solution is steamed for two hours, filtered wheu hot, allowed to stand for 

 twenty-four hours or till sedimentation has occurred, and filtered again. 

 For a liquid medium there is added to this '25 per cent, of a freshly 

 prepared 1 per cent, solution of neutral red and the sugar, — when glucose, 

 dulcite, or adonite is used, '5 per cent, is added, in the case of other 

 sugars 1 per cent. The fluid is distributed in Durham's fermentation 

 tubes and sterilised in the steamer for ten minutes on two successive days, 

 care being taken not to overheat the medium. 



For bile-salt agar 1"5 to 2 per cent, agar is dissolved in the stock 

 solution in the autoclave, if necessary cleared with white of egg and 

 filtered. Neutral red and a sugar are added, as in the case of the 

 liquid medium. It is well to sterilise it in flasks containing 80 c.c, 

 this being an amount sufficient for three large Petri capsules. When 

 this medium is used for examining urine or feces, plates are 'inoculated 

 as with Drigalski's medium {infra) ; for its use in water examinations, 

 see p. 152. 



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. Litmus is often used instead of 

 neutral red. 



Drigalski and Conradi's Medium. — This is one of the media used for 

 the study of intestinal bacteria, and especially for the isolation of the 

 typhoid group of organisms, (a) Three pounds of meat are treated with 

 two litres of water overnight ; the fluid is separated as usual, boiled 

 for an hour, filtered, and there are added 20 grms. Witte's peptone, 

 20 grms. nutrose, 10 grms. sodium chloride ; the mixture is then 

 boiled for an hour, 60 grms. finest agar are added, and it is placed in 

 the autoclave till melted (usually one hour) ; it is then rendered slightly 

 alkaline to litmus, filtered, and boiled for half an hour. (6) 260 c.c. 



