MACCONKEY'S BILE-SALT MEDIA 43 



on the agar during its exposure to the air. The plates are usually 

 inoculated by means of a glass spatula made by bending three inches of a 

 piece of glass rod at right angles to the rest of the rod. This part is 

 dipped in the infective material and smeared in all directions over the 

 surfaces of three or four plates successively without any intervening 

 sterilisation. The plates are again exposed to the air after inoculation 

 for half an hour and then incubated for twenty-four hours. At the end 

 of such a period b. coli colonies are 2 to 6 mm. in diameter, stained 

 distinctly red, and are non-transparent. Colonies of the b. typhosus are 

 seldom larger than 2 mm., they are blue or bluish- violet in colour, are 

 glassy and dew-like in character, and have a single contour. Sometimes 

 in the plates b. subtilis and its congeners appear, and colonies of these 

 organisms have a blue colour. Their growth is, however, more exuberant 

 than that of the typhoid bacillus, being often heaped up in the centre, 

 and the contour of the colony is often double. 



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. dysenteries, etc. 

 The characteristic ingredients are bile salts and various sugars. The 

 stock solution is the following: Commercial sodium taurocholate, 0*5 

 gramme ; Witte's peptone, 2'0 grammes ; distilled water, 100 c.c. 

 For a liquid medium there is added to this '5 per cent of a freshly 

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

 glucose is used 0'5 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. As with Drigalski's medium, it is well to sterilise it in flasks 

 containing 80 c.c., this being an amount sufficient for three Petri 

 capsules. When this medium is used for examining urine or faeces, 

 plates are inoculated as with Drigalski's medium (supra} ; for its use in 

 water examinations see p. 136. 



In the bile-salt bouillon the formation of both acid and gas is 

 observed if such formation occurs, and in the bile-salt agar acid produc- 

 tion is recognised by the red colour of the colonies of the acid-producing 

 organisms. 



MacConkey's original medium was a 1 per cent bile-salt lactose agar 

 with no indicator, and was used for the detection of intestinal bacteria 

 in water. Such a medium is unfavourable to all the common spore- 

 bearing organisms found in water, and by incubating at 42 C. tubes, in 

 which there is probably a mixed infection from such a source, the growth 

 of most other water bacteria is inhibited. B. coli and b. typhosus, on 

 the other hand, grow readily. With the former the surface colonies are 

 broad, irregular, and flat, of opaque colour, and with a small spot of 

 yellow or orange in the centre, and the colony is surrounded by a haze ; 

 the deep colonies are lens-shaped, of orange colour, and are likewise 

 surrounded by a haze. With the typhoid organism at the end of forty-eight 

 hours the surface colonies are small, round, raised, and semi-transparent, 

 while the deep colonies are lens-shaped, white, and have no surrounding 



1 Neutral red gives a deep crimson with acids and a yellow -red with 

 alkalies. 



