CULTURE REACTIONS ON SPECTAL MEDIA 355 



growth usually of a brownish tint, sometimes with . a moist 

 surface, which rapidly spreads and becomes thicker. The 

 appearance on potato, however, varies much with the different 

 strains and also with the reaction of the potato. 



Culture Reactions on Special Media. — A great variety of 

 media has been used for the appreciation of special characters 

 in the b. coli. These reactions depend upon the capacities of the 

 organism to orginate chemical changes in a variety of substances. 



A. Fermentative Reactions on Carbo-hydrates. — B. coli shows 

 great powers of splitting . up carbo-hydrates with the formation 

 of acids, especially lactic acid, and gases, chiefly carbon dioxide 

 and hydrogen. 



Fermentation of Sugars. — As stated on page 81, litmus or 

 neutral-red peptone water, or dextrose-free bouillon in Durham's 

 tubes is used, the sugar to be employed being added in the 

 proportion of half to one per cent. The fermentative capacities 

 of the b. coli are very wide. It produces acid and gas in 

 lactose, glucose, lsevulose, galactose, maltose, raffinose, mannite, 

 dulcite, sorbite, and very frequently in cane sugar (saccharose). 1 

 It produces a similar change in the glucosides, salicin, and 

 arbutin. In cultures in gelatin made from fresh meat sometimes 

 bubbles of gas appear from the fermentation of the dextrose 

 present in the meat (Fig. 108 C), and if melted gelatin be infected 

 and shaken up, bubbles of gas form round the colonies developing 

 at room temperature. 



The reactions of b. coli in some media other than simple 

 sugar solutions likewise depend on sugar fermentation, and of 

 these are the following : — 



Curdling of Milk. — If the b. coli be grown in milk, preferably 

 litmus milk, acid is produced from the lactose present which 

 further curdles the milk. If litmus milk be used, the acid 

 reaction should be permanent when growth is allowed to go on 

 for some days. 



Measuring of Gas Formation. — As has been said, the gases produced 

 by the b. coli in fermenting sugars are chiefly carbon dioxide and 

 hydrogen. Many observers attach considerable importance, first, to the 

 amount of gas formed from a given quantity of glucose in a given time, 

 and, second, to the ratios of the two gases to one another, in such a 

 fermentation. For the observation of this, MacConkey recommends the 

 following method : fermentation tubes (p. 80, Fig. 32, e), with the 

 closed limb graduated, containing 2 per cent, peptone and 1 per cent. 

 glucose in tap water, are inoculated and incubated for forty-eight hours 

 at 37° C. The tube is allowed to cool and the total amount of gas noted. 



1 A strain of b. coli fermenting cane sugar was formerly referred to as 

 b. coli communior, but this differentiating term has been discarded. 



