352 TYPHOID FEVER 



for the separation of the organism, the surface colonies are 

 somewhat large, and it may be irregular in outline, but 

 the deep colonies are smaller and lenticular in shape, and 

 under a low power of the miser oscope appear rather dense to 

 transmitted light. A similar growth occurs on blood serum. 

 On potatoes, in forty-eight hours, there is a distinct film of 

 growth 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 have been used for the appreciation of special characters 

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

 organism to originate 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 gas, chiefly carbon dioxide 

 and hydrogen. 



Gelatin Shake Cultures. If a tube of gelatin be melted, 

 infected with b. coli, shaken up, allowed to solidify, and kept 

 at room temperature, distinct growth of the organism occurs, 

 and round each little colony, bubbles of gas form, which in time 

 coalesce and give the tube a readily recognised appearance. 

 This phenomenon is due to the organism fermenting the sugars 

 originally present in the meat, and is thus to be grouped with 

 other carbo-hydrate reactions. 



Fermentation of Sugars. As stated on page 80, 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 

 glucose, lactose, laevulose, 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. 



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 



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

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



