CULTURE REACTIONS ON SPECIAL MEDIA 353 



for some days. A similar reaction is observed if litmus whey is 

 used (p. 51). 



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. 81, Fig. 36, c), with the 

 closed limb graduated, containing 2 per cent, peptone (Witte) 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. The bulb is then filled with 2 per cent, sodium hydrate solution, 

 the opening closed with the thumb and thoroughly shaken. After the 

 gas has been collected in the closed arm the thumb is removed and the 

 ratio of the hydrogen left to the original gas volume is read off. 



Voges and Proskauer's Reaction. This is a reaction which is 

 not given by the classical type of b. coli, but as it occurs with 

 many members of the coli group it may be described here. It 

 also depends on carbo-hydrate fermentation. A glucose peptone 

 solution tube is inoculated and growth allowed to take place for 

 three days. A solution of caustic potash is added and the tube 

 allowed to stand for twenty-four hours at room temperature. A 

 red fluorescent colour is produced, causing the medium to 

 resemble a weak alcoholic solution of eosin. 



B. Action on Neutral-Red. When b. coli is grown on neutral- 

 red lactose bouillon, a rosy red colour, the effect of the lactic 

 acid upon the dye, is at first seen. Frequently this is succeeded 

 by the appearance of a green fluorescence due to a direct action 

 of the organism upon the dye. This is evidenced by the fact 

 that the neutralisation of the lactic acid by an alkali does not 

 lead to a reproduction of the original alkaline tint in the 

 indicator. The reaction, however, varies with composition of 

 the medium, the important factors being the percentage of 

 sugar and the reaction. 



C. Production of Indol. The b. coli produces indol in 

 peptone water. The methods have been given on page 82, and 

 for the detection of the reaction the use of Ehrlich's rosindol 

 test is preferable ; (if the nitroso-indol reaction be used, a small 

 quantity of a nitrite must be added). Two peptone tubes 

 should always l>e inoculated, and if the reaction is not obtainable 

 in one after two or three days' growth, the other should be 

 incubated for from six to seven days and then tested. Where a 

 faint reaction is obtained, it is well to corroborate the presence of 

 indol by dissolving the rosindol out with amyl-alcohol as described. 



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