460 A MANUAL OF BACTERIOLOGY 



straw-yellow or brownish-yellow, moist, thick growth, 

 with darkening of the potato, but if the potato is not 

 fresh and acid in reaction the growth may be colourless. 

 Milk is a good culture medium, and is curdled in from one 

 to seven days. This curdling is principally due, not to an 

 enzyme, but to the formation of a considerable amount of 

 lactic acid, though a milk-curdling enzyme has been 

 described by Savage 1 as being formed under certain 

 conditions. The gas which is produced in culture media 

 under anaerobic conditions consists of hydrogen and 

 carbon dioxide. Under aerobic conditions marsh gas is 

 stated to be also formed. The ratio of H to C0 2 is about 

 2 : 1 for dextrose and lactose. In broth it produces a 

 general turbidity without film formation, and the culture 

 gives the indole reaction on the addition of a nitrite in 

 twenty-four to forty-eight hours. Lead-acetate is not 

 darkened. 



The table, pp. 450 et seq., gives the fermentation reac- 

 tions of B. coli and of a number of organisms belonging 

 to the typhoid-colon group, for which the author is indebted 

 to Professor Castellani. 



B, coli is an active fermenter of many carbohydrates, 

 alcohols, and glucosides, 2 e.g. glucose, lactose, galactose, 

 mannitol and dulcitol, but not of adonitol. Cane-sugar 

 may or may not be fermented ; sometimes only acid is 

 formed, sometimes both acid and gas are produced. To 

 the variety producing both acid and gas from cane-sugar 

 Durham gave the name B. coli communior. Neutral red 

 in glucose broth is changed to a fluorescent yellow, and 

 Houston describes a typical B. coli as " flaginac," i.e. pro- 

 ducing fluorescence in neutral red glucose broth (fl), acid 

 and gas from lactose (ag), indole in peptone-water (in), 



1 Journ. PatJtol. and Bad., November, 1904. 



2 See Twort, Proc. Roy. Soc. Lond., B, vol. Ixxviii, p. 329 ; MacConkey, 

 Journ. of Hygiene, vol. v.. 1905, p. 333, and vol. vi, 1906, p. 385. 



