THE COLON-TYPHOID GROUP 89 



Cultural Characters. On gelatin plates colonies appear 

 in one or two days, the growth being thin and lilmy, 

 irregular in shape, translucent at the margins, and moist 

 in appearance. Jn gelatin stab or shake cultures bubbles 

 of gas are produced. The gelatin is not liquefied. 



On serum and on agar greyish-white, moist, shining 

 growths form. On potato, if acid, a yellowish growth is 

 obtained, but the growth may be colourless if the potato 

 is not fresh. Milk is rendered acid, and curdled in from 

 one to three days at 37 C. without subsequent digestion 

 of the casein and without subsequent production of alka- 

 linity. When grown in peptone-water containing one 

 of the following substances glucose, laevulose, maltose, 

 galactose, arabinose, raffinose (?), lactose, mannite, 

 sorbite, dulcite or dextrin (cane-sugar, sometimes) acid 

 and gas are produced. The gas production varies in 

 amount. From the fact that an organism may lose its 

 power of forming gas from sugars while retaining its 

 power of forming gas from alcohols, Penfold concludes 

 that two enzymes are concerned (see also p. 28). The 

 gas produced from dextrose consists of hydrogen 2 volumes 

 and carbon dioxide 1 volume. Present American opinion 

 places no reliance on this formula. B. coli produces indole 

 in broth or peptone-water cultures, generally within two 

 days, but sometimes a week is necessary before it can be 

 demonstrated. It reduces nitrates to nitrites. In 

 neutral-red lactose media the acid formed produces a 

 rose-red colour. On solid media no further change of 

 colour takes place, but in neutral-red lactose broth, the 

 dye is reduced with production of a greenish-yellow 

 fluorescence. 



In the Voges-Proskauer reaction the organism to be 

 tested is grown for three days in 2 per cent, glucose broth. 

 Two or 3 c.c. of strong caustic potash are added, and a 

 pink eosin-like colour developing on exposure to air for 

 twenty-four hours constitutes a positive reaction. The 

 colon bacillus does not give this reaction. Rivas devised 

 a quick method of performing this test: 1-4 c.c. of a forty- 

 eight-hour culture is boiled with 5 c.c. of 10 per cent, sodium 

 hydrate solution, the reaction being hastened by blowing 

 air through the liquid or by shaking. 



By means of fermentation and other reactions over a 

 hundred types of colon bacilli can be distinguished. It 



