Fig. 15.—B. Coli Commune. Stained to show Flagellae.
Lortet found it, along with other organisms, in the mud of the Lake of Geneva, at a spot where the water was chemically very pure. Dr. A. C. Houston, the bacteriologist of the Metropolitan Water Board, enumerates sixteen varieties of this organism, 80 per cent. of which produced acid and gas in lactose-peptone cultures, indol in peptone-water cultures, and when grown in milk produced acid and clot. The bacterium (Fig. [15]) resembles that of typhoid fever, and has frequently been mistaken for it. It is, however, much more resistent to destructive influences. It is a short bacillus, possessing flagellae, by which it moves more or less rapidly.
B. coli forms short rods 0·8 µ wide, 1 to 3 µ long. It moves somewhat slowly by means of flagellæ, which may be demonstrated by staining with Loffler’s method.[78] It grows equally well in absence or presence of air, that is, it is a facultative anaerobe. Although it will grow at room temperature, the optimum growth is at 37° C. In plate cultures the appearance of the colonies below the surface of the gelatin is quite different from that of the surface colonies. The former are small round colonies, about the size of a pin head; the latter spread into a whitish iridescent film, with irregular edges.
B. coli does not liquefy the gelatin. When grown in nutrient solutions containing sugars, it produces much acid, and at the same time gases are given off, consisting of CO2 and hydrogen. If the growth in this solution be allowed to continue a secondary fermentation ensues, and the culture eventually becomes alkaline.
Indol is produced by B. coli, and may be demonstrated by adding to 10 c.c. of the culture, 1 c.c. of a 1/50 per cent. solution of pure potassium nitrite; then adding a few drops of concentrated sulphuric acid, when, if indol be present, a red coloration (nitroso-indol) is produced. This bacterium reduces nitrates to nitrites. Cultivated in a 1 per cent. solution of peptone, to which 1/10000 per cent. of potassium nitrate has been added, after four hours at 37° C., the presence of nitrite may be shown; after the growth has continued for seventeen hours, the nitrite is further reduced to ammonia. Among other products of B. coli, Harden found lactic, formic, acetic, and succinic acids, ethyl-alcohol, CO2 and hydrogen.
In Germany, W. Lembke and H. Becker have specially investigated the bacterial flora of the dog’s intestines. Lembke, in 1896[79] cultivated the bacteria from the fæces of the dog, fed in various ways—bread, meat, and fat diet—and found B. coli constantly present, although the form of the individuals, as well as the colonies, and the intensity of the indol and gas formation, showed great variations.
The other species of bacteria present varied with the kind of food; this has a great influence on the flora of the intestines, which was found to be very different when the dogs were fed on bread to what it was when they were fed on meat.
Lembke describes two other species of bacteria closely resembling B. coli, one of which he calls B. coli anindolicum, which, as the name implies, gives no indol reaction; the other, B. coli anaerogenes, is non-motile, possesses no flagellæ, and differs from B. coli by the absence of gas production in the fermentation of sugars.