With the object of ascertaining the effect of the various species of bacteria contained in the dung upon skins, a large number have been isolated, and the effect of pure cultures in different media has been tried upon skin.[77] A number of the results have been published in the Journal of the Society of Chemical Industry. Professor H. Becker, who has done a great deal of this part of the work, is of opinion that the principal organisms concerned in the bating exist in the dog’s intestines, and belong to the group of coli bacteria. These are very widely distributed bacteria, and are found in the large intestines of mammals, and, as a consequence, in almost all soils, and in the mud of rivers and lakes. The principal variety is B. coli commune. [77] The subject is not so far removed from direct human interest as one might suppose. For many years Metchnikoff, in Paris, has been studying the bacterial flora of the human intestines. His theory is, that old age is caused by the poisonous products of these intestinal bacteria, and he proposes to counteract the effects of these poisonous organisms, by introducing into the system large quantities of lactic acid bacteria, either in the form of tabloids, or in the form of sour milk cultures. 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. [78] For the demonstration of flagellæ, the material is taken from a culture on solid media--a young agar culture is best. Make a dilution by Soyka’s method (p. 103), dry and fix on a cover-glass as described, p. 88, then proceed as follows:--