Eberth, and after him several other authors, have, in cases of typhoid fever, observed, in the closed follicles of the intestines, and in the lymphatic glands and spleen, bacilli which were distinguished by their peculiar arrangement and defective reaction to aniline dyes. Gaffky has followed out the investigation and given a complete account of this form.

The bacteria are small rods with rounded ends (see Fig. 147) which are usually single or in pairs in the tissues, but may grow into longer threads. They possess very active motility, produced by means of numerous cilia, which are situated not only at the ends but on the sides of the bacilli. Spore-formation has not been conclusively demonstrated in the bacillus. The bacilli grow on various nutrient media, but especially on potato. They form on potato a thin layer, which to the naked eye only differs from the remainder of the surface in presenting a more moist appearance, but a small portion is seen under the microscope to present vast numbers of the active bacteria. This growth on potato is very characteristic, and, occurring as it does at the ordinary temperature, is very important as showing that these bacteria are not obligate parasites, but that they can grow outside the body. It has also been shown that they grow vigorously in milk (Wolffhiigel), and that they may be preserved and even grow in water.

The typhoid bacillus is difficult to stain, giving almost no colour with watery solutions of ordinary aniline dyes. With Loeffler's alkaline methyl-blue, and with Ziehl's carbolic acid and fuchsine solution, they are readily stained. Sections should be left twenty-four hours in the solutions. The preparations should be washed with water and not with alcohol.

The bacilli have not been unequivocally inoculated in animals, but the toxines from cultures have been found to produce symptoms, whether with or without the bacilli.

In man the bacilli have been found in the intestinal contents, in the closed follicles of the intestine, the lymphatic glands, and the spleen. They have been found also in the blood of typhoid patients. In the tissues mentioned they grow in little masses with intervals between. 1 hey are not found in every case, diminishing apparently as the disease advances. Hence they are uniformly met with in recent cases, in which numerous clumps of bacilli will be found in the swollen Peyer's patches and glands. In later cases, where ulceration has occurred, they will be found in the deeper layers of the patches, in the mucous membrane and muscular coat beneath the ulcer. According to Wright and Semple the typhoid bacilli are present in the urine in most cases of typhoid fever, and they are frequently absent from the stools. The bacillus coli communis has been mistaken for the typhoid bacillus in the stools. Sometimes the urine is actually turbid with typhoid bacilli. This seriously raises the question whether the intestine is really the main seat of the bacillus.

Fig. 147. - Bacillus of typhoid fever. x about 1000.

As the bacilli extend to the blood, it may be that the general symptoms of typhoid fever are due to this; but, as the extension is apparently not great, it is more probable that the bacilli form a poison in the intestine, glands, and spleen, which reaching the blood, produces these symptoms.

The propagation of typhoid fever occurs by multiplication of the bacilli outside the body and their subsequent ingestion. The faeces and urine we have seen may contain large numbers of bacilli. Considering how readily they propagate in milk and other media outside the body, it is not difficult to understand how they should frequently infect the ingesta and produce the disease in man. There are many epidemics which have been traced to contaminated milk.