Etiology.— Bruce, in 1887, demonstrated the presence in the spleen in undulant fever of a special bacterium the— Micrococcus melitensis— and by a series of experiments proved that it was the cause of the disease. Unfortunately the bacterium occurs only sparsely in the general circulation (unless in the earlier stages, when the temperature is high), and therefore to search for it in the later stages of the disease does not aid in diagnosis; but pathologically Bruce's discovery is of great importance, as it enables us to say positively that undulant fever is a distinct disease, altogether different from either typhoid or malaria. The organism is present in abundance in the spleen pulp, and also in the lymphatic glands, in which it persists longer than elsewhere, and from both of which it can be separated by cultivation. Bruce found it in the spleen in ten fatal cases. His results have been confirmed by Hughes, Gipps, Wright, Durham, Bassett-Smith, and many others. Injections of pure cultures give rise to a similar disease in monkeys and other animals, from whose blood the micrococcus can be recovered, cultivated afresh, and, on injection into other animals, again give rise to the disease. In five recorded instances inoculation— intentional and accidental— of cultures of the micrococcus into man has been followed by the characteristic symptoms of undulant fever after an incubation period of from five to fifteen days.
The Micrococcus melitensis measures 0.33 μ in diameter. It occurs generally singly, often in pairs, sometimes in fours, but never, unless in culture, in longer chains. According to Gordon it possesses one to four flagella. It is readily stained by a watery solution of gentian violet, and is best cultivated in a 1½-per-cent. very feebly alkaline peptonized agar beef jelly; in this medium, soon after inoculation, it appears as minute, clear, pearly specks. After thirty-six hours the cultures become a transparent amber; later they are opaque. No liquefaction occurs.
At one time believed to be a delicate organism, recent investigations have shown that the micro-
