1908-9.] The Pathogenesis of Micrococcus melitensis. 
553 
Rat , Mouse. 
No serious observations as to the susceptibility of these small rodents 
appear to have been made until I first took up the subject on my arrival 
in Malta in April 1906. Speaking generally, the pathogenic effects observed 
in these animals after inoculation with M. melitensis are identical with 
those already observed in the larger rodents — rabbits and guinea-pigs— 
when once the organism has adapted itself to life in the body tissues of 
the rat and the mouse, and has acquired a certain degree of virulence for 
them — with this single but noteworthy difference, viz. that the rat and the 
mouse appear to be ill adapted for the elaboration of specific agglutinins to 
M. melitensis. These anti-bodies are consequently formed in very small 
amounts or not at all. Out of the scores of experimental rats and mice 
that I have examined, one white rat alone gave evidence of the presence of 
agglutinin, and even in this animal the titre of the serum did not rise 
above 1 : 40. 
Experiments were first undertaken in connection with the sewer rat. 
A black sewer rat, captured two days previously, apparently healthy and 
whose blood failed to yield evidence of the presence of specific agglutinins 
when tested against M. melitensis, was injected subcutaneously at the root 
of the tail with half of a two-days-old culture (emulsified in 075 c.c. 
normal saline solution). The strain of M. melitensis employed had been 
recently isolated from a fatal case of Malta fever in man by one of my 
colleagues. 
The animal was not obviously affected, but on the fourth morning after 
inoculation was found dead. Cultivations from the heart blood yielded a 
few colonies of M. melitensis; from the spleen, numerous colonies; and from 
the urine, innumerable colonies. 
With the culture thus obtained another sewer rat was inoculated intra- 
cerebrally (1 loopful of two-days agar culture in 01 c.c. normal saline). This 
animal was found dead the following morning, and a pure culture of 
M. melitensis was recovered from the spleen. 
These two experiments are somewhat discounted by the fact that sewer 
rats usually die off quickly in captivity — that is, in two or four days — and 
the successful recovery of the coccus from the cadaver might only have been 
possible by reason of post-mortem multiplication. 
Consequently, white rats were substituted for the sewer rats, and a 
series of intracerebral passages (always using spleen cultures) to the number 
of four were carried out. The experimental animals all died within twenty- 
four hours. The micrococcus was successfully isolated from the spleen of 
