306                         Pasteurellosis in White Mice

Yusef [1935] employed the precipitation test to overcome certain obvious
difficulties in the agglutination test. He established three serological groups
in dealing with most of the strains used previously by Cornelius (loc. cit) , and
discussed certain unaccountable discrepancies when comparing his results with
the findings of the previous worker.

Smillie [1920] isolated Pasteurella organisms in wild rats. Attempts to
correlate the above by serological methods with Past. pestis, were
unsuccessful.

Thus it is apparent that the species specificity of the known members of
of the pasteurella group still remains unsettled. This is, perhaps, one of the
reasons why Topley and Wilson [1937] have stated that " there is still a disposi-
tion to treat the Pasteurella group as a convenient refuge for any small,
gram-negative bacillus whose identity has not been accurately ascertained ".

Conflicting as the opinions are in regard to classification, they are sufficient
to emphasise the fact that a very close relationship exists between these or-
ganisms and to demonstrate their distinct characters, if they do exist, most
delicate and reliable serological tests must be employed. A further difficulty,
however, in the way of serological study of these organisms arises from the fact
that they are but poor antigens, both for the excitation of antibody produc-
tion and for serological reaction in vivo.

In view of the opinions stated in the foregoing paragraphs, it would seem
that there is sufficient evidence to show that the mouse organism possesses the
common characters of the Pasteurella group. As regards its type, the mor-
phological, cultural, and biochemical characters indicate that it is similar to
Pasteurella muricida, Meyer and Batchelder [1926]. They have described in
detail the tests of 34 strains of rat pasteurella obtained in the course of plague
control work in Oakland and Alameda. They observed that maltose was
seldom split by fresh strains while old cultures repeatedly acquired the
property of fermenting it. But the organism under study fermented mal-
tose in about 24 to 48 hours. Such variations, however, are not uncommon
and are likely to appear within the boundaries of any species or strain.

The stalactile formation observed was suggestive of similarity with
Past. petis but this was ruled out by the absence of capsule formation and abi-
lity to ferment sucrose.

In discussing its relationship with Past. pseudotuberculosis rodentium
it would seem necessary to go a little further in detail. The mouse pasteurella
possesses the character of true animal pasteurella in its ability to ferment suc-
rose—a character sometimes exhibited by Past. pseudotuberculosis ro-
dentium strains [Topley & Wilson, 1937]. It is also non-motile but again Past.
pseudotuberculosis rodentium Group III is non-motile to which the organism
under study appears to have some cultural similarity but no serological affi-
nity. The absence of cross titre between mouse serum and Past. pseudotu-
berculosis rodentium antigens as compared with the demonstrable agglutination