A spontaneous respiratory disease of mice incited by Friedländer like bacilli has been described. The bacilli inducing the disease, while morphologically and culturally indistinguishable from the usual varieties of Friedländer bacilli, are antigenically distinct from the common type strains known. The bacilli grow better in cultures at 23°C. than at 37°C.

The disease in mice displayed an incubation period of about 48 hours. About 50 per cent of an exposed population succumbed to septicemic and acute hemorrhagic, pneumonic processes. Among the exposed animals were some individuals which remained apparently well and carried the pathogenic bacilli in their nasal passages.

The several manifestations of the spontaneous disease were reproduced by instilling small numbers of the cultured bacilli into the nasal passages. 48 hours after inoculation, certain mice had already succumbed; the deaths continued to occur, so that by the end of the 2nd week, 70–80 per cent of the animals had died. Among the survivors certain carriers of the bacilli in the nasal passages occurred; a few appeared entirely refractory to the infection. The succumbing mice showed at autopsy and by culture septicemia and diffuse pneumonic inflammation.

No fluctuation in virulence could be detected in bacilli derived from mice while the disease was spreading spontaneously at the periods of epidemic rise, interepidemic interval, or postepidemic quiescence. Moreover, the bacilli cultured from the nares of apparently healthy carriers were equally pathogenic with those taken from the blood and lungs of animals succumbing quickly.

No rough colony variants were cultured at any phase of the spontaneous disease, although they were readily obtainable by artificial culture. The variant strains proved stable and of low virulence.

This Friedländer bacilli infection in mice takes several clinical courses, depending on variations in host reaction and not depending on bacterial variation. The particular type of infection manifested is determined by the degree of resisting power displayed by the infected animals at the moment that the infection occurred and progressed.

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