The Interrelationships between the various members of the B. enteritidis—B. paratyphosus B Group of Bacteria

IN a series of investigations on the epidemic spread of bacterial infection among mice (Topley, 1919, 1921), it has repeatedly been noted that, if an epidemic be started among a mouse-population by feeding certain animals on cultures of B. gaertner and subsequently introducing other susceptible mice into the cage, a varying proportion of these latter may succumb to infection with an organism morphologically and culturally identical with B. gaertner, but differing sharply from it in regard to its agglutination reactions. This organism has been referred to in previous communications as belonging to the B. suipestifer group, using that term to include B. aertrycke and other closely related types. The object of the investigations already reported, and of others still in progress, has been to gain some knowledge of those biological laws which must govern the spread of epidemic disease. It has therefore been necessary to attempt an answer to the question whether we should regard an epidemic, during which mice have died from infection with each of these bacterial types, as a homogeneous outbreak, or whether we must consider each infection separately. While we are far from suggesting that the serological differences which have been demonstrated are without significance, yet we believe that for broader purposes of analysis the whole series of deaths which occur under such circumstances should be taken together, a procedure which has already been adopted in considering the results of earlier experiments (Topley, 1921). We have been led to this view by a consideration of the available evidence with regard to the interrelationships among the complex paratyphoid group, and of the ascertained facts regarding the serological complexity of such bacterial species as the Pneumococcus and the Meningococcus. We have also, in the course of the past three years, carried out a large number of experi-