' Bacillus piliformis ' (Tyzzer) and Tyzzer’s disease of the laboratory mouse I. Propagation of the organism in embryonated eggs

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Abstract
A contagious disease of laboratory mice characterized by focal necrotic lesions of the liver was first described by Tyzzer (1917). This author described a pleomorphic sporing organism associated with the disease and called it ''Bacillus piliformis''. He found it intracellularly in hepatic cells bordering the necrotic foci and also in epithelial cells of the large intestine of mice showing liver lesions. Many cells contained large numbers of organisms, and Tyzzer concluded that multiplication was intracellular. B. piliformis has not been cultivated on bacteriological media and hitherto the only sources available lor experimental study of Tyzzer''s disease have been infected mouse liver and brain. With such material it is difficult to produce liver lesions in mice by parenteral injection unless the animals are treated with cortisone. This communication describes the isolation and serial passage of two strains of B. pilformis in embryonated eggs. One strain was propagated for 30 serial passages, 26 of which were made in eggs by yolk sac injection and the others in mice. A non-sporing variant obtained from this strain produced hepatic lesions in embryos and mice similar to those produced by the parent sporing strain. No evidence was obtained of extracellular growth of the organism and intracellular growth occurred almost exclusively in epithelial cells of the yolk sac endoderm and hepatic cells of the embryo. Although pleomorphic, B. piliformis has a distinctive morphology which was described in detail by Tyzzer. All the forms described by this author were observed in embryonated eggs. The vegetating form of the organism rapidly lost its infectivity in vitro and also in the egg following death of the embryo. This loss of infectivity proved a limitation to certain in vitro studies and no method of halting it in the fluid state was discovered. However, it proved possible to obtain sufficient survival of the non-sporing strain for inoculation purposes by preserving crude tissue suspensions at -75[degree]C. The spores survived heating at 56[degree]C but were usually inactivated at 65[degree]C. They survived well at room temperature and the organism was usually recovered by mouse and egg passage from inoculated eggs kept at this temperature for over a year. Penicillin was found to be highly effective in halting the progress of infection in eggs. In the course of the work there were indications of a cyclic variation in the susceptibility of embryonated eggs to infection, and on one occasion a significant difference was demonstrable between eggs obtained from different pens. The results obtained when yolk sac and chick embryo liver infected with B. piliformis were injected into mice are described in another communication (Craigie 1966).