Susceptibility of the Sage Brush Vole, Lagurus curtatus, to Listeria monocytogenes.

Once isolated, Listeria monocytogenes is easily grown on ordinary bacteriological media; however, initial isolation from clinical specimens may be unsuccessful unless the organism is present in large numbers, or the specimen is refrigerated in broth for several days to several months. In the absence of refrigeration, organisms were not recovered from some specimens showing histological evidence of L. monocytogenes (Gray et al., J. Bacteriol. 55:471, 1948). Inoculation of animals with suspected material and subsequent recovery of L. monocytogenes has not been rewarding (Seeliger, Listeriosis, Hafner Publishing Co., New York, 1961). Although the white mouse, rabbit, and guinea pig can be infected experimentally with L. monocytogenes, the lethal dose is too large to be of value for recovery of small numbers of organisms. Dunaeva (J. Microbiol. Epidemiol. Immunobiol. 28:1268, 1957) reported that one L. monocytogenes organism was lethal to the Russian steppe lemming. As the lemming was not available, the native sagebrush vole, Lagurus curtatus, was drawn to our attention. A limited number of L. curtatus was recently acquired, enabling us to describe the lethal response of this vole to small doses of Listeria. The voles were divided into groups of four with individuals of each group selected at approximately the same weight. For comparative purposes, Swiss Webster mice (Rockland Farms) averaging 16 g each were inoculated with lower dilutions of the same culture used for the voles. All animals were inoculated intraperitoneally with 0.2 ml of the diluted culture. Livers of dead animals were examined grossly for focal lesions which were microscopically and culturally confirmed. The inoculum was prepared from L. monocytogenes 19303 grown overnight in Brain Heart Infusion (BHI; Difco) at 37 C. Different doses of organisms were prepared by serial dilutions in 1:10 BHI maintained in an ice bath. Different dilutions were selected to provide an estimated low dosage of organisms contained in a 0.2-ml volume for injection. Concurrently with injection, 0.2 ml of each dilution being inoculated was spread on each of four plates of Tryptose Agar (Difco) containing 1% dextrose. Colonies were counted after 48 hr at 37 C. Listeria do not clump or chain; consequently, the colony counts are highly reproducible.