FAILURE OF ANIMAL PASSAGE TO INCREASE THE VIRULENCE OF LISTERIA MONOCYTOGENES.

Silverman, Sidney J. (U.S. Army Biological Laboratories, Fort Detrick, Frederick, Md.), James F. Drawdy, and Donald A. Kautter. Failure of animal passage to increase the virulence of Listeria monocytogenes. J. Bacteriol. 86:92-94. 1963.-Three strains of Listeria monocytogenes, JHH, A4413, and 53P380, were serially passed through mice, guinea pigs, or monkeys via the intracerebral, intraperitoneal, and intraspinal routes, respectively. The first two strains had been maintained on laboratory medium for some time; strain 53P380, received in the brain of a sheep that had died of listeriosis, was never inoculated into laboratory media. The virulence of the strains was tested by the intraperitoneal or respiratory route. An apparent increase in virulence for mice of strain JHH was observed after animal passage. However, if the same strain maintained on Tryptose Agar and Brain Heart Infusion broth was injected in a suspension of normal mouse brain or spleen and liver, there was no difference in the virulence of the strain between animal-passaged and media-grown cells. An adjuvant effect was noted also with brothgrown cells suspended in mineral oil (Bayol F) or in alcohol or acetone extracts of normal mouse brain. A decrease in virulence for guinea pigs occurred with strain 53P380 after a series of passages by the intraperitoneal route in the same animal.