The broad host range antibiotic resistance plasmid pAM,B1 was transferred from Streptococcus faecalis to 9 of 15 Listeria monocytogenes strains by conjugation. L. monocytogenes transconjugates could transfer the plasmid either among L. monocytogenes strains or back to S. faecalis. Transfer between the various strains occurred without any detectable plasmid DNA rearrangements. The pAMP1 replicon was stable in L. monocytogenes-it was retained without antibiotic selection when the bacteria were grown in culture media or passed in mice-and the presence of pAMP1 had no major effect on L. monocytogenes virulence. These data suggest that pAM,B1 or its derivatives might serve as useful L. monocytogenes cloning vehicles. The data presented also demonstrate that pAMP1 is compatible with two different native L. monocytogenes plasmids and that Listeria species harbor native plasmids in addition to the 38.5-megadalton plasmid pRYC16 previously reported by Perez-Diaz et al. (J. C. Perez-Diaz, M. F. Vicente, and F. Banquero, Plasmid 8:112-118, 1982). Of 29 L. monocytogenes strains screened, 7 contained plasmid DNA. Four strains had similar if not identical plasmids that were 34 megadaltons in size, whereas three other strains contained either a 53-, 44-, or 32-megadalton plasmid; none of these plasmids has the same restriction pattern as pRYC16. DNA homology experiments indicate that the various plasmids are related and suggest that there may be a common set of sequences present in all of the plasmids examined.

In the mouse system, acquired resistance to Listeria monocytogenes can only be demonstrated after immunization with viable microorganisms. A successful state of immunity cannot be elicited with formalin-killed organisms or bacterial cell-derived products. Viable, serologically cross-reactive organisms (not mouse pathogenic) do not induce a state of immunity as measured by acquired resistance. The duration of immunity, once established, is dose independent, and the absolute interval of its existence is not extended by secondary challenge with large numbers of viable organisms. The decline of immunity in actively immunized animals is not altered by antigenic challenge with formalin-killed cells or cell products. This indicates that the cellular requirements for the development of host resistance are similar for induction as well as maintenance. In vitro measurements of cellular immunity by migration inhibition indicate that formalin-killed organisms as well as cell products were recognized by actively sensitized lymphocytes obtained from immune animals.

A water-soluble extract of Listeria monocytogenes strain 10403 acts as a mitogen on cultured mouse spleen lymphocytes. This mitogen induced a response six to nine times that of controls, as measured by [3H]thymidine incorporation. The mitogen extract was derived from washed bacterial cells which were mechanically disrupted with a French press. The extract was centrifuged at 105,000 x g and filtered through a 0.22-,um filter. Similar levels of lymphocyte stimulation were observed in lymphocyte cultures prepared from spleens of nude mice, indicating the effect of this mitogen on B-cells. The mitogenic property of this extract was destroyed by heating to 560C. This heat treatment does not destroy the antigens in the extract, which stimulate spleen cell cultures obtained from specifically immune mice. Similarly prepared extracts from Staphylococcus epidermidis and Salmonella typhimurium did not show similar levels of mitogenic activity. The mitogenic property of the L. monocytogenes extract was present in two strains of Listeria tested and was not associated with mouse virulence.

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Progressive myelopathy in the German shepherd dog is a degenerative neurologic disease of unknown etiology. Results presented in a previous study indicated a depression in the response to thymus-dependent mitogens by peripheral blood leukocytes obtained from dogs with progressive myelopathy. Data presented here indicate that this depressed response to mitogens was associated with the presence of peripheral blood suppressor cells. Suppressor cell activity was detected in dogs that were severely affected with PM, but was not apparent in dogs that were mildly affected. Peripheral blood leukocytes obtained from dogs with progressive myelopathy suppressed the mitogenic response to autologous lymph node cells as well as allogeneic normal canine peripheral blood cells. The suppressor cells had the capacity to suppress mixed leukocyte reactions. Suppressor cell activity was radioresistant. Both nylon wool-adherent and -nonadherent peripheral blood leukocyte populations contained suppressor activity. Suppressive activity diminished after incubation of the suppressor cells with indomethacin, suggesting that suppression may be mediated by the release of prostaglandins. Although a role for peripheral blood suppressor cells in the disease process has not yet been established, it is possible that this abnormal regulatory activity reflects an attempt by the host to control an autoimmune event.