NATURALLY-OCCURRING VIRULENCE-ATTENUATED ISOLATES OF LISTERIA-MONOCYTOGENES CAPABLE OF INDUCING LONG-TERM PROTECTION AGAINST INFECTION BY VIRULENT-STRAINS OF HOMOLOGOUS AND HETEROLOGOUS SEROTYPES

Experimental infections of mice with strains of Listeria spp. isolated from contaminated food sources allowed discrimination of strains into those either exhibiting high, attenuated or low virulence. Compared t o the highly virulent L. monocytogenes strain EGD, an attenuated strai n such as L99 persisted for shorter times (5 versus 10 days) in the in fected host. Using a tissue culture cell model of infection, we found that, although strain L99 was capable of accumulating actin like its v irulent counterpart following invasion, it was unable to generate the polarized actin tails required for intracellular and cell-to-cell move ment. Immunoblot analysis using specific antiserum to the ActA polypep tide, a molecule that is necessary for movement of the bacterium withi n the eucaryotic cell, indicated that a slightly truncated form of thi s polypeptide was produced in the L99 strain. Despite its reduced viru lence, the attenuated strain L99 was just as effective in generating p rotection in immune mice as the highly virulent strains, albeit with a 1000-fold higher infective dose. Based on the results obtained from t his study, we suggest that one of the mechanisms accounting for widesp read resistance in humans to infection by Listeria may be due to asymp tomatic infections by naturally occurring strains attenuated for virul ence.