Enhanced in vitro engulfment of Listeria monocytogenes by rabbit polymorphonuclear leukocytes in the presence of sera from immune rabbits.

The role of immune serum in the engulfment of Listeria monocytogenes by polymorphonuclear leukocytes (PMNLs) of rabbits was documented employing an in vitro assay. Three serovarieties of L. monocytogenes, viz. serovars 1/2a, 4a and 4b, were separately incubated with PMNLs of nonimmune rabbit and high titre homologous or heterologous antisera. Normal rabbit serum was used as control. The number of L. monocytogenes per neutrophil was counted in stained smears in each assay and opsonic indices were calculated. Higher opsonic indices, i.e. 2.50, 2.09 and 2.56 for the three strains respectively, were obtained when bacteria were engulfed in the presence of homologous antisera as compared to when the bacterial cells were incubated with heterologous antisera, opsonic indices being in the range of 0.87 to 1.63. These results are indicative of a possible role of specific serum factors in at least the internalization of L. monocytogenes by PMNls of rabbits and therefore in the host defenses against Listerial infections.