INTRACELLULAR SURVIVAL AND MULTIPLICATION OF VIRULENT AND LESS VIRULENT-STRAINS OF STREPTOCOCCUS-BOVIS IN PIGEON MACROPHAGES

The intracellular fate of pigeon S. bovis strains ingested by macropha ges was studied in vivo and in vitro. During in vivo experiments, hist ological and electron microscopical examinations demonstrated numerous cocci, which appeared to be actively multiplying, within splenic macr ophages of pigeons experimentally inoculated with a highly virulent S. bovis serotype 1 strain. In pigeons inoculated with a low virulence s erotype 3 strain, intracellular cocci were only occasionally observed. For in vitro experiments, pigeon peritoneal macrophages were inoculat ed with a S. bovis serotype 1 or serotype 3 strain and incubated. Foll owing an initial decrease, an increase in the number of intracellular bacteria was observed in tests performed with the S. bovis serotype 1 strain, demonstrating intracellular multiplication. Macrophages in the se experiments had all died after 7 h of incubation, possibly indicati ng that the intracellular replication of S. bovis resulted in the rele ase of substances toxic for macrophages. In experiments performed with the S. bovis serotype 3 strain, the number of intracellular bacteria continuously decreased, reflecting killing of organisms. Significant c hanges in the number of adhering macrophages in S. bovis serotype 3 in oculated cultures were not observed. These results indicate S. bovis i n pigeons is a facultative intracellular bacterium and intracellular m ultiplication may be involved in virulence.