The effect of phagocytosis on the growth and survival of avian tubercle bacilli in embryonic chicken tissue cultivated in vitro

Abstract

1. Explants of embryonic chick tissue were infected with avian tubercle bacilli of different degrees of virulence to study (a) some of the factors on which control of infection in tissue cultures depends and (b) the viability of bacilli ingested by wandering cells.2. Embryonic lung cultures could control infection for 2–3 weeks owing to the intense phagocytic activity of the many wandering cells which they contained. The tissue appeared unharmed by infection with bacilli of either virulent or avirulent strains.3. The wandering cells largely protected the much less phagocytic fibroblasts from intracellular infection.4. This protective action of wandering cells towards less phagocytic cell types was limited by three factors: (1) the density of the infection relative to the volume of the explant, (2) the growth-rate of the infecting strain and (3) the proportion of wandering cells which the tissue contained.5. Even under the most favourable conditions control of bacillary growth in the tissue was only temporary, largely owing to the gradual disappearance of the wandering cells during repeated subcultivation.6. As control of bacillary growth was lost, the lung fibroblasts became packed with bacteria, but they appeared undamaged and might even undergo normal mitotic division. Eventually the ingested bacilli seemed to kill the cells by proliferating until they burst the cytoplasm.7. Embryonic avian tissue cultures though infected with a highly virulent strain never showed the rapid, widespread necrosis observed by Maximow in cultures of adult rabbit tissue infected with a virulent bovine strain. Possible explanations of this disparity are discussed.8. A special method was devised for testing the viability of bacilli phagocyted by macrophages. It showed that phagocyted bacilli, whether virulent or avirulent, survived for at least 21 days; when the host wandering cells were killed the ingested organisms were capable of immediate and profuse growth on suitable nutritive medium.