In plasma cultures, with or without tissue, tubercle bacilli form characteristic streptothrix-like colonies of loosely twisted skeins of threads.
In plasma tissue cultures embryonal connective tissue and epithelial cells phagocytyze tubercle bacilli freely.
Splenic tissue cultures from fourteen day or older embryos if inoculated with isolated bacilli will phagocytyze and prevent the development of all or nearly all bacilli present. If heavy inoculations of tubercle bacilli with many bacillary clumps are made in splenic cultures the bacilli develop more freely than in similar connective tissue cultures from heart tissue in spite of marked phagocytosis. In such cultures the first change seen is a clustering of small round cells, interpreted as lymphocytes, around bacillary clumps and colonies followed by a similar clustering of larger parenchymatous polygonal cells with one or several nuclei, interpreted as epithelioid cells, which phagocytyze any bacilli with which they may come in contact. The larger cells containing bacilli then fuse to form larger multinuclear cells and these by further fusion form typical giant cells with quiescent nuclei in characteristic groupings and bacilli in degenerated protoplasm away from the nuclear groups.
Cells which have phagocytyzed bacilli may digest them or may be unable to do so, in which case the bacilli develop within the cells and eventually cause complete cell degeneration.