FATE OF LISTERIA-MONOCYTOGENES IN MURINE MACROPHAGES - EVIDENCE FOR SIMULTANEOUS KILLING AND SURVIVAL OF INTRACELLULAR BACTERIA

The intracellular survival of the ubiquitous pathogen Listeria monocyt ogenes was studied in primary cultures of bone marrow-derived mouse ma crophages. Bacteria were able to grow rapidly in these cells, with an apparent multiplication rate of about 40 min. Electron microscopy demo nstrated that intracellular bacterial replication was the consequence of simultaneous intracellular killing and replication of bacteria in t he same cells. Within the first hour following phagocytosis, most bact eria were destroyed in the phagosomal compartment to which they were c onfined. This was due to early transfer of hydrolytic enzymes to phago somes, undoubtedly via phagosome-lysosome (P-L) fusion, as demonstrate d by a quantitative analysis after staining for a lysosomal marker, ac id phosphatase. One hour after infection, about 14% of the bacteria we re free in the cytoplasm, in which they multiplied and induced actin p olymerization and spreading to adjacent macrophages, as in epithelial cells. By using the 2,4-dinitroanilino)-3'-amino-N-methyldipropylamine staining procedure, direct evidence is presented that all phagosomes were acidified immediately after phagocytosis, thus indicating that in traphagosomal bacteria were exposed to an acidic environment that migh t favor vacuolar lysis by listeriolysin O. Intracellular growth in mac rophages, therefore, appears to be the result of a competition between the expression of the hydrolytic activity of these cells following P- L fusion and the capacity of L. monocytogenes to escape from the acidi fied phagosomal compartment before P-L fusion has occurred. The findin g that concomitant intracellular killing and survival of L. monocytoge nes occurs in the same macrophages might explain the high immunogenici ty observed in vivo with live bacteria, as opposed to killed bacteria.