Interaction of virulent and avirulent Listeria monocytogenes with cultured mouse peritoneal macrophages.

The interaction between smooth and rough Listeria monocytogenes and mouse peritoneal macrophages in culture was investigated. Initially, antibiotics were deleted from the culture medium, and no attempt other than the removal of unphagocytized bacteria by extensive washings was made to control extracellular growth. Under these conditions the monolayers were rapidly destroyed within an 8-h period, and this was associated with increases in the intracellular population of both strains. Extracellular viability counts revealed that washings failed to reduce the bacteria in the medium to less than 10% of the original inoculum. Continuous phagocytosis of Listeria which grew logarithmically in the maintenance media appears to account for the observed changes in the number of intracellular bacteria. The data also indicate that it is primarily the free bacteria in the culture medium which are responsible for the cytotoxic effects. In other experiments streptomycin-penicillin solutions were added to the maintenance media after an initial period of phagocytosis. In the presence of antibiotics, the total number of macrophages per field remained relatively constant, and no morphological alterations in the leukocyte cultures were observable. Extensive intracellular multiplication of either strain was not evident in fixed and stained cover slips. Viable intracellular counts reveal that after 24 h there is almost total killing of the rough variant, whereas the smooth strain tended towards complete survival.