The impact of the Listeria monocytogenes large plasmid on its interaction with HeLa cells and Acanthamoeba polyphaga trophozoites

Plasmids have been studied in Listeria monocytogenes, and plasmids with different sizes and functions have been reported. However, the role of large plasmids in the ecology of L. monocytogenes, in particular their impact on bacterial interactions with eukaryotes, has not been fully understood. In this study, the potential role of L. monocytogenes large plasmid DNA for invasion and intra-cellular growth in eukaryotic cells was determined by comparing the growth of the plasmid-cured versus wild type in HeLa cells and Acanthamoeba polyphaga trophozoites. Forty nine bacterial isolates were tested and 55% of environmental isolates contained plasmids. However, plasmid DNA was not found in clinical isolates. Both the plasmid-cured and the wild type bacteria showed similar behaviors in co-culture with HeLa cells and A. polyphaga trophozoites. After 10 successive passages through HeLa cells, the majority of bacteria recovered lost plasmid DNA. Our findings suggested that L. monocytogenes may lose large plasmids while growing in the enriched intra-cellular environment in eukaryotic cells. The result of this study indicated that plasmid-associated determinants have no significant impact on bacterial survival during co-culture with A. polyphaga and HeLa cells under conditions tested.