Frequency and conditions of spontaneous plasmid transfer from E. coli to cultured mammalian cells

After exposing mammalian cell cultures to E. coli carrying the complete cDNA of poliovirus 1 in a plasmid, formation of polioviruses ensued even when no eukaryotic transcription signals were contained in the plasmid. The age of the bacteria greatly influenced the frequency of gene transfer, which was low if the bacteria were harvested in the exponential stage, reaching a maximum at the stationary phase and declining afterwards. Since the gene transfer was inhibited by cytochalasin B, phagocytosis of the bacteria could be a first step for the transfer; however, other mechanisms could not be ruled out.