Fate of the DNA in plasmid-containing Escherichia coli minicells ingested by human neutrophils

Escherichia coli minicells containing the plasmid pSC101 (approximately 10 kb) or pBR322 (approximately 4 kb) were opsonized and incubated with human neutrophils. The neutrophils responded to the minicells as they would to native E coli: they ingested the minicells, discharged their granule contents into the minicell-containing phagosomes, and expressed a respiratory burst. After one hour of incubation, the fate of the ingested plasmid DNA was examined. No DNA degradation was detected by trichloroacetic acid precipitation or agarose gel electrophoresis. Moreover, when pBR322 recovered from ingested minicells was transformed into E coli, no mutations in either of the antibiotic resistance genes carried by the plasmid were detected out of many thousand transformants screened. These findings confirm the surprisingly limited effect of neutrophils on ingested DNA.