Parachlamydia acanthamoeba enters and multiplies within human macrophages and induces their apoptosis

Parachlamydia acanthamoeba is an obligately intracellular bacterium that naturally infects free-living amoebae. It is a potential human pathogen and may survive in human macrophages. We studied P. acanthamoeba entry into, and multiplication within, human monocyte-derived macrophages. After 8 h of incubation, 80% of macrophages were infected with a mean of 3.8 P. acanthamoeba organisms per cell. Electron microscopy demonstrated that parachlamydiae were in an intracellular vacuole. After infection with living organisms, the number of parachlamydiae per macrophage increased 4 times from day 0 to day 4, whereas heat-inactivated parachlamydiae were eliminated during the same period. Quantitative PCR confirmed that P. acanthamoeba replicates within macrophages. Transcriptional activity of P. acanthamoeba was detected by reverse transcription-PCR targeting the gene encoding ADP-ATP translocase (tlc). P. acanthamoeba exerted a cytopathic effect on macrophages. When macrophages were infected with living bacteria, their number decreased significantly from day 0 to day 4 due to apoptosis, as shown by annexin-V binding and electron microscopy. This study shows that P. acanthamoeba enters and multiplies within human macrophages before inducing their apoptosis.