Invasion of Phagocytic and Non-Phagocytic Cells by ‘Candidatus Berkiella cookevillensis’

Abstract

Bacterium ‘Candidatus Berkiella cookevillensis’ (strain CC99) was isolated from an amoeba in cooling tower biofilm. CC99 is an obligate intracellular pathogen growing within vacuoles closely associated with the host cell nucleus and typically lysing its amoeba host in 3-4 days. CC99 that lyse from amoebae can infect both THP-1 and U937 human macrophage-like cell lines, but whether bacteria from human cell lines are transmissible to other human cells had not been investigated. In this study, CC99 bacteria lysed from amoebae productively infected both THP-1 and HeLa human epithelial cells, as evidenced by nuclear-associated vacuoles filled with bacteria. At least 50% of the cell populations were positive for CC99 at 72 hours as determined by histochemical staining. Furthermore, CC99 DNA was detected in these treated cells using quantitative PCR (qPCR). However, when CC99 was grown in either THP-1 or HeLa cells, no bacteria were detectable in these cells either by histochemical staining or by qPCR at 72 hours. These results suggest that although bacterium CC99 may be transmissible to human cells when lysed from amoebae, CC99 propagated in human cells may not be directly transmissible to other human cells. Therefore, this study has important implications for the safety procedures surrounding amoeba-derived infections, such as Legionella spp.