Infection and nuclear interaction in mammalian cells by ‘Candidatus Berkiella cookevillensis’, a novel bacterium isolated from amoebae

Abstract

Background ‘ Candidatus Berkiella cookevillensis’ and ‘ Ca. Berkiella aquae’ have previously been described as intranuclear bacteria of amoebae. Both bacteria were isolated from amoebae and were described as appearing within the nuclei of Acanthamoeba polyphaga and ultimately lysing their host cells within 4 days. Both bacteria are Gammaproteobacteria in the order Legionellales with the greatest similarity to Coxiella burnetii. Neither bacterium grows axenically in artificial culture media. In this study, we further characterized ‘ Ca. B. cookevillensis’ by demonstrating association with nuclei of human phagocytic and nonphagocytic cell lines. Results Transmission electron microscopy (TEM) and confocal microscopy were used to confirm nuclear co-localization of ‘ Ca. B. cookevillensis’ in the amoeba host A. polyphaga with 100% of cells having bacteria co-localized with host nuclei by 48 h. TEM and confocal microscopy demonstrated that the bacterium was also observed to be closely associated with nuclei of human U937 and THP-1 differentiated macrophage cell lines and nonphagocytic HeLa human epithelial-like cells. Immunofluorescent staining revealed that the bacteria-containing vacuole invaginates the nuclear membranes and appears to cross from the cytoplasm into the nucleus as an intact vacuole. Conclusion Results of this study indicate that a novel coccoid bacterium isolated from amoebae can infect human cell lines by associating with the host cell nuclei, either by crossing the nuclear membranes or by deeply invaginating the nuclear membranes. When associated with the nuclei, the bacteria appear to be bound within a vacuole and replicate to high numbers by 48 h. We believe this is the first report of such a process involving bacteria and human cell lines. Electronic supplementary material The online version of this article (10.1186/s12866-019-1457-z) contains supplementary material, which is available to authorized users.

The inside scoop: Comparative genomics of two intranuclear bacteria, “Candidatus Berkiella cookevillensis” and “Candidatus Berkiella aquae”

“…CC99 is also an obligate intracellular parasite of mammalian cell lines. Both bacteria replicate within BCVs that are closely associated with or within the nucleus, resulting in lysis of the host cell [21]. Although, this genomic study provides insight into gene products that contribute to the metabolism and virulence of these bacteria, it still remains unclear why CC99 but not HT99 is able to infect mammalian cells.…”

“…Both CC99 and HT99 infect and replicate within protozoa, including Acanthamoeba polyphaga and Dictyostelium discoideum (Fig 1) [17]. CC99 can also infect and replicate in mammalian cells (S1 Fig), including phagocytic and non-phagocytic cell lines [21], also associating with or entering the nucleus. During infection of host cells, both bacteria exhibit replication within a bacteria-containing vacuole (BCV) that interacts with or enters the host cell nucleus.…”

“…Within one hour after internalization, both bacteria are visible in the host cell cytosol enclosed in a vacuole. As infection progresses, the BCV traffics through the cytoplasm and eventually invades or closely associates with the nucleus of the host [21]. Within BCVs, the bacteria replicate to large numbers and eventually escape after lysis of the host cell.…”