Observations on the transmission of Dientamoeba fragilis and the cyst life cycle stage

Abstract

Little is known about the life cycle and mode of transmission of Dientamoeba fragilis. Recently it was suggested that fecal–oral transmission of cysts may play a role in the transmission of D. fragilis. In order to establish an infection, D. fragilis is required to remain viable when exposed to the pH of the stomach. In this study, we investigated the ability of cultured trophozoites to withstand the extremes of pH. We provide evidence that trophozoites of D. fragilis are vulnerable to highly acidic conditions. We also investigated further the ultrastructure of D. fragilis cysts obtained from mice and rats by transmission electron microscopy. These studies of cysts showed a clear cyst wall surrounding an encysted parasite. The cyst wall was double layered with an outer fibrillar layer and an inner layer enclosing the parasite. Hydrogenosomes, endoplasmic reticulum and nuclei were present in the cysts. Pelta-axostyle structures, costa and axonemes were identifiable and internal flagellar axonemes were present. This study therefore provides additional novel details and knowledge of the ultrastructure of the cyst stage of D. fragilis.

Keywords: Dientamoeba fragilis; cyst; electron microscopy; transmission.

Graphical abstract

Figure 1.

Viability of D. fragilis in-vitro following incubation at acid pH. Dientamoeba fragilis trophozoites were incubated in PBS at different pH levels (2–7) and negative control condition with a Loeffler slope (pH7-Slope) for 0–6 h. The graphs show the resulting total cell counts relative to T0 (a), viable proportion (b) and relative viable cell counts (c). The error bars represent standard error of mean cell counts, performed in triplicate.

Figure 2.

Transmission electron micrographs of a D. fragilis cyst showing the cyst wall and other organelles. (A) Transverse section across a mononucleated cyst showing the outer fibrillar cyst wall (o), peritrophic space (ps) and the nucleus of the encysted parasite (n). (B) Basal body structure (bb). (C) Developing hydrogenosomes which contain a dense core and a less electron-dense outer layer (h). (D) The inner membrane of the cyst wall (i). Scale bar represents 200 nm.

Figure 3.

Transmission electron micrographs of a D. fragilis cyst showing the whole cyst and other organelles and structures. (A) Transverse section across a binucleated cyst showing the outer cyst wall (o), the peritrophic space (ps), the 2 nuclei (n) nucleolus (nu). Note the hydrogenosomes (h) and the ‘cauliflower-like’ hydrogenosomes (hc) that lie in the space between the margins of the 2 nuclei. (B) A closer view of the outer cyst wall (o). Note the weakened area of the inner membrane (w) lying directly next to the ESV-like structures (v). (C) Hydrogenosomes (h), glycogen granules (g) and the inner membrane of the cyst (i). (D) Electron micrograph of the outer cyst wall (o) showing the dark striations (s) and coated in a glycocalyx (gl), the inner membrane (i), the peritrophic space (ps). Note the developing hydrogenosomes with the darker inner core and less electron-dense outer layer (h). (E) Smooth endoplasmic reticulum (er), inner cyst membrane (i) and peritrophic space (ps). (F) Golgi complex of D. fragilis cyst: weakening of the inner membrane (w): outer cyst wall (o).