A microscopic description and ultrastructural characterization

1 2 Dientamoeba fragilis is a pathogenic trichomonad found in the gastrointestinal tract of 3 humans and is implicated as a cause of diarrhoea. Despite its discovery over a century 4 ago, there has been no recent thorough description of this parasite by microscopy. 5 SEM, TEM, confocal and light microscopy were therefore used to characterize D. fragilis 6 populations growing in xenic culture. Two different populationssmooth and ruffled cells 7 were identifiable by SEM. No flagella, pelta structures, undulating membrane or 8 pseudocyst-like forms were present. The organelles in D. fragilis were analysed by TEM; 9 like Trichomonas and Histomonas, D. fragilis contains hydrogenosomes that presumably 10 represent the site of anaerobic respiration. The nuclear morphology of D. fragilis 11 trophozoites grown in vitro and trophozoites from clinical isolates were also compared by 12 confocal microscopy and light microscopy. The majority of cells grown in culture were 13 mononucleate while most cells in permanent stained fecal smears were binucleate. The 14 two nuclei of D. fragilis are morphologically indistinguishable and contain equivalent 15 amounts of DNA as determined by DAPI staining. The approximate cell and nuclear 16 volume of four isolates of D. fragilis were measured and shown to be comparable to other 17 trichomonads. In addition, the discovery of a Virus-Like Particle (VLP) is also reported 18 for the first time in D. fragilis. This study therefore provides extensive and novel details 19 of the ultrastructure of a neglected protozoan parasite that is an emerging cause of human 20 disease. 21 22