Entamoeba hartmanni

This small amoeba is commensal in man, colonizing the colon. It is morphologically identical to Entamoeba histolytica, for which it is often mistaken. Size and appearance after staining are factors in a correct diagnosis.

Trophozoite

Size: from 5 to 12μm;  usual range, 8-10μm. Trophozoites parasitized by Sphaerita may be larger and similar in size to Entamoeba histolytica.

Motility: similar to Entamoeba histolytica.

Nucleus: not visible in fresh specimens. In fixed, stained specimens, the diameter ranges from 1.5 to 3μm.

-     Karyosome: small and compact, in central, sub-central or eccentric position, as for Entamoeba histolytica.

-     Peripheral chromatin: small, evenly distributed granules;  in some cases, granules are irregular and unevenly distributed.

Wet mounts stained with MIF, Lugol's iodine solution, or Bailenger's stain as well as permanent stained smears reveal a nucleus that is characteristically Entamoeba, thereby avoiding confusion with Endolimax nana trophozoites which often are of the same size.

Cytoplasm:  may contain small vacuoles or inclusions, but in general has a uniform appearance.

With trichrome staining, this trophozoite stains less intensely and appears more delicate than Entamoeba histolytica.

Cyst

Size: from 4 to 8 μm, smaller than Entamoeba histolytica (12-14 μm); this is a fundamental element for the differential diagnosis.

Shape: round, sometimes oval.

Appearance: clean and refractile in fresh specimens, very different from the appearance of Endolimax nana.

Vacuoles: small, do not stain well with Lugol's iodine solution.

Chromatoid bodies: numerous, elongated, with rounded ends, smaller than those of Entamoeba histolytica. May also be pointed or spherical.  Present even in mature cysts. Given the abundance of chromatoid bodies, their presence often prevents the nuclei from being seen.

Nucleus/i: The mature cyst can have from 1 to 4 nuclei, each with a diameter of 1-2 μm.  However, specimens usually contain cysts with only 1 or 2 nuclei. The structure of the nucleus (karyosome and peripheral chromatin) is similar to that of the trophozoite.

Images