Entamoeba coli

Large cosmopolitan amoeba, commensal in man, that lives and multiplies in the large intestine.

Trophozoite

Size: from 15 to 50 μm; usual range, 20-30μm.

Motility: visible only in fresh, unfixed stool specimens. The trophozoite slowly forms a pseudopod, then withdraws it and remains immobile maintaining a round shape. After a few moments, a new pseudopod forms in a new position, and so on. The result is an anarchic, sur place (non-progressive) movement, without a defined direction. In some cases, it is possible to observe the simultaneous, explosive formation of 3-5 small, rounded pseudopods extend simultaneously, but even in this case the amoeba remains within the microscopic field.

Cytoplasm: in mobile trophozoites, the ectoplasm (hyaline and transparent) is clearly distinct from the endoplasm which appears unevenly granular with vacuoles often containing starch granules, bacteria, yeast but also cysts of parasites, such as Giardia intestinalis, or of other amoebae. The cytoplasm is often parasitized by Sphaerita.  As a general rule, the cytoplasm does not contain red blood cells, except in the rare case of patients with intestinal hemorrhage.  In these cases, Entamoeba coli trophozoites can be hematophagous.

Nucleus: 5-7 μm, often visible on fresh wet mount, although it is often impossible to clearly see the characteristics of the karyosome and peripheral chromatin. In these cases, it is appropriate to examine specimens that are fixed (with SAF) and temporarily or permanently stained.

-     Karyosome: large, diffuse or granular, generally eccentric.  Achromatic granules (chromosomes) can often be seen around the karyosome.

-     Peripheral chromatin: the nuclear membrane is thick and irregular, with coarse granules of chromatin often unevenly arranged.

With trichrome staining, the Entamoeba coli trophozoite (above all its nucleus) sometimes stains more intensely than trophozoites of Entamoeba histolytica or other amoebae.

It is not always easy for the inexperienced microscopist to distinguish between nuclei of Entamoeba coli and Entamoeba histolytica.  Therefore, for a correct diagnosis of the species, it is important to observe many trophozoites. Entamoeba coli is a non-pathogenic amoeba, so its differential diagnosis from Entamoeba histolytica is of fundamental importance to avoid patients being treated unnecessarily for amoebiasis, when the cause of their intestinal symptoms is quite different (for example, colon cancer). 

Cyst

Size: 10 to 35 μm; usual range, 15-20 μm. Immature cysts are generally larger.

Shape: generally round, often oval or irregular. The cyst is surrounded by a thick double wall, hard to see in fresh specimens, that has a shell-like appearance which is clearer, more refractile and more uniform in thickness than that of Entamoeba histolytica.

Nucleus/i: depending on the stage of maturation, there can be from 1 to 8 nuclei. It is possible to observe cysts with 16 or 32 nuclei (supernucleate cysts).

-     Cysts with 1 nucleus: large nucleus, with diffuse karyosome that displays clear karyokinetic activity. In most cysts, a large (iodophilic) glycogen vacuole is present;  in these cases, the nucleus is oval and confined to the periphery of the cyst. With the maturation of the cyst, the vacuole shrinks and finally disappears. Chromatoid bodies with pointed ends are visible in the space between the vacuole and the cyst wall.

-     Cysts with 2 nuclei: the nuclei are usually oval and located at diametrically opposite poles with respect to the vacuole.  In contrast, in binucleate cysts of Entamoeba histolytica, the nuclei are generally paired.

-     Cysts with 4 nuclei:  at this stage of maturation, the glycogen vacuole is normally absent and the nuclei are dispersed within the cyst.  The nuclei can have variable sizes, some are irregular and evidently undergoing division, with coarse peripheral chromatin and a karyosome composed of granules of dispersed chromatin. Tetranucleate cysts of Entamoeba coli can be mistaken (by inexperienced microscopists) for mature cysts of Entamoeba histolytica.  Therefore, it is important to remember that a tetranucleate Entamoeba coli cyst is larger (>14 μm) than a mature cyst of Entamoeba histolytica, can be variable in shape, and has nuclear peripheral chromatin and karyosome composed of irregular granules.

-     Cysts with 8 nuclei:  these cysts are fully mature. In fresh mount preparations, the nuclei exhibit thin peripheral chromatin with a small karyosome eccentrically located. In mature cysts, which represent the infective stage, chromatoid bodies are rarely seen and the glycogen vacuole is always absent.

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