Differentiating E. histolytica Cysts from the Commensal Amoeba Cysts

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Differentiating E. histolytica Cysts from the Commensal Amoeba Cysts

As mentioned previously, observation of cysts is often a better way to differentiate the amoeba. The most important characteristics to look for are: number and appearance of nuclei, presence and appearance of chromatoid bodies, and presence of glycogen. Chromatoid bodies are dark staining, rod-like structures in the cytoplasm.
As shown in Table 5, E. histolytica cysts are most similar to E. harmanni and E.coli cysts. The major point of differentiation is that E. histolytica cysts have 1-4 nuclei visible, but E. coli cysts generally have up to 8 or more. E. hartmanni have 1-4, but they are notably smaller.
Cysts are stained with trichrome unless otherwise noted.
Table 5. Differentiating Amoeba Cysts.
OrganismImageNuclear CharacteristicsCytoplasm
E. histolytica
Spherical
10-20 μm;
(usually 12-15 μm)

Trichrome; arrow points to chromatoid body (12)

Wet mount; arrow points to nuclei seen (13)
1-4 nuclei
Peripheral chromatin: fine, evenly distributed
Karyosome: small, compact, usually central
May contain chromatoid bodies which are blunt or rounded with smooth edges
E. coli
Spherical or oval
10-35 μm

Iodine stain - 6 nuclei visible (14)
Mature cyst: 8 nuclei (occasionally as many as 16)
Peripheral chromatin: course, granular
Karyosome: large, eccentric or central
May contain chromatoid bodies which appear splinter shaped, with rough edges
E. hartmanni
Spherical
5-10 μm; (usually 6-8 μm)

(15)
1-4 nuclei (2 nuclei common)
Peripheral chromatin: fine, evenly distributed granules
Karyosome: small, compact, central
Usually contain chromatoid bodies which are elongated, blunt ends, smooth
E. nana
Oval
5-10 μm; (usually 6-8 μm)

(16)
Usually 4 nuclei
Peripheral chromatin: none
Karysome: smaller than the troph, but larger than the Entamoeba spp.
Small granules or inclusions common
Chromatoid bodies: rare but fine linear chromatoidals can occasionally be seen
I. butschlii
Oval or round
5-20 μm;
(usually 10-12 μm)

(17)
Mature cyst: 1 nucleus
No peripheral chromatin
Karyosome: large, eccentric, refractile granules may be on one side
Occasional granules
No chromatoid bodies
Most notable: large, compact, well defined glycogen
12. DPDx. "Laboratory Diagnosis of Amebiasis: Entamoeba histolytica and Entamoeba dispar - E. histolytica/E. dispar cyst showing a chromatoid body with bluntly rounded ends (arrow)." CDC.gov, 2021, https://www.cdc.gov/dpdx/resources/pdf/benchAids/Entamoeba_benchaid.pdf
13. DPDx. "Laboratory Diagnosis of Amebiasis: Entamoeba histolytica and Entamoeba dispar - E. histolytica/E. dispar cyst with three visible nuclei (arrows)." CDC.gov, 2021, https://www.cdc.gov/dpdx/resources/pdf/benchAids/Entamoeba_benchaid.pdf
14. Osman, Iqbal. "Entamoeba coli cysts with many nuclei." Wikimedia Commons, 20 Sep 2012, https://commons.wikimedia.org/w/index.php?curid=33628234
17. DPDx. "Parasites - Nonpathogenic (Harmless) Intestinal Protozoa - Iodamoeba buetschlii cyst stained with trichrome." CDC.gov, 23 Sep 2020, https://www.cdc.gov/parasites/nonpathprotozoa/