Differentiating Characteristics of Pathogenic Amoebae (After Dobell and O’connor).
Motile Stage
| Entamoeba histolytica | Entamoeba coli | Endolimax nana | Iodamoeba bütschlii | |
| Size | 20-30µ | 20-30µ | 6-12µ | 9-13µ |
| Motility | Very characteristic. Flows in almost straight line across field. Later becomes less active pushing out a few, large blunt, blade-like pseudopodia which are perfectly hyaline being composed entirely of ectoplasm. | Usually sluggish but may show considerable activity when freshly passed. The movement consists chiefly in changes of shape without progression. | Slow progressive movement when freshly passed. Later slight changes of shape. Soon rounds and dies. | Slight motility with movements similar to E. coli. Quickly degenerates and dies. |
| Cytoplasm | Endoplasm finely granular and uniform in appearance. May contain red blood cells but bacteria probably never seen in normal individuals. Ectoplasm clear and well developed. | Endoplasm had a bulky granular appearance and contains numerous food vacuoles charged with bacteria and vegetable débris. Never contains red blood cells. No sharp line between endoplasm and ectoplasm. | Endoplasm finely granular with numerous minute food vacuoles containing bacteria. No sharp line as a rule between the endoplasm and ectoplasm. | Endoplasm finely granular and homogeneous. Usually contains numerous food vacuoles charged with minute bacteria. Cysts of this amoeba have previously been described been described as “iodine cysts.” |
| Nucleus | 4-7µ. A delicate vesicle inconspicuous or invisible. Stained shows fine beads of chromatin lining wall. Karyosome small, spherical and central. | 4-7µ. Distinguishable. Stained shows larger beads of chromatin lining wall. Karyosome spherical, eccentric and larger than that of E. histolytica. | 1-3µ. Vesicular with delicate membrane. Stained shows wall free from chromatin. All chromatin contained in large, irregular, eccentric karyosome. Variations in shape of karyosome characteristic. | 2-3.5µ. Small vesicle with distinct membrane. Stained shows wall free from chromatin as a rule. Typically chromatin contained in large, central spherical karyosome. Zone between nuclear wall and karyosome filled with single layer of small granules. |
Encysted Stage
| Entamoeba histolytica | Entamoeba coli | Endolimax nana | Iodamoeba bütschlii | |
| Size | 7-15µ | 15-20µ | 7-9µ | 9-12µ |
| Shape | Round. | Round. | Usually oval. | More or less rounded. |
| Wall | Thin. | Thicker than E. histolytica. | Thin. | Relatively thick. |
| Nuclei | Typically shows four nuclei. May show one to four. Nuclei at rest structurally similar to that of adult nucleus. | Typically shows eight nuclei. May show one to twenty. Structurally similar to adult nucleus. | Very small. May show from one to four nuclei. Rarely eight. Structurally similar to adult type of nucleus. | One relatively larger nucleus. Differs structurally from the adult type in that the granules in the clear zone become massed at one pole giving an eccentric karyosome. |
| Chromatoids | Large chromatoids common. | Large chromatoids may be present but usually absent. | Absent. | Absent. |
| Glycogen | Diffuse but not abundant. | Relatively abundant in the early stages. Scanty or absent in mature cysts. | Rarely present. | Dense glycogen mass is characteristic. |
Walker also notes the advantages of examining a specimen with a ⅔ inch objective as encysted amoebae are easily picked up. In opposition to the usual recommendation of text-books to report only on motile amoebae, he recommends the making of a differential diagnosis on nonmotile encysted forms. This however is now generally accepted by experienced workers as true.
The preëncysted E. histolytica has a nucleus much resembling that of E. coli. The presence of the same chromidial bodies one notes in the cysts is an aid in recognizing this stage. The 4 nuclei of the cysts are much smaller than the nucleus of the preëncysted or vegetative stage.