Entamoeba Dispar

Entamoeba Dispar

Why E. histolytica infection produces a symptomatic invasive disease in relatively few infected individuals (~1%) has perplexed clinicians and scientists for generations. Explanations usually entail one of two general theories: 1) E. histolytica is normally a commensal which on occasion can switch to an invasive pathogen, or 2) E. histolytica is actually two morphologically identical species. Although the two species hypothesis was first proposed in 1925 by Brumpt, it was mostly ignored until the late 1970's. Substantial biochemical and molecular data has accumulated over the last 2-3 decades which indicate that the non-pathogenic isolates of E. histolytica are genetically distinct from the pathogenic isolates (Diamond and Clark, 1993).

Comparison of Entamoeba histolytica Isolates
CRITERIA	NON-INVASIVE	INVASIVE
In Vitro Culture	xenic	axenic
ConA Agglutination	-	+
Complement Resistance	-	+
Zymodemes (isoenzymes)	I & III	II
Numerous Antigenic Differences (eg., GIAP Epitopes)	1-2	1-6
Numerous DNA Sequence Differences (eg., rRNA)	2.2% sequence diversity
RFLP/DNA Probes	B133, cEH-NP1	P145, cEH-P1

Entamoeba histolytica isolates from pathogenic (invasive) and non-pathogenic (non-invasive) patients exhibit many phenotypic and genotypic differences (Table). Some of the first noted differences were the in vitro growth characteristics, agglutination with concanavalin A (ConA), and resistance to complement lysis. Pathogenic strains have the ability to grow in axenic cultures (without bacteria) whereas the non-pathogenic strains required bacteria for in vitro growth. The ConA agglutination and complement resistance imply that the outer surfaces of the pathogenic and non-pathogenic strains are different. Several antigenic differences in surface proteins have been noted and non-pathogenic isolates have been reported to lack a particular glycoconjugate on their surface (Bhattacharya et al, 2000).

Isoenzyme analysis revealed different zymodemes for the pathogenic and non-pathogenic isolates. Similarly, numerous antigenic differences between pathogenic and non-pathogenic isolates have been described. A well-characterized antigenic difference is in a surface protein referred to as galactose-inhibitable adherence protein (GIAP) or the Eh-lectin. A panel of monoclonal antibodies recognize epitopes shared between isolates (ie, 1 and 2) and epitopes only found on pathogenic isolates (ie, 4-6). These antibodies have been adapted for the differentiation of the two species (Jackson and Ravdin, 1996). GIAP is also implicated to be involved in the ConA agglutination, the resistance to complement and adherence.

Analysis of DNA and sequencing of several genes also revealed genotypic differences between the pathogenic and non-pathogenic isolates. The most striking variation is the 2.2% difference between the ribosomal RNA gene sequences of pathogenic and non-pathogenic isolates. Unlike some of the other differences (Gilchrist and Petri, 1999), the rRNA would not potentially contribute to virulence. Furthermore, rRNA sequences of humans and mice differ by less than 2.2% indicating that the pathogenic and non-pathogenic strains are quite diverged. These differences led to the formation of a new species call E. dispar (Diamond and Clark, 1993). It is not clear whether E. dispar is truly non-pathogenic in the strictest sense (see discussion on pathogenicity vs virulence) since the parasite is capable of producing small focal intestinal lesions in experimental animals. Slight morphological differences between E. dispar and E. histolytica have also been noted (Espinosa-Cantellano et al, 1998).

Historical Highlights of the Genus Entamoeba

1875	Lösch demonstrated that dysentery is due an ameba, which he named Amoeba coli.
1891	Councilman and Lafleur demonstrated role of ameba in tissue invasion.
1903	Schaudinn renamed species to Entamoeba histolytica.
1925	Brumpt proposed two species to distinguish pathogenic and non-pathogenic infections: E. dysenteriae and E. dispar.
1957	Burrows classified E. hartmanni as a distinct species instead of the 'small race' of E. histolytica as previously advocated by Brumpt in 1926.
1973	Martinez-Palomo demonstrated differences in agglutination between pathogenic and non-pathogenic isolates.
1978	Sargeaunt and Williams demonstrated zymodeme differences between pathogenic and non-pathogenic isolates.
1988-93	Several investigators demonstrated antigenic and DNA differences between pathogenic and non-pathogenic isolates.
1993	Diamond and Clark proposed E. dispar as the name for non-pathogenic E. histolytica.
1997	WHO endorsed two species.

References

Bhattacharya A, Arya R, Clark CG, Ackers JP (2000) Absence of lipophosphoglycan-like glycoconjugates in Entamoeba dispar. Parasitology 120, 31-35.

Diamond LS, Clark CG (1993) A redescription of Entamoeba histolytica Schaudinn, 1903 (Emended Walker, 1911) separating it from Entamoeba dispar Brumpt, 1925. J. Euk. Microbiol. 40, 340-344.

Espinosa-Cantellano M, Gonzalez-Robles A, Chavez B, Castanon G, Arguello C, Lazaro-Haller A, Martinez-Palomo A (1998) Entamoeba dispar: ultrastructure, surface properties and cytopathic effect. J. Euk. Microbiol. 44, 265-272.

Gilchrist CA, Petri WA Jr (1999) Virulence factors of Entamoeba histolytica. Curr. Opin. Microbiol. 2, 433-437.

Jackson TFHG, Ravdin JI (1996) Differentiation of Entamoeba histolytica and Entamoeba dispar infections. Parasitol. Today 12, 406-409.