Entamoeba bangladeshi: An insight.

Molecular tools have the potential to differentiate microscopically similar gut micro-eukaryotes that may have significantly different relationships with the human host. Using broad range Entamoeba primers to amplify a section of the eukaryotic 18S small subunit ribosomal RNA gene a novel member of the Entamoeba family (Entamoeba bangladeshi) has recently been identified. The goal of this review is to place this species in the context of what is already known about this genus and to discuss the tools and data needed to elucidate the host-microbe relationship.

INTRODUCTION

The Entamoeba genus branched early from the main eukaryotic lineages at some point however it began to exploit the ecological niche provided by the metazoan gut. Protozoan cysts have been isolated from the fossilized fecal material of dinosaurs, and current molecular methods have led to the identification of Entamoeba species in a wide range of vertebrates.[12] Adaption to the anerobic gut environment has resulted in the secondary loss of the capacity to metabolize oxygen[3] and some, but not all Entamoeba species have adopted a parasitic lifestyle and cause host disease. Entamoeba histolytica the causative agent of amebaisis in humans and a major cause of morbidity and mortality in children under five has for this reason been the focus of the most study.[45] Evidence is now building however that the second member of the Entamoeba genus Entamoeba moshkovskii is also associated with human disease.[67]

An ongoing longitudinal study of enteric diseases common in a disadvantaged Bangladesh community provided the ideal opportunity to examine the diversity of commensal and parasitic Entamoeba species occurring in children at that geographical location.[891011] The study involved the routine microscopic characterization of surveillance and diarrheal stool for Entamoeba cysts and the deployment of molecular tools to distinguish the species known to infect humans at this geographical location (E. histoytica, Entamoeba dispar and E. moshkovskii). This work, therefore, resulted in a pool of samples known to contain cysts morphologically identical to E. histolytica but which were molecularly distinct.[10]

ENTAMOEBA BANGLADESHI

Despite the advances in the specificity and sensitivity of molecular diagnostic methods to detect known pathogens the cause of many diarrheal illnesses in children can often not be identified.[121314] Despite the well-known limitations of microscopy as a diagnostic tool for E. histolytica its affordability and accessibility has resulted in its continued use.[151617] Microscopic analysis of both surveillance and diarrheal stool specimens identified samples, which contained cyst and trophozoites indistinguishable from E. histolytica, but which were clearly differentiated from known Entamoeba species by immuno- and DNA based-diagnostic tests.[10] The initial focus of the work of Royer et al. was to discover new Entamoeba species in these samples, which could potentially be pathogenic.[10]

MICROSCOPIC AND PHYLOGENETIC ANALYSIS

As described above cysts and trophozoites from the selected fecal samples appeared similar to those of E. histolytica by light microscopy. The amoeboid trophozoites of the novel isolates were cultured from positive stool samples by standard techniques and examined by transmission electron microscopy. This technique revealed no additional features (such as the hydrogenosomes present in the amoeboid Dientamoeba fragilis) which would allow Entamoeba bangladeshi to be distinguished visually from other members of the Entamoeba genus.[101819] Primers directed against a small subunit rRNA region conserved throughout the Entamoeba family were used to amplify a variable section of the gene, and its sequence confirmed that E. bangladeshi was a novel species that was most similar to the other members of the Entamoeba family, which infect humans, E. histolytica and E. dispar.[1020]

VIRULENCE CHARACTERISTICS

Children in this study location are constantly exposed to enteric pathogens, and identification of stool samples with multiple parasites are common even during routine surveillance suggesting that a sub-clinical disease is common.[14] E. bangladeshi has been isolated from the stools of both asymptomatic children and those experiencing diarrhea. Additional clinical and epidemiological studies such as those that linked the E. moshkovskii species with human disease are needed to discern the true role of E. bangladeshi in the human host.[621] New avenues for exploration of the role this micro-eukaryote may play in human health have been discovered during the recent massive parallel sequencing studies of human gut microbiota. It is unknown at present what (if any) subtle impact of colonization with E. bangladeshi could have on human gut flora and the intestinal immune system.[22] Microbial organisms previously regarded as commensal passengers in the human intestine have been found to impact immune homeostasis-leading to the view that the commensal gut microbial community is a “virtual organ,” which can have large effects on host well-being.[2324] The two known pathogenic Entamoeba species despite being present in very low copy numbers in the human gut obviously can have a drastic effect on the indigenous microflora as they cause diarrheal illness.[25] It is intriguing to speculate that other Entamoeba species which do not cause clinical disease may still have a large impact on microflora composition.[26]

PROTECTIVE IMMUNITY

Current evidence suggests that short term mucosal immunity develops in response to Entamoeba infections.[2728] However this protection is species-specific with little or no protection conferred by infections with members of the different species and co-infections are common.[29] Antibodies based on the immunodominant Gal/GalNAc lectin of E. histolytica were unable to recognize E. bangladeshi positive samples in immunodiagnostic ELISA assays.[10] It is therefore unlikely that an infection with E. bangladeshi although it may confer for a limited period protection against recolonization with itself will offer any protection against succeeding pathogenic E. histolytica or E. moshkovskii infections.

CONCLUSION

Molecular characterization of E. bangladeshi is necessary to appreciate the ecological niche it occupies in human health and disease. Sequencing of E. bangladeshi and E. histolytica isolates is needed to evaluate the natural diversity of this species in this genus. Finally new tools are needed to rapidly identify this species for high-throughput epidemiologic studies in humans and to determine the host range and environmental reservoirs of this potential parasite.