Enterocytozoon bieneusi, a human pathogen

Abstract

Although brought to the forefront in the 1980s with the AIDS pandemic, microsporidia infecting humans are still little known. Enterocytozoon bieneusi, by far the most frequent microsporidia species causing diseases in humans, is responsible for intestinal illness in both non- and immunocompromised patients. This species presents an astonishing genetic diversity with more than 500 genotypes described, some of which have a strong zoonotic potential. Indeed, E. bieneusi infects a broad array of hosts, from wild to domestic animals. This emerging eukaryotic pathogen has thus been associated with foodborne/waterborne outbreaks. Several molecular assays have been developed to enhance its diagnosis or for epidemiological purposes, providing valuable new data. Here, we propose an overview of the current knowledge on this major species among the microsporidia, so far rather neglected in human medicine.

Keywords: Enterocytozoon bieneusi; foodborne pathogen; intestinal pathogen; microsporidia; opportunistic infectious diseases; outbreak; waterborne pathogen; zoonosis.

Figure 1.

Enterocytozoon bieneusi life cycle. Following ingestion of the environmental spore (A), the coiled polar tube rapidly discharges, injecting the spore contents (sporoplasm and nucleus) into the cytoplasm of a host cell (B). The parasites are located in the cytoplasm of intestinal lining cells (enterocytes) between the nucleus and the luminal surface. The proliferative plasmodial stage contains multiple elongated nuclei that will undergo divisions and are adjacent to electron-lucent inclusions, associated with a relatively simple cytoplasm. Then, the multinucleated sporogonial plasmodia gradually enlarges and the cytoplasm becomes more complex, containing notably electron-dense discs, and all will be the precursors of the polar filament and annexes. During sporogonial division, deep invagination of the parasite cell surface will delineate individual sporoblast cells containing individual nuclei and polar tube complexes, which will then mature into spores. The latter are released from ruptured host cells and are excreted with faeces into the environment. (C) Humans become contaminated through contact with infected animals or other individuals and also through water or food contaminated by infected animal droppings.

Figure 2.

Phylogenetic analysis and host specificity in Enterocytozoon bieneusi. (A) Groups (1–15) and Subgroups (1a, 1b, 1c, …) of E. bieneusi genotypes based on the phylogenetic analysis of nucleotide sequences of the Internal Transcribed Spacer (ITS). Fifteen major genetic Groups are recognized. Group 1 is further divided into nine subgroups and Group 2 in three subgroups. Groups 3–15 contain fewer genotypes and are not further subdivided. Group 1 has a low level of host specificity, and only a few genotypes have restricted host ranges, suggesting it is of zoonotic concern. Group 2 genotypes were once considered to be adapted to ruminants, but some have subsequently been found in other animals and humans. Therefore, support for host specificity no longer can be sustained for Group 2 genotypes. Therefore, Group 2 is considered of increasing zoonotic concern. Genotypes belonging to Groups 3–15 are more host-specific, but little information is available for these groups, so data should be taken with caution. Current data support host adaptation in most genotypes of these groups, suggesting limited zoonotic potential. (B) Multilocus sequence typing (MLST)-based phylogenetic analysis is more discriminant and identifies within Group 1 seven subpopulations (SP) presenting host specificities, whereas they were not distinguished within ITS-based Subgroups. The ITS-based host range analysis demonstrated the potential existence of host adaptation in Group 1. The MLST-based population genetic analysis would be more precise in assessing subpopulation formation and host-specific differences. For example, SP1 contains mainly isolates identified with ITS as zoonotic genotypes (D, type IV, …), whereas SP2 comprises isolates defined with ITS as anthroponotic genotype A. However, further research is needed as host specificity and zoonotic potential of SP7 are poorly understood, and primers currently used for MLST failed to amplify mini- and micro-satellites in isolates from the other Groups (2-15).

Figure 3.

Different stains of Enterocytozoon bieneusi spores in faecal smears from patients with intestinal microsporidiosis (scale bar 5 µm, original magnification ×1,000). (A) Weber's modified trichrome stain: spores appear pink on a green background with a belt-like stripe that girded the spore diagonally or equatorially. (B) Calcofluor-white stain: spores appear under blue fluorescence with increased intensity on the periphery or through the spore. (C) Indirect immunofluorescence assay with monoclonal antibodies against E. bieneusi (Bordier Affinity Products, Switzerland): spores appear under green fluorescence.