Encephalitozoon hellem

A Microbial Biorealm page on the genus Encephalitozoon hellem

Encephalitozoon hellem; Scanning electron micrograph showing an eukaryotic cell bursting and releasing spores of Encephalitozoon hellem to the extracellular medium. From Centers for Disease Control; Division of Parasitic Diseases.

Classification

Higher order taxa

Superkingdom, Kingdom, Phylum, Suborder, Family, Genus:

Eukaryota, Fungi, Microsporidia, Apansporoblastina, Unikaryonidae, Encephalitozoon

Species

Encephalitozoon hellem

Description and significance

Encephalitozoon hellem was initially dectected and isolated in 1991 from three AIDS patients who were suffering from keratoconjunctivitis among other ailments. Conjunctival scrapings and corneal tissue samples were taken from each of the three patients and were tested using SDS-PAGE analysis and Western Blotting (4). All three isolates had identical banding patterns and appeared similar to another species of Encephalitozoon, E. cuniculi, and the new AIDS related microsporidian was then given its name, Encephaliozoon hellem.

In 1994, another AIDS patient experiencing a foreign body sensation in his left eye had a conjunctival swab tested using a calcofluor stain and a fluorescent-antibody stain with murein antiserum raised against E. hellem. A cross-reaction occured with another species of Encephalitozoon, so a PCR test was performed. It was found that the amplified rRNA generated digestion patterns by restriction endonuclease FokI that were indentical to the digestion patterns of E. hellem (3). These observations allowed researchers to conclude that Encephalitozoon hellem causes keratoconjunctivitis in addition to other disseminated infections in immunosuppressed patients. Doctors and researchers soon found that patients treated with albendazole and topical fumagillin responded rapidly with no opthalmologic signs (3).

Encephalitozoon hellem is a unicellular microsporidian that is thought to be fecal borne. It is a spore forming parasite whose spores measure approximately 1x1.5-2.0 microns in diameter (4). E. hellem has been named one of the four most common human microsporidian parasites, with its human hosts consisting mainly of immunodificient patients. The genotyping of this microsporidian has been helpful in understanding other forms of human microsporidosis and their transmission (5). Microsporidia such as E. hellem is culturable from affected tissues and useful for species-species diagnosis. While the culturing of these microbes is time-consuming and requires expert skill, species-species identification of microsporidia is important in order to study the various levels of drug responses of the differing species of Encephalitozoon(3).

Genome structure

Encephalitozoon species have the smallest genomes reported to date among all single-celled eukaryotes. It is suggested that this is because of an early divergence from microsporidia that are no longer supported by current phylogenic data. The genome size of is approximately 2.39 Mb (7). Analysis of the Internal Transcribed Spacer (ITS) sequence of the rRNA genes and SSU rRNA sequences of various isolates indentified 3 types of genotypes of E. hellem, 1A, 1B, and 1C. Each of these genotypes have 1253 bp, 1313 bp, and 1373 bp respectively and all have identical ITS sequences. These three genotypes differ in small insertions or deletions and some point mutations (8). While ITS sequences served as a valuable marker in researching E. hellem, the polar tubule protein (PTP) gene proved to be more valuable as it found an additional genotype, 2B, having 1421 base pairs. Genotype 2B has very different ITS and SSU rRNA sequences and extensive differences in PTP sequences(5).

Later studies have revealed two more genotypes, 2A and 2B. While six different genotypes have been discovered, the significance of E. hellem's genetic diversity is unclear. The number of E. hellem isolates is very limited leaving researchers without enough information to compare genotype distribution among humans and other hosts such as birds (5).

The polar tubule protein gene, called PTP1, is the best known target so far for genotyping Encephalitozoon hellem. PTP1 is an intronless gene that exists as a single copy per haploid genome. It is very rich in glycine and proline and contains a hydrophilic central domain of six tandem repeat units. The length of the protein depends on the number of repeats present, but is approximately 43 kDa in size (9).

Pulsed-field gel electrophoresis (PFGE) is most often used for typing Encephalitozoon hellem and has found two karyotypes A and B with 7 and 8 chromosomal bands respectively. PFGE is currently the most discriminative tool to research the intraspecific diversity of E. hellem (8).

Cell structure and metabolism

Encephalitozoon hellem is a unicellular, intracellular microsporidian species (1). It is a parasite whose gram positive spores measure approximately 1x1.5-2.0 microns. Each spore has a unique polar tubule organelle that makes six to eight coils inside the cell. These polar tubules are the ultrastructures of E. hellem(4). On the opposite end of the polar tubule is a polar vacuole that responds to environmental stimuli such as pH and pressure changes. Other organelles include ribosomes and nuclei, but no mitochondria (11). Inside each cell there is a monokaryotic nuclei and thick endospores w/ irregulary shaped exospores (4). The thick spore wall is composed of an electron dense undercoat and a thick electron dense lucent layer of chitin (11). The cells' nuclear envelope is punctuated by rare nuclear pores about 50-60 nm and does not break down upon nuclear division. Instead, the nuclear envelope elongates into a dumbell shape with the two daughter nuclei at each end linked by a long straight bundle of microtubules (10).

There are also amorphous plaques associated with the nuclear envelope close to the poles of the spindle called electron-dense spindle plaques (ESP). These ESPs are the focus of microtubule arrangement. Based on the organization of the cell, it can be assumed that both the spindle and the cytoplasmic microtubules are involved in the nuclear division process (10).

Researhers have found that E. hellem cells synthesize single cytosolic neutral aminopeptidase activites that preferentially cleave leucine and arginine. This activity is associated with the microbes sporoplasm being passed down the polar tubule for host cell invasion and also for the regulation of parasite development. The maximal activity occurs at a pH of 7.2. The aminopeptidase has been estimated by direct fluorogenic analysis to have a molecular mass of 72 kDa and based on other studies they have been found to be potential targets for chemotherapeutic agents in parasitic diseases (13).

Ecology

Describe any interactions with other organisms (included eukaryotes), contributions to the environment, effect on environment, etc.

Pathology

E. hellem was first isloated in HIV patients, but has also been found to infect mice, birds, and even bats. When isolating E. hellem in the HIV patient, there were 24 monoclonal antibodies that were used against not only E. hellem, but also two other species of Encephalitozoon, E. intestinalis and E. cuniculi. The antibodies did not react with either of these two, indicating antibody specificity across the three species. Antigenic diversity of the different karyotypes of E. hellem was demonstrated as well when two monoclonal antibodies reacted with one karyotype B and not karyotype A (1).

Experiments exposing chickens to E. hellem found that the microbe was detectable in the feces of the animals for up to 19 days, showing that E. hellem is potentially fecal borne and that chickens are a very likely host. While a definite route of infection is not well documented, it has been assumed that transmission of E. hellem most likely occurs by direct contact or by the ingestion of contaminated food or water (2).

Once a host has been found, E. hellem, like other microsporidia, act as spore forming parasites that use their unique polar tubule as a means of infecting their hosts. A mature, resistant spore will extrude the polar tubule and through this, the sporoplasm will be injected into the host cell. The sporoplasm is then able to multiply inside of the host cell via binary fission or multiple fission. Like the photo above illustrates, the spores will continue to multiply until the host cell cytoplasm is filled, causing it to burst and release more spores into the surroundings that will mature and carry on the infection to other cells (6).

Studies have found that the glycosylation of the ultrastructure gene PTP1, can be modified by O-linked mannosylation indicating that mannose pretreatment of host cells may decrease infection by E. hellem. This suggests that the O-mannosylation of PTP1 mau have a functional significance for the ability of E. hellem to invade their hosts (12).

The clinical manifestations of E. hellem include keratoconjunctivitis, infection of respiratory and genitourinary tract, and disseminated infection. With E. hellem being one of the four most common human microsporidian parasites, hosts are usually HIV patients but infections have also been reported to infect immunodificient patients that do not have HIV (6).

Application to Biotechnology

Encephalitozoon hellem is not known to produce any enzymes that are beneficial to the environment or its inhabitants. However, studies performed on E. hellem have shown potential to be beneficial to clinical and biological research. For example, culturing of the microbe is useful for species-species diagnosis (3). It is also believed that different sources of E. hellem isolates could be important to finding new variants and genotyping other isolates may be important to evaluating the actual variability in the species (8). Also, the genotyping of this microsporidian has been helpful in understanding other forms of human microsporidosis and their transmission (5).

Current Research

Enter summaries of the most recent research here--at least three required

References

1. Mo L., Drancourt M. Antigenic diversity of Encephalitozoon hellem demonstrated by subspecies-specific monoclonal antibodiesJ. Eukaryotic Microbiology 49, 249-54 (2002)

2. Fayer, R. et. al Detection of Encephalitozoon Hellem in Feces of Experimentally Infected ChickensJ. Eukaryotic Microbiology 50, 5743-575 (2003)

3. Didier, E.S. et. al Diagnosis of disseminated microsporidian Encephalitozoon hellem infection by PCR-Southern analysis and successful treatment with albendazole and fumagillinJ Clinical Microbiology, 34, 947–952 (1996)

4. Didier, E.S. et. al Isolation and characterization of a new human microsporidian, Encephalitozoon hellem (n. sp.), from three AIDS patients with keratoconjunctivitisJ Infect. Dis. 163,617-21 (1991)

5. Xiao, L. et. al Genotyping Encephalitozoon hellem Isolates by Analysis of the Polar Tube Protein GeneJ Clinical Microbiology 39, 2191–2196 (2001)

6. Microsporidiosis Centers for Disease Control & Prevention National Center for Infectious Diseases Division of Parasitic Diseases

7. Delarbre, S. et. al Genetic diversity in the microsporidian Encephalitozoon hellem demonstrated by pulsed-field gel electrophoresis J. Eukaryotic Microbiology 48, 471-474 (2001)

8. Haro, M. et. al Intraspecies Genotype Variability of the Microsporidian Parasite Encephalitozoon hellem J. Clinical Microbiolgy 41, 4166–4171 (2003)

9. Peuvel, I. et. al Polymorphism of the gene encoding a major polar tube protein PTP1 in two microsporidia of the genus Encephalitozoon Parasitology 121, 581-7 (2000)

10. Bigliardi, E. et. al Mechanisms of microsporidial cell division: ultrastructural study on Encephalitozoon hellem J Eukaryotic Microbiology 45, 347-51 (1998)

11. Shadduck, J., Greely, E. Microsporidia and Human Infections Clinical Microbiology Reviews 2, 158-165 (1989)

12. Xu, Y. et. al, Glycosylation of the major polar tube protein of Encephalitozoon hellem, a microsporidian parasite that infects humans Infect Immun 72,6341-50 (2004)

13. Millership, JJ. et. al Characterization of aminopeptidase activity from three species of microsporidia: Encephalitozoon cuniculi, Encephalitozoon hellem, and Vittaforma corneae J Parasitology 88, 843-8 (2002)

Edited by Tiffany Myer, student of Rachel Larsen