Organism: Entamoeba histolytica

by GP

Entamoeba histolytica is an anaerobic protozoan parasite that primarily infects digestive tract in humans and primates.  This pathogenic microorganism is the fourth leading cause of deaths and third leading cause of morbidity due to protozoan infections worldwide (1).  Transmission of E. histolytica occurs when a person ingests food or water that is contaminated with infected feces or during sexual intercourse (2).  This sort of transmission is more prevalent in developing countries with poor sanitary conditions.  The infection of E. histolytica is usually called amebiasis or amoebiasis (1,2).  Amebiasis occurs in the large intestines, which causes an internal inflammation. The internal inflammation is caused by trophozoites, which are infected cells that enter the intestines. The trophozoites come into contact with human cells, which induce a rapid influx of calcium in the cell.  This causes all membrane movement to stop.  The internal workings of the cell are disrupted, organelles lyse and, cells die. Consequently, the amoeba vacuums up the dead cell.

Figure 1. The Life Cycle

of E. histolytica

The course of infection for this parasite starts with mature cysts entering the human body, by ingesting mature cysts in fecally contaminated food or water.  Once inside the body, trophozoites develop in the large intestine and produce cysts. In some cases, the trophozotic cysts could remain undetected in the intestinal lumen and pass through by feces.  The cysts that exit the body by feces in cyst form can survive outside the body for several weeks.  In other cases, the trophozoites can pass the mucosal barrier or enter the bloodstream that causes the infection to occur (2, Figure 1). The symptoms of the disease include diarrhea and abdominal pain for mild cases.  For severe cases of the disease the symptoms may include stomach pain, blood and mucus in feces, and high fever (2).

Figure 2. Entamoeba histolytica with four nuclei

Often E. histolytica is misdiagnosed or mistaken for other similar infections.  When diagnosing amebiasis, stool samples are usually taken and it is difficult to distinguish if the infection is caused by E. histolytica due to the similarity to other infections and its symptoms as well.  However, a way to determine the presence of the parasite is to examine the number of nuclei (Figure 2). For example, the mature cysts have four nuclei, which is a telltale sign of Entamoeba histolytica cysts. Other than stool samples, enzyme-linked immunosorbent assay (ELISA) can be used to evaluate the sensitivity level of the parasite in cells.  ELISA is a useful tool in medicine to measure the interaction between antibodies and antigens from foreign microorganisms. ELISA is used to examine the trophozoites because at this stage in the cell cycle of Entamoeba histolytica, they are in the intestinal tract and can be easily extracted than cysts from stool samples.  The trophozoites are treated with different detection kits for ELISA and the level of expression of each kit are further analyzed in PCR, and this helps to know which substrate and substance identify the pathogenic parasite.  Thus, knowing which enzyme substrate and serum substances to use for identifying Entamoeba histolytica, the diagnosis of the disease, amebiasis, cannot be confused by other pathogens that share similar characteristics.  Stool samples examined under microscopy only show that parasites are present, but cannot tell what type of parasite it is.  Treating amebiasis with proper diagnosis and antibiotics, such as auranofin, can save lives and inform the public about how the parasite spreads. Diagnostic steps can be taken, like using stool samples and ELISA, to ensure the right diagnosis is made.

After diagnosing the patient with amebiasis, there are several treatments for Entamoeba histolytic, but the rise of antibiotic resistance is a major concern.  The treatment of amebiasis involved metronidazole, which E. histolytica is now resistant to (1) (Oh NO!).  Have no fear; new drugs are being made to target specific sites of amebiasis.  Drug screen tests have identified auranofin as an effective drug against the parasite.  This drug is originally used for treating rheumatoid arthritis.  To understand how the drug is targeting E. histolytica, transcriptional profiling and thioredoxin reductase assays show the drug targets the parasite’s thioredoxin reductase.   Thioredoxin is a protein that acts like an antioxidant and helps reduce other proteins by cysteine thiol-disulfide exchange. This is an important biological process because preventing thioredoxin reduction in E. histolytica means disulfide bonds in cells are not reduced.  Since the disulfide bonds are not reduced, sensitivity to reactive oxygen species (ROS) is increased in which cell structures can be damaged (1).

Along with disrupting thioredoxin reductase, the role of adherence to mucosal membrane is another target site for drugs (3).  As mentioned above, the course of infection mainly pertains to the large intestines, thus the parasite seems to adhere and degrade the mucosal barriers.  Research by Ravdin et al. found that the adherence of E. histolytica to target cells requires microfilament function, which lyse cells, and has a specific amebic receptor that is attracted to N-acetyl-d-galactosamine (GALNAc) (3).  The adherence of amebas to a functional group, CHO, on cells show carbohydrate specificity and it is a key component of the contact-mediated killing of target cells.  GALNAc inhibits the amebic cytolysis of target cells, so cells cannot be killed.  Since the cells cannot be killed, the parasite cannot adhere to the mucosal barriers and cannot enter the bloodstream (3,4).  To target the adherence site and adherence receptors on Entamoeba histolytica and cell surface, respectively, drug therapies can be made to specifically target that region, like using GALNAc.

All the aspects of Entamoeba histolytica is known in terms of being the first human amoeba to have its genome sequenced and analyzed.  The Sanger Institute is the first to sequence the genome and with their help, further insight of the amoeba parasite can provide evolutionary DNA markers and mechanisms of infection.  The human amoeba parasite, Entamoeba histolytica, infects 50 million people around world and 70,000 deaths occur.  All hands are on deck to fight Entamoeba histolytica and prevent misdiagnosis.

Citations

1. Sharon L Reed, et al. "A High-Throughput Drug Screen For Entamoeba Histolytica Identifies A New Lead And Target." Nature Medicine 18.6 (2012): 956-960. Academic Search Premier. Web. 15 Nov. 2012.
2. Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 02 Nov. 2010. Web. 15 Nov. 2012. 
3. Ravdin, Jonathan I., and Richard L. Guerrant. "Role of Adherence in Cytopathogenic Mechanisms of Entamoeba Histolytica." Journal of Clinical Investigation 68.5 (1981): 1305-313. Print.
4. Barbara J. Mann, et al. "Identification Of Entamoeba Histolytica Thiol-Specific Antioxidant As A Galnac Lectin-Associated Protein." Molecular & Biochemical Parasitology 127.2 (2003): 113. Academic Search Premier. Web. 15 Nov. 2012.
5. Mirelman, David. "Comparison of Use of Enzyme-linked Immunosorbent Assay-based Kits and PCR Amplification of RRNA Genes for Simultaneous Detection of Entameba Histolytic and E. Dispart." Journal of Clinical Microbiology 35.9 (1997): 2405-407. Print.