Cryptosporidium parvum
A Microbial Biorealm page on the genus Cryptosporidium parvum
Classification
Higher order taxa
Eukaryota; Alveolata; Apicomplexa; Coccidia; Eucoccidiorida; Eimeriorina; Cryptosporidiidae; Cryptosporidium; Cryptosporidium parvum
Species
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NCBI: Taxonomy |
Cryptosporidium parvum
Description and significance
Cryptosporidium parvum is part of the phylum Apicomplexa which contains many important parasites such as Toxoplasma, Plasmodium, Babesia, Cyclospora, Isopora, and Eimeria. C. parvum is an eukaryotic intracellular pathogen that infects both humans and livestock causing the disease cryptosporidiosis (2). The parasite is a serious global health concern. It is especially a problem in underdeveloped regions of the world where sanitation is an issue (6). This parasite is also particularly dangerous to children and immuno-deficient patients. Transmission is common through ingestion of contaminated water sources. The disease has the potential to infect on a large scale. In 1993, 400,000 people in Milwaukee were ill from drinking water contaminated with this parasite (7). Important epidemiological studies have been done on the species. However, while extensive researches have been done on this particular parasite, no known cure is currently available for cryptosporidiosis (2). Ever since the Milwaukee incident, extra consideration has been given to control and prevent cryptosporidiosis outbreak (7).
Cryptosporidium parvum and Cryptosporidium hominis are two closely related pathogens. They are among the 15 species in genus Cryptosporidium that cause intestinal diseases in human and animals. C. parvum and C. hominis were previously known as C. parvum genotype 1 and C. parvum genotype 2, respectively. However, it is now understood that the two species have different transmission cycles and invade a different range of hosts. (8) The genomic sequences of C. hominis and C. parvum show that they only have 3-5% divergence and no large insertions, deletions or rearrangement. The similarity between the two suggests that their phenotypic differences are caused by polymorphism in coding regions and differences in gene regulation (1).
Genome structure
The complete genome of C. parvum has been sequenced and compared to other apicomplexans. The information below is obtained from the shotgun sequencing of the Iowa "type II" isolate of C. parvum. C. parvum 's genome contains eight linear chromosomes with a total of 9.1Mb of DNA sequence. Its genome is relatively compact when compared to the genome of Plasmodium falciparum with 23 Mb of sequence in 14 chromosomes. C. parvum's genome has shorter intergenic regions, fewer introns, and fewer genes which contributes to its compactness (2). The sub-telomeric regions of the C. parvum genome do not have the clusters of large antigenically diverse protein families that are found in Plasmodium (5). It is estimated that C. parvum contains 3807 protein encoding genes, much less than the estimated 5300 genes of Plasmodium. This is due to the absences apicoplast and mitochondrial genome and having fewer genes encoding for metabolic functions and variant surface proteins. The breakdowns of the genes are as follows. The mean gene length excluding introns is 1795bp, the percent of coding DNA is 75.3%, the percent of genes with introns is 5%, the mean length of intergenic regions is 566bp, and the gene density is 2382 bp per gene. And there is a 30% GC content in the genome, and 23.9% GC content in the intergenic regions. There are 45 tRNA genes, six 5S rRNA genes, and five 5.8S, 18S, and 28S rRNA units (2).
Cell structure and metabolism
C. parvum relies on the host for many nutrients (5). C. parvum metabolism is very streamlined when compared to Plasmodium. The absence of mitochondria genes suggests that this parasite relies on glycolysis as the source of energy production. C. parvum is capable of taking up and catabolizing monosugars such as glucose and fructose. They can also synthesize and cabtabolize polysaccharides such a trehalose and amylopectin. As an anaerobic organism, it needs to conserve its ATP usage. It uses a pyrophosphate-dependent phosphofructokinase. There are various end products to C. parvum's glycolysis which includes lactate, acetate, and ethanol. Ethanol is likely to be produced through two independent pathways. The first is from the combination of pyruvate decarboxylase and alcohol dehydrogenase. The second way is from acetyl-CoA with a bifunctional dehydrogenase (adhE) that has acetaldehyde and alcohol dehydrogenase activities (2).
Modular fatty acid synthase (CpFAS1), polyketide synthase homolog (CpPKsI), and several fatty acyl-CoA synthase, and fatty acyl elongase are found. This suggests that C. parvum may participate in fatty acid metabolism. However, there are no enzymes of the fatty acid oxidative pathway except 3-hydroxyacyl-CoA dhydrogenase which indicates that fatty acid is most likely not an energy source (2).
C. parvum has a simplified purine metabolism. It has only retained an adenosine kinase and enzymes that catalyzed the conversion of adenosine 5'-monophosphate (AMP) to inosine, xanthosine, and guanosine 5'-monophosphates (IMP,XMP, and GMP). And unlike other apicomplexans, C. parvum relies on adenosine for purine salvage. Also, they rely on pyrimidine salvage. The parasite has generally lost its ability to synthesize amino acids de novo, but does retain the ability to convert select amino acids. They appear to rely on amino acid uptake from the host by using a set of amino acid transporters (2).
Ecology
When the parasite is ingested, C. parvum resides in the columnar brush border epithelial cells. It is an intracellular parasite since it lives inside the host cell membrane, but it is extracytoplasmic because it lives on the surface of the intestinal epithelium. This particular niche is thought to allow the parasite to be able to evade the immune surveillance while being able to take advantage of the solutes transported across the host microvillus membrane. It has been found that C. parvum has up to 9 genes that encode for families of putative sugar transporters and 11 genes that code for amino acid transporter (2).
Pathology
C. parvum is responsible for intestinal, tracheal, and pulmonary cryptosporidiosis. Often transmissions occur by the ingestion of contaminated public water supplies, making transmissions common in places such as water parks and public pools. The most common symptom is severe diarrhea, but in cases of pulmonary and tracheal cryptosporidiosis coughing and fever can also be seen (9). For the most part, the diarrhea is self limiting. However, immuno-compromised individuals can develop uncontrollable diarrhea leading to severe dehydration that is potentially fatal (2).
C. parvum is cystforming apicomplexans which mean it forms oocysts as part of its life cycle. Oocysts are highly resistant to environmental stresses. It is resistant to conventional chlorine treatment of the community water supplies which makes prevention of cryptosporidiosis difficult. The oocysts are shed from the infected host in their feces, and the feces can contaminate both water and food. The disease is transmitted through the feces oral route which means that the disease is obtained through the ingestion of contaminated water or food. Researchers have been unable to successfully culture the parasite continuously in the lab which makes the studying of this organism difficult. Also, the parasite cannot be genetically manipulated causing further limitations in the studies of the organism. (2)
Another important aspect of the pathology of C. parvum has to do with its cell divisions. Apicomplexans undergo many different types of cell division. The life cycle consists of an intracellular, asexual phase of multiplication followed by the differentiation of gametes, fertilization, meiosis and the release of infectious oocysts in the gut lumen (6). One of the unique aspects of their life cycle is merogony. During this stage of the lifecycle, the parasite replicates its DNA several hundred times before splitting into hundreds of daughters cells simultaneously. This process is important to apicomplexan pathogenesis, and it means that large number of parasites can be generated in just one cell cycle (3).
Some research has been done using antibodies as a possible treatment. Passive antibody therapy has been looked into as a possible treatment for cryptosporidiosis since antimicrobial drugs in immuno-compromised hosts are ineffective. AIDS patients are extremely vulnerable to this disease. One of the benefits of the antibody therapy is that for cryptosporidiosis, the antigens can be taken orally unlike other disease where a systemic administration is required (4).
Current Research
As mentioned earlier, the research on C. parvum has been greatly hindered due to unsuccessful lab culturing of the parasite. There have continuing efforts to try and understand the reason behind this problem and to search for the best conditions to culture C. parvum. It has recently been shown that C. parvum preferentially infects dividing cells. In an experiment where C. parvum was grown on monolayer cells, a high percentage of cells in the S/G2/M phases of the cell cycle were infected. In one experiment, while 6.2% of the total monolayer cells were infected, 43.8% of the cells in S/G2/M phases from the same sample were infected. Other experiments yield similar results. These data are applicable only to in vitro growth of the parasite because similar conclusions were not reached when examining natural infections. Natural infections were not limited to the small intestine dividing cells that are located in the crypts. Host cell mitotic cycle might not be the primary limitation of C. parvum growth, and further studies needs to be done to find methods to achieve continuous asexual multiplication (6).
Another important aspect of studying C. parvum is being able to quickly identify it. C. parvum, one of the fifteen Cryptosporidium species, is closely related to C. hominis. It is important to be able to identify the parasites from each other at the species level in clinical samples. Being able to track the species with ease is crucial for epidemiologic investigations. Although the most reliable way to differentiate C. parvum and C. hominis is through sequencing, it is very time consuming. A new Luminex-based assay has been tested, and it has proven to be rather effective. The assay uses species specific probes linked to carboxylated Luminex microspheres that hybridize to a Cryptosporidium sp. microsatellite-2 region. The two species differ in this region by one nucleotide substitution. This rapid assay is both cost effective and reliable. And it works with as few as 10 oocysts per 300 microliters of stools. This new assay could help to better track the spread of diseases and provide epidemiologic information (8).
Efforts have been put forth in trying to prevent cryptosporidiosis by disinfecting water supplies. Conventional chemical disinfectants such as chlorine or ozone are impractical due to the disinfection byproducts. Alternative water treatment methods have been investigated. Catalyzed-solar radiation is being tested as a way of eliminating C. parvum spores. In the study, Bacillus subtitlis spores were used in place of C. parvum oocysts. It was shown that 1.5 hours of exposure to close to 1 sun-intensity solar radiation with 5mM ferrous ion and 70mM hydrogen peroxide decreased the spore viability by 49 percent. The study of solar photocatalysis could be potentially useful in disinfecting water supplies especially in rural and underdeveloped areas (10).
References
Pain, A., Crossman, L., and Parkhill, J. “Comparative Apicomplexan genomics”. Nature Reviews Microbiology. 2005. Volume 3. p. 454-455. http://www.nature.com/nrmicro/journal/v3/n6/full/nrmicro1174_fs.html
Abrahamsen, M., Templeton, T., Enomoto, S., Abrahante, J., Zhu, G., Lancto, C., Deng, M., Liu, C., Widmer, G., Tzipori, S., Buck, G., Xu, P., Bankier, A., Dear, P., Konfortov, B., Spriggs, H., Lyer, L., Anantharaman, V., Aravind, L., and Kapur, V. “Complete Genome Sequence of the Apicomplexan, Cryptosporidium parvum”. Science. 2004. Volume 304. p. 441-445. http://www.sciencemag.org/cgi/content/full/304/5669/441
Rider, S., Cai, X., Sullivan, W., Smith, A., Radke, J., White, M., and Zhu, G. “The Protozoan Parasite Cryptosporidium parvum Possesses Two Functionally and Evolutionarily Divergent Replication Protein A Large Subunits”. J. Biol. Chem. 2005. Volume 280. p. 31460-31469. http://www.jbc.org/cgi/content/full/280/36/31460
Casadevall, A., Dadachova, E., Pirofski, L. “Passive Antibody Therapy for Infectious Diseases”. Nature Reviews Microbiology. 2004. Volume 2. p. 695-703 http://www.nature.com/nrmicro/journal/v2/n9/full/nrmicro974_fs.html
Pain, A., Crossman, L., Sebaihia, M., Cerdeño-Tárraga, A., and Parkhill, J. “Strength in Diversity”. Nature Reviews Microbiology. 2004. Volume 2. p. 358-359. http://www.nature.com/nrmicro/journal/v2/n5/full/nrmicro889_fs.html
Widmer, G., Yang, Y., Bonilla, R., Tanriverdi, S., and Ciociola, K. “Preferential infection of dividing cells by Cryptosporidium parvum”. Parasitology. 2006. Volume 133. p. 131-138. http://journals.cambridge.org/action/displayFulltext?type=6&fid=454979&jid=&volumeId=&issueId=02&aid=454977&fulltextType=RA&fileId=S0031182006000151
“Cryptosporidiosis Control and Prevention”. Center for Disease Control and Prevention. http://www.cdc.gov/ncidod/dpd/parasites/cryptosporidiosis/crypto_control_prevent.htm
Bandyopadhyay, K., Kella, K., Moura, I., Cristina, M., Carollo, C., Graczyk, T., Slemenda, S., ohnston, S., Silva, A. “A rapid microsphere assay for identification of Cryptosporidium hominis and Cryptosporidium parvum in stool and environmental samples”. 2007. Published online ahead of print. http://jcm.asm.org/cgi/reprint/JCM.00138-07v1?view=long&pmid=17652477
"Foodborne Pathogenic Microorganisms and Natural Toxins Handbook, Cryptosporidium parvum”. U.S. Food and Drug Administration. http://www.cfsan.fda.gov/~mow/chap24.html
Guisar R., Herrera, M., Bandala, E., Garcia, J., and Corona-Vasquez, B. “Inactivation of waterborne pathogens using solar photocatalysis”. Journal of Advanced Oxidation Technologies. 2007. Volume 10. p. 435-438. http://portal.isiknowledge.com/portal.cgi?DestApp=WOS&Func=Frame
Edited by student of Rachel Larsen Angela Wang
Edited by KLB
