Shigella
A Microbial Biorealm page on the genus Shigella
methyllobacillius flagellatus\
Background Information
History
Shigella organisms are a group of gram-negative pathogens, which were initially recognized as the causal agents of shigellosis (also known as bacillary dysentery) in the 1890s. Shigella became an official genus in the 1950s, consisting of four species: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. Each of these species have their own responsibilities. S. dysenteriae serotype 1 causes deadly epidemics, S boydii is restricted to the Indian subcontinent, and S. flexneri and S. sonnei are prevalent in developing and developed countries, respectively. S. flexneri, an enteroinvasive gram-negative bacterium, is responsible for the worldwide endemic form of bacillary dysentery.
Description
Shigiella is a non spore forming gram negative bacteria that aids in the facilitation of intracellular pathogens. It is able to survive the proteases and acids of the intestinal tract and infections to hosts can be caused from a very low dose. As little as 10 to 100 bacteria are needed to cause infection.
Genome structure
The four different species of Shigella vary greatly in the genomic structure. The largest species S. sonnei contains 4,825,265 base pairs. S. flexneri contains 4,607,203 base pairs, S. boydii contains 4,519,823 base pairs and the smallest species S. dysenteriae contains 4,369,232 base pairs.
Structure and Life Functions
Cell Structure
Shigiella is a non spore forming gram negative bacteria that aids in the facilitation of intracellular pathogens. It is able to survive the proteases and acids of the intestinal tract and infections to hosts can be caused from a very low dose. As little as 10 to 100 bacteria are needed to cause infection.
Life Cycle
The Shigella life cycle begins with penetration of colonic mucosa. This results in degradation of the epithelium and acute inflammatory colitis in the lamina propria. This causes leakage of blood, inflammation in the colon, and mucus into the intestinal lumen.
Metabolism
Shigella pathogens use a mixed acid fermentation pathway to metabolize substrates. Products of this anaerobic pathway include ethanol, acetic acid, lactic acid, succinic acid, formic acid, and CO2.
Pathology
Transmission
Fecal-oral transmission is the main path of Shigellosis infection however other modes of transmission include ingestion of contaminated food or water, contact with a contaminated inanimate object, and sexual contact. Outbreaks of Shigellosis infection are common in places where sanitation is poor.
Frequency
Shigella spp. Infects around 450,000 individuals just in the United States yearly, and of those 450,000 cases approximately 6,000 infected people require hospitalization to treat the aliment. Of the various strains of shigella, S. sonnei is the cause of 78% of infections, and S. flexneri, and S. boydii combined are responsible for the rest of the remaining 22% of cases. The occurrence of S. dysenteriae is rare in the United States, it is however more common in developing countries with poor sanitary conditions, and water purification systems. Worldwide there are approximately 165 million cases of shigella annually, with 98% of those cases occurring in third world, developing nations. In those developing nations shigella was responsible for 1 million deaths. Unlike in the United States there are a fair amount of cases especially those resulting in death are due to the infection of S. dysenteriae it accounts for 30% of infections. Developing countries are some 20 times more likely to develop a case of shigella then more developed countries. In developed countries the number of fatal cases is around 1%, and in countries of the Far and Middle East the fatality is more along the lines of 20% of cases result in death. The majority of cases of shigella are reported in the summer months. The majority of shigella cases occur in children 15 years old and under accounting for 50% of reported cases. This is most like due to poor personal hygiene and hand washing technique, or lack there of. It is difficult to have an extremely accurate gauge of the actual number of cases that occur because 90-95% of shigella infections are typically asymptomatic so they may go unnoticed, and thus unreported.
Current Research
High Prevalence of Antimicrobial Resistance among Shigella Isolates in the United States Tested by the National Antimicrobial Resistance Monitoring System from 1999 to 2002, Sivapalasingam, S., Nelson, J. M., Joyce, K., Hoekstra, M., Angulo, F. J., and Mintz, E. D.
-This information on shigella's resistance to various antibiotics will help in treating shigellosis. Click this hyperlink for the full text of this article.
References
Hale, Thomas L. Genetic Basis of Virulence in Shigella Species. Dept. of Enteric Infections, Walter Reed Amry Institute of Research. Washington, D. C.: American Society for Microbiology, 1991. 206-224. 10 Nov. 2006 <http://mmbr.asm.org/cgi/reprint/55/2/206.pdf>.
Hale, Thomas L., and Gerald T. Keusch. "Shigella." GSBS At UTMB. The Graduate School of Biomedical Sciences at UTMB. 10 Nov. 2006 <http://www.gsbs.utmb.edu/microbook/ch022.htm>.
Sivapalasingam, S. "High Prevalence of Antimicrobial Resistance among Shigella Isolates in the United States Tested by the National Antimicrobial Resistance Monitoring System from 1999 to 2002." PubMed Central. New York, NY: NYU School of Medicine, 2006. 17 Nov. 2006 <http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=16377666#N0x84ceb98.0x92357c0#N0x84ceb98.0x92357c0>.
Sureshbabu, Jaya, and Poothirikovil Venugopalan. "Shigella Infection." EMedicine From WebMD. 12 Sept. 2006. WebMD. 10 Nov. 2006 <http://www.emedicine.com/ped/topic2085.htm>.
Toebe, Carol. "Microbial Metabolism." CCSF. City College of San Francisco. 17 Nov. 2006 <http://cloud.ccsf.edu/Departments/Biology/ctoebe/metab.htm>.
Edited by Deidre DeSilva, Kayleigh Erazmus, and Megan Harney under Dr. Kirk Bartholomew of Sacred Heart University, Fairfield, CT.
