Escherichia coli O157:H7
A Microbial Biorealm page on the genus Escherichia coli O157:H7
Bacteria; Protebacteria; Gammaproteobacteria; Enterobacteriales; Enterobacteriaceae; Escherichia; E. coli (3)
Description and significance
Escherichia coli (E. coli) were first discovered in 1885 by Theodor Escherich, a German bacteriologist. E. coli in human large intestine can assist with waste processing and food absorption, but not all E. coli are helpful to humans, other stains are infectious. A pathogenic strain of E. coli is E. coli O157:H7, a member of the enterohemorrhahic E. coli group and lives in cattle’s intestines. E. coli O157:H7 produces Shiga toxins, causing severe illness by contaminated meat (2). Escherichia coli O157:H7 was first identified as a human pathogen in 1982 in the United States of America, following an outbreak of bloody diarrhea associated with contaminated hamburger meat. In 2006, there was an outbreak involving raw spinach, with 199 illnesses, 102 hospitalizations, 31 hemolytic uremic syndrome, a severe kidney condition, and three deaths in the US (1). Prevention of E.coli O157:H7 contamination is by washing hands thoroughly after using the bathroom, before preparing or eating food, and contact with animals. Cook meats thoroughly at a temperature of at least 160°F/70˚C. Lastly, avoid raw milk, unpasteurized dairy products, and unpasteurized juices; and avoid swallowing water when swimming or playing in lakes, ponds, streams, and swimming pools (1).
E. coli has only one circular chromosome and a circular plasmid. The complete sequence of the O157:H7 chromosome is 5,498,450 bp in length. Since the strain contains a large virulence plasmid of 92,721 bpm and a cryptic plasmid of 3306 bp the whole genome size is 5, 594, 477 bp (4).
Cell structure, metabolism & life cycle
E. coli serotype O157:H7 is a mesophilic, Gram-negative rod-shaped (Bacilli) bacterium, which possesses adhesive fimbriae and a cell wall that consists of an outer membrane containing lipopolysaccharides, a periplasmic space with a peptidoglycan layer, and an inner, cytoplasmic membrane. Some strains are piliated and capable of accepting and transferring plasmid to and from other bacteria (1). Though it has extremely simple cell structure, with only one chromosomal DNA and a plasmid, it can perform complicated metabolism to maintain its cell growth and cell division.
E. coli possesses operons for transport and utilization of sucrose, urease, and sorbose, but the sorbose operon is disrupted by the insertion of a Mu-like phage. The strain also possesses a glutamate-fermentation system and two aromatic acid degradation systems that are not present in E. coli K-12 (4).
Ecology (including pathogenesis)
Escherichia coli can be commonly found in lower intestines of human and mammals and help with digestion processes. However, different strains of E. coli like E. coli O157:H7 is one of the most infective strains that can cause food poisoning. E. coli O157:H7 is found in the intestines of healthy cattle and are used as reservoir. The Shiga toxin released from E. coli requires highly specific receptors on the cells' surface in order to attach and enter the cell; species such as cattle, pig, and deer which do not carry these receptors may harbor toxigenic bacteria without any ill effect, shedding them in their feces. Shiga toxin is the main virulence factor of E. coli O157:H7 infection (5). Shiga toxin released from E. coli is responsible for foodborne illnesses that cause severe damage to the lining of the intestine leading to hemorrhagic colitis and kidney failure. Shiga toxin targets an organ such as the kidney and binds to receptors on cell membranes. Shiga toxin enters cells and stops the cells from producing proteins it needs to function. Without the ability to sustain its function, the cell dies through either apoptosis or necrosis. By killing cells crucial for organ function, Shiga toxins can have a debilitating effect on the body (5). Implicated foods are typically those derived from cattle (e.g., beef, hamburger, raw milk); however, the infection has also been transmitted through contact with infected persons, contaminated water, and other contaminated food products (1). Some important symptoms are bloody diarrhea, stomach cramping, vomiting, and loss of appetite, abdominal pain, and fever and kidney failure. Infection with E. coli O157:H7 is diagnosed by detecting the bacterium in the stool which is essential for public health purposes, such as finding outbreaks (6).
According to CDC, in 1999 E. coli O157:H7 was responsible for an estimation of 73,480 cases of illness, 2,168 hospitalizations, and 61 deaths annually in the USA. Right now there has been only seven cases of E. coli outbreak .
2.Escherichia coli O157:h7 and other shiga toxin-producing Escherichia coli (stec) . (2011, july 8). Retrieved from http://www.cdc.gov/nczved/divisions/dfbmd/diseases/ecoli_o157h7/
3.Escherichia coli O157:H7. NCBI taxonomy browser. 10/22/2011. http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi
4.Hayashi, T., Makino, K., Ohnishi, M., Kurokawa, K., Ishii, K., Yokoyama, K., Han, C., & Ohtsubo, E. (2001). Complete genome sequence of enterohemorrhagic Escherichia coli O157:h7 and genomic comparison with a laboratory strain k-12. DNA Research, 8, 11–22.
5.Hurley BP, Thorpe CM, Acheson DWK. 1998. Shiga toxin translocation across intestinal epithelial cells is enhanced by neutrophil transmigration. Infect Immun. 69:10: 6148-6155.
6.Mohawk, K., & O'Brien, A. (2010). Mouse models of Escherichia coli O157:h7 infection and shiga toxin injection. Journal of Biomedicine and Biotechnology, 2011, 1-17.