What is e. coli infection?
Escherichia coli (E. coli) is a rod-shaped, gram-negative bacterium and a member of the Enterobacteriaceae family. Its cytoplasm is enclosed by an inner membrane, a periplasmic space, a peptidoglycan layer, an outer membrane, and, finally, a capsule. Most strains produce two types of projections, flagellae for motility and fimbriae (pili) for cellular adhesion and genetic transfer. There is no nucleus. The genome consists of a single circular chromosome that is usually complemented by multiple plasmids. There are no intracellular organelles, and respiratory processes occur at the cellular membrane.
E. coli are found as normal flora in the gastrointestinal tracts of mammals and are the most common facultative anaerobes in the human intestinal tract. The traits that transform these benign inhabitants into disease-causing pathogens are called virulence factors. The virulence factors of E. coli may be divided into adhesins, toxins, and capsules. Adhesins consist of fimbriae or outer membrane proteins that allow the bacteria to bind to host cells and exert their disease-causing effects. Toxins are proteins made by E. coli that can be released to damage, or even kill, host cells. Capsules can enable the bacteria to elude the immune system and invade host tissues.
Plasmids can be transmitted between various strains of E. coli and other bacteria by a process called conjugation. In order for a bacterium to conjugate, it must possess F (fertility) factor, which is a specific type of plasmid that contains genes for plasmid DNA replication and pili construction. A bacterium with F factor can use its F factor–generated pili to hold onto another bacterium and inject selected portions of genetic material into the bacterial partner. This genetic material can add new virulence factors or antibiotic resistance attributes to the recipient bacterium.
The diseases caused by E. coli may be divided into intestinal and extraintestinal. The E. coli strains causing intestinal diseases are of several different types. Enteropathogenic E. coli (EPEC) cause disease by adhering to intestinal epithelial cells with an outer membrane adhesin (intimin) and special pili, both of which are plasmid-mediated. The exact mechanism by which these virulence factors alter the intestine—resulting in watery diarrhea, low-grade fever, and vomiting—is unclear. Enterotoxigenic E. coli (ETEC) cause illness using a combination of mucosal adherence and toxin production. The enterotoxin is similar to cholera toxin and alters ionic transfer in the intestinal cells, producing copious amounts of watery diarrhea. The illness can vary greatly, from a lack of symptoms to severe diarrhea with cramps, nausea, and dehydration. The adhesins and toxins appear to be mediated by both chromosomal and plasmid genes. Some E. coli have acquired genes from Shigella dysenteriae via conjugation, and these strains can produce Shiga toxins (STEC). The toxins permit intestinal invasion, resulting in painful, bloody diarrhea indistinguishable from shigellosis; such strains are referred to as enterohemorrhagic (EHEC). In about 5 percent of cases, Shiga toxins enter the bloodstream, causing damage to red blood cells, endothelial cells, and kidney cells; this is called the hemolytic-uremic syndrome (HUS). The life-threatening HUS is usually associated with the O157:H7 strain of E. coli and is seen more often in children than in adults. The O157:H7 strain often colonizes cattle, and humans may then acquire the infection from eating beef or fresh vegetables contaminated by cattle manure.
The extraintestinal diseases caused by E. coli vary widely. E. coli is the most common cause of urinary tract infections (UTIs). The strains that cause UTIs are different from those strains that colonize healthy individuals. These uropathogenic E. coli possess fimbriae the bind to cells lining the urinary tract. They are also encapsulated and produce a toxin (hemolysin). The E. coli may ascend the urinary tract through the urethra to the bladder and kidney. The route is more common in women because of a shorter urethra and can be facilitated in both men and women by the use of a urinary catheter. Infection of the kidney can also occur via the bloodstream. E. coli infection can follow surgery, especially when the intestinal tract is violated. Surgical wound infection, abscesses, and peritonitis are possible. Because ducts connect the gallbladder and pancreas directly with the intestinal lumen (cavity), E. coli often play a prominent role in cholecystitis and pancreatitis. Newborns with undeveloped immune systems may experience ear infections, bacteremia, or meningitis caused by E. coli. The strains producing neonatal meningitis have a K1 capsule, which may facilitate passage into the brain. Because E. coli are so common and possess many virulence factors, they can produce many additional types of infection.
Mild cases of diarrhea caused by EPEC strains usually can be managed with fluids and other supportive therapies, but the duration of illness may be made shorter with the use of antibiotic therapy. ETEC diarrhea is treated with loperamide and oral antibiotics. STEC and HUS are treated with supportive care. HUS may require renal dialysis.
Extraintestinal infection is treated by antibiotic therapy. Since E. coli are becoming increasingly resistant to antibiotics, susceptibility testing on the particular strain of E. coli causing the infection, isolated from diagnostic cultures, must be performed.
Escherichia coli is named after Theodore Escherich, who described the bacterium in a paper published in 1885. Frederich Blattner of the University of Wisconsin completed the sequencing of the 4,288 genes of the E. coli genome in 1997. Nearly half of these genes were newly identified.
Epidemiological studies have assisted in the understanding of the origins of E. coli infections. Intestinal illness can be prevented with improved farming methods, better food processing and handling, expanded sewage and sanitation facilities, purification of drinking water, and hand washing. The prevalence of hospital-acquired infections, such as UTIs, can be reduced by limiting the use of urinary catheters and the employment of closed drainage systems when catheters are necessary.
The treatment of extraintestinal E. coli infection, and some intestinal infections, depends upon the use of one or more effective antibiotics. Resistance is a rapidly growing problem and can be controlled only through reduction of the inappropriate use of antibiotics and the development of new agents.
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