Escherichia coli, commonly called Ecoli, normally occurs in nature, including the intestinal tracts of humans and other vertebrates. In intestine it constitutes the major part of the normal enteric microflora and live as harmless strain. However, there are a few pathogenic strains of E.coli that can cause enteric and urogenital infections in man. Several different types of pathogenic E.coli are capable of causing diarrheal disease.
CELLULAR MORPHOLOGY
Ecoli is short, oval rod that occurs singly or in pairs. It is motile with peritrichous flagella. It does not possess spore and capsule. It is Gram-negative.
CULTURAL CHARACTERISTICS
PHYSICAL REQUIREMENTS
E. coli is a facultatively anaerobic bacterium. It grows well in a temperature range of 25 to 40°C with an optimum 37°C. Its growth is maximum when the pH is slightly alkaline, range being 6.8 to 7. 2. It does not require additional supply of carbon dioxide.
NUTRITIONAL REQUIREMENTS
E.coli can grow on simple laboratory media, such as nutrient Agar and nutrient broth. Selective and differential media for the isolation and differentiation of this bacterium include EMB agar, MacConkey agar, brilliant green agar, bromothymol blue lactose cystine agar, deoxycholate citrate agar, and crystal violet neutral red bile agar. It produces lactose-fermenting colonies on these media.
COLONIAL MORPHOLOGY
E. coli produces round, smooth, low convex, moist, shiny, entire, gray, translucent colonies on nutrient agar. In nutrient broth it produces a uniform turbidity and a heavy deposit that disappear on shaking.
BIOCHEMICAL CHARACTERISTICS
E. coli catabolizes glucose and other carbohydrates with the formation of acid and gas. It's characteristic biochemical activity is the fermentation of lactose with the production of acid and gas. It gives positive indole test and positive methyl red test.
ANTIGENIC STRUCTURE
E. coli contains two types of antigens: somatic 0 antigen and flagellar H antigen. The 0 antigen consists of polysaccharide chains associated with a core oligosaccharide anchored to a lipid complex. More than 164 different 0 antigens have been discovered and more are expected to be discovered.
The H antigen is protein in nature. More than 42 H antigens have been described in E. coli. A few isolates of this bacterium have been reported nonmotile and nonflagellated and thus H-negative.
A third type of antigen has also been described associated with the pili of E. coli. However, the clinical significance of this antigen is not very clear. Another antigen associated with the surface of E. coli is its capsular layer. This antigen is called K antigen. Approximately 100 types of K antigen have been described.
E. coli responsible for diarrheal diseases has serotype characters and are generally fit into one of four categories as shown in the following:
* Enterotoxigenic E. coli(ETEC)
* Enteropathogenic E. coli (EPEC)
* Enterohemorrhagic E. coli (EHEC)
* Enteroinvasive E.coli(ElEC)
PATHOGENESIS
E. coli is responsible for causing diarrheal diseases in man of all ages. White harmless strains of E. coli occur widely in nature, including the intestinal tracts of humans and other vertebrates, pathogenic types are a frequent cause of enteric and urogenital tract infections. Pathogenic types enter the human body orally by ingesting food or water contaminated with the human excreta. The colonization of the bacteria takes place in the intestinal tract where they produce endotoxins and exotoxins depending upon the pathogenic type. This leads to the diarrheal disease.
CLINICAL MANIFESTATIONS
The onset of the E. coli infections is rapid and duration is short. As mentioned above, there are four weII-documented pathogenic types of E. coli. A discussion of the clinical picture of E. coli diarrhea requires separate consideration of these pathogenic types.
ENTEROTOXIGENIC E. COLI INFECTION
ETEC produces a diarrheal disease in humans that is mediated through a heat labile toxin (LT) and/or a heat-stable toxin (ST). Toxin production is genetically encoded by transferable DNA plasmids. The mode of action of LT is almost same as that of cholera toxin since it acts on the enterocytes via CAMP. This leads to the massive secretion of water and lot of electrolytes in the lumen of the intestine. ST causes activation of guanylate cyclase and subsequently elevates the levels of cGMP. This results in the secretion of water and electrolytes. The clinical picture of the disease is indistinguishable from severe cholera. The symptoms of the illness include severe watery diarrhea, nausea, mal aise, vomiting, fever, and abdominal pain. Adults traveling in tropical region typically experience travelers’ diarrhea that is an acute but milder disease. Enterotoxigenic E. coli also causes diarrhea of varying severity in children, especially in nonindustrialized countries.
ENTEROHEMORRHAGIC E. COLI INFECTION
This is a dangerous type of E. coli infection that is caused by ingesting undercooked burgers, unpasteurized apple juice or cider, salad, salami, and unpasteurized milk. Many strains of EHEC are commonly found involved in majority of the cases of this infection include 0157:H7, 026:H11, and 0111 :H8. These strains produce toxins that have effects similar to those produced by Shigella dysenteriae. These enterotoxins cause necrosis of the mucous membrane of the intestine, anemia, stomach cramps and bloody diarrhea, and hemolytic uremic syndrome leading to kidney failure.
ENTEROPATHOGENIC E. COLI INFECTION
EPEC colonizes the epithelial surface of the mucous membrane of the intestine and subsequently causes necrosis and diarrhea. Evidence suggests that EPEC infection resembles Salmonella infection and that the bacteria show less invasive potential. The outbreaks of this illness occur sporadically, primarily in children. Diarrheal disease involving newborns is called infantile diarrhea.
ENTEROINVASIVE E. COLI INFECTION
EIEC produces an illness in adults and infants similar to that produced by Shigella species. It colonizes the epithelial surface of the intestine and causes epithelial cell death. The symptoms include fever, abdominal cramps, watery diarrhea, followed by gross dysentery with bloody stool and mucus.
LABORATORY DIAGNOSIS
Bacteriological diagnosis of E. coli infections mainly depends on the isolation and identification of E. coli from the clinical specimens. Since the patient discharges numerous E. coli cells in stool, the most useful clinical specimen is stool. lsolation of the bacterium is carried out on EMB or MacConkey agar or any other suitable medium. Pink, dark centered colonies with green metallic sheen are characteristically produced on EMB agar whereas red colonies are developed on MacConkey agar. Further identification is made on the basis of biochemical properties. Since the biochemical properties of the pathogenic strains of E. coli are identical to those of commensal nonpathogenic strains of the intestine, special tests are required to detect specific pathogenic groups of E. coli. These tests include enterotoxigenic test, serotyping, and DNA probing.
THERAPY
Oral administration of fluids and electrolytes is highly recommended in the therapy of E. coliEscherichia coli, commonly called Ecoli, normally occurs in nature, including the intestinal tracts of humans and other vertebrates. In intestine it constitutes the major part of the normal enteric microflora and live as harmless strain. However, there are a few pathogenic strains of E.coli that can cause enteric and urogenital infections in man. Several different types of pathogenic E.coli are capable of causing diarrheal disease.
PREVENTION
E. coli infections can be prevented by taking into consideration the measures found successful in the prevention of salmonellosis or other enteric diseases.
