Escherichia coli is a gram-negative, rod-shaped, facultative anaerobic bacterium. This microorganism was first described in 1885 by the German pediatrician, Theodor Escherich. Most E.coli strains are part of the normal flora of the gastrointestinal tract of humans and animals. However there are some strains that have evolved into pathogenic E.coli by acquiring virulence factor through plasmids, and bacteriophages etc. Enterotoxigenic, Enteroinvasive, Enteroadherent, Enteroaggregative, Enterohemorrhagic, and Enteropathogenic strains are the six groups that constitute pathogenic E.coli (Kaper et al., 2004). Enterohemorrhagic E.coli (EHEC) group possess E.coli strains that produces Shiga toxin (Stxs) resulting in hemorrhagic colitis and hemolytic uremic syndrome (HUS) in humans. EHEC serotypes such as O26:H11, O911:H21, O111:H8, O157:NM, and O157:H7 are frequently associated with human disease (Paton et al., 1999). Enterohemorrhagic E.coli serotype O157:H7 was recognized in 1982 as a human pathogen associated with outbreak of bloody diarrhea in Oregon and Michigan, U.S.A (Riley et al., 1983). E.coli O157:H7 has become one of the most important foodborne pathogens. The most important means of transmitting the organism is foodborne i.e. ingesting contaminated foods, especially those of bovine origin, others include consumption of sprouts, lettuce etc. Cattle and poultry broilers have been identified as reservoir hosts of E.coli O157:H7, and they are asymptomatic (i.e. they do not show any symptoms). Escherichia coli O157:H7 thrives in diverse environments, from soil, sewage, and water ecosystems to the host gastrointestinal tract, and it can survive for long period of time in water. E.coli O157:H7 is a major pathogen for public health problems in many countries including Nigeria (Nweze 2009; Esumeh et al., 2011; Ekundayo et al., 2014; Igwe et al., 2015).
Scientific classification of E coli O157:H7
Domain ---------------Bacteria
Kingdom---------------Monerans
Phylum ---------------Proteobacteria
Class -----------------Gammaproteobacteria
Order-----------------Enterobacteriales
Family ---------------Enterobacteriaceae
Genus ----------------Escherichia
Species ---------------coli
Strain----------------O157:H7
Pathogenicity of Escherichia coli O157:H7
Enterohaemorrhagic E.coli (EHEC) are capable of targeting the intestinal epithelium and releasing toxins that create damage to other areas and/or the organs of the body. The main toxins associated with E.coli O157:H7 are the Shiga toxins (Stx) which can be encoded on multiple prophages within the genome of the pathogen. The toxin acts on the lining of the blood vessels i.e. endothelial cells. The B subunits of the toxin bind to the globotriasylceramide (Gb3) receptors of the cell where they are internalized. The A subunit of Shiga toxin is an N-glycosidase that modifies the RNA component of the ribosome to inactivate it and so bring a halt to protein synthesis leading to the death of the cell. The endothelial cells has to continually renew itself, so this killing of cells leads to a breakdown of the lining and to haemorrhage. The first response is usually bloody diarrhea. These toxins target other Gb3 receptors found within the central nervous system and organs such as the pancreas resulting in severe vascular damages, strokes, seizures or death.
Infections caused by Escherichia coli O157:H7
Human infection caused by E.coli O157:H7 can present a broad clinical spectrum ranging from asymptomatic cause to death. Most cases start with non-bloody diarrhea and self-resolve without further complications. However, some patients progress to bloody diarrhea in 1-3days. In 5-10% of those patients, the disease can advance to the life threatening HUS or thrombocytopenic purpura (TTP) (Banatvala et al., 2001). There is no specific treatment for E.coli O157:H7 infections, thus highly effective prevention and control of E.coli O157:H7 infections are essential.
Survival of E.coli O157:H7 in the Gastrointestinal Tract
Acid resistance (AR) - is the ability of bacteria to protect themselves from extreme low pH (<pH 3.0). The low pH in the stomach (pH 1.5-3.0) is one of the first host defenses against food borne enteric pathogens. Acid resistance lowers the infectious dose of enteric pathogens (Schlech et al., 1993). E.coli O157:H7 has a low infectious dose, thereby increasing the chance to colonize the intestines and cause infections.
Environment survival - Escherichia coli O157:H7 can survive and persist in numerous environments such as soil, water, and food as well as in animal reservoirs. Escherichia coli O157:H7 must adapt to variations or extreme changes in temperature, pH, and osmolarity to survive in varied environments.