الفهرس 
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Abstract
the present study is designed to investigate the following: evaluation of the sensitivity and specificity of PCR technique in comparison with biochemical reaction by API 20 E and conventional culture methods by isolation of E coil from suspected samples collected from mastitic bovine milk identification of isolates using conventional methods and adaption of PCR technique for detection of E coil to give accurate diagnosis by using specific primers. Escherich, (1885) was the first who identified E. coli in 1885, and the bacterium was known as bacterium coli commune. De. et al., (1956) identified enterotoxins secreted by E. coli able to mediate increased fluid secretion in gut leading to diarrhoea based on virulence properties of E. coli, their difference in epidemiology and distinct O:H serotypes In recent years, there has been considerable increase in the incidence of drug resistance in bacteria due to extensive and indiscriminate use of antimicrobial agents for therapy, prophylaxis or growth promotion. The emergence of drug resistant strains following continuous feeding of antibiotics to livestock has led to loss of their efficacy (Smith and Hall, 1966 and Singh et al., 1992).
The organism typically colonizes the infant gastrointestinal tract within hours of life and thereafter, both E. coli and the host derive mutual benefit (Drasar and Hill, 1974). Wilson et al. (1983) reported that the microorgansim originally called bacterium coli was isolated from the feces of infants by Escherich over a century ago.
Martin et al. (1986) stated that raw milk was first recognized as a vehicle oftransmission of E. coli. Levine, (1987) stated that In the debilitated or immunosupressed host or when gastrointestinal barriers are violated even normal non pathogenic strains of E. coli can cause infection. Pathogenic E. coli are one of the most important groups of bacteria causing diarrhoea and extraintestinal infections in humans and animals.Jones, (1990) stated that millions of non-pathogenic E. coli bacteria are living in the normal intestinal microflora of both humans and animals. E. coli adheres to the mucus overlaying the large intestine where they constitute a useful microbial barrier because of their antagonism to pathogenic bacteria. E. coli is massively excreted with the faeces in the environment. E. coli is ubiquitous in the cow’s environment. It is obvious that challenge of E. coli udder infection will remain so long as cows produce faeces.
Varnam and Evans (1991) stated that subdivided the pathogen strains of E.
coli subdivided based on clinical signs, mechanisms of pathogenesis, biochemical and serological markers into five groups : enterpathogenic (EPEC), enterotoxigenic ETEC), enteroinvesive (EIEC), enteroaggregative, diffuse adherent (Eagg- DAEC), and enterohaemorrhagic (EHEC).
Pathogenic E. coli are one of the most important groups of bacteria causing extraintestinal infections. Since that, it has become a pathogen of major concern in both food and dairy industries, because of its ability to cause severe illness to humans and animals (Abdul-Raouf et al., 1993).Cullor, (1996) stated that E. coli is a Gram-negative, non-spore-forming rod, which belongs to the family Enterobacteriaceae. Gram-negative bacteria have a cell wall that typically consists of three layers, the cytoplasmic membrane, the peptidoglycan layer and the outer membrane. The outer cell membrane contains phospholipids, membrane proteins and lipopolysaccharide (LPS). Lipopolysaccharide comprises lipid-A, the lipopolysaccharide core and repeated polysaccharide units, called O-antigens. He stated that on the outer surface, bacteria may have fimbriae, which protrude from the cell wall. The surface mayalso be covered with a thick polysaccharide layer, called a capsule. Based on the different structures of O-antigens, K-antigens (capsular) and H-antigens (flagellar), E. coli can be divided into O:H:K serotypes.Lipid-A is the lipophilic, inner part of LPS, which causes the toxic effects of LPS, also known as endotoxin (Cullor, 1996; Hogan and Smith, 2003). E. coli is part of the normal intestinal flora of humans and animals, and is the most common facultative anaerobic bacterial species in the gut. The bacteria are constantly excreted in the faeces to the environment. Pathogenic E. coli bacteria can cause intestinal and extra-intestinal infections in mammalian and avian hosts (Cullor, 1996; Nagy and Fekete, 1999). Cullor, (1996) stated that E. coli is the predominant cause of urinary tract infection in humans, and also causes invasive diseases, such as bacteraemia and meningitis, in humans and animals.
China and Guffaux,(1999) stated that E. coli is a member of the family of Enterobacteriaceae. of primary importance is their ability for fermentation of lactose. Over 700 antigenic types or serotypes of E. coli have been recognised based on O, H, and K antigens. Two classes of coliforms have to be distinguished strains that are harmless (non-pathogenic strains) and strains that cause a wide variety of typical clinical infections (pathogenic strains).Lehtolainen, (2004) mentioned that E. coli strains involved in acute clinical mastitis have, however, no specific virulence factors.
2. Mastitis overview :- Mastitis is defined as an inflammatory reaction of the mammary gland (International Dairy Federation, 1987). In practice, whether a case of mastitis is classified as clinical or subclinical often depends on how carefully the cow is observed when diagnosis is made The extent of these changes is determined by the severity of the infection. (International Dairy Federation, 1987; Harmon, 1994 and Pyörälä, 2003).