الفهرس 
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Abstract
This study was performed to investigate prevalence of E.coli and Salmonella in broiler chicks in north delta in Egypt by traditional method and molecular one, in addition to antibiotic susceptibility of these isolates and study pathogenicity of E. coil in relation to their serotypes and virulence genes content.
Traditional investigation including culture and biochemical examination for 54 broiler flocks during first two weeks of age resulted in 28 isolates were positive for E. coli while only 4 isolates positive for Salmonella.
Serological investigation of E. coli showed that 10 isolates were nontypable and 18 isolates serotyped for 9 serotypes including (4) O115,(3) O142,(2) O158,(2) O55,(2) O125,(2) O114,(1) O27,(1) O20 and (1) O15 while Salmonella serotyped as S.enteritidis .
Molecular examination for 18 E. coli isolates by Pho A gene resulted in only 15 isolates confirmed as E. coli then these 15 isolates investigated for 3 virulence genes (iss, fim H and iut A) resulted in 14/15 have iss ,3/15 have fim H and 7/15 have iut A .
Molecular identification of S. enteritidis by using sefA gene, all 4 isolates were positive. Sequence analysis of (sefA) of four S. Enteritidis isolates was conducted and revealed that 4 isolates (MK610458 S. Enteritidis 1), (MK610459 S. Enteritidis 2),( MK610460 S. Enteritidis 3 ) and (MK610461 S. Enteritidis 4) were identical to each other and other strains and the percentage of identity was (99 to 100 %).
Susceptibility of fifteen E. coli isolates to different antibiotics recorded as the following gentamicin, ofloxacin, and enrofloxacin recorded the highest susceptibility by 8/15 (53.3%) followed by ciprofloxacin and florfenicol by 4/15 (26.7% ) and doxycycline, neomycin, colistin, and cefotaxime 1/15 (6.6%) while all isolates recorded 100% resistance to Ampicillin, Amoxycillin-clavulanic acid and tetracycline .
Antibiotic sensitivity of Salmonella was recorded as follow highest susceptibility was observed to enrofloxacin, ofloxacin, florphenicol, gentamicin and tetracycline (100%) followed by doxycycline and neomycin (50%). while exhibited 100 % resistance to ampicillin, amoxycillin-clavulanic acid and colistin, spiramycine
and cefotaxime (75%).
Pathogenicity testing for 13 selected E. coli isolates according to their serotypes and virulence genes content were applied in 2 modes (One day old SPF chicks and Embryonated chicken SPF Eggs ) and resulted in , SPF one day old chick model, two isolates were classified as highly virulent while the other 3 isolates were moderately virulent and the remaining 8 isolates recorded mild airsaculitis without mortalities that were avirulent strains. On the other hand, all thirteen tested isolates were very virulent by inoculation into SPF embryonated chicken eggs.