| Colibacillosis, caused by avian pathogenic Escherichia coli (APEC) is a common disease of poultry resulting in significant economic losses in poultry industry in many parts of the world. We investigated 243 E.coli strains collected from visceral organs of poultry having died from colibacillosis from 18 provinces in China from 1995 to 2005, most of which belong to common O serogroups, such as 078, 02 and 01.The Pathogenicities of 243 avian Escherichia coli strains isolated from different regions of China were evaluated in commercial day-old chickens. Six birds per isolate were inoculated intratracheally with 0.1ml of a 24-hour Luria-Bertani (LB) broth culture containing 107 colony-forming units (CFU) of E.coli. Based on mortalities and lesions occurring during a 7-day postinoculation period, 83.5%, 14.0% and 2.5% of 243 isolates were characterized as high, intermediate and low in pathogenicity, respectively. Among them, 87.9% (109/124), 10.5% (13/124), 1.6% (2/124) of 078 serogroups isolates and 82.4% (42/51), 13.7% (7/51), 3.9% (2/51) of 02 serogroups isolates were high, intermediate and low in Pathogenicity, respectively. These results suggested that, in all Escherichia coli isolates tested in this study, more than 80% were high pathogenic ones, while less than 20% were intermediate to low in pathogenicity.In high pathogenic E.coli isolates, 61.6% (125/203), 27.1% (55/203), 8.9% (18/203) and 2.5% (5/203) of the strains were resistant, intermediate resistant, slow-resistant andsensitive to chicken sera. In intermediate pathogenic E.coli isolates, 29.4% (10/34), 44.1% (15/34), 17.6% (6/34) and 8.8% (3/34) of those isolates were resistant, intermediate resistant, slow-resistant and sensitive to chicken sera. 3 of 6 isolates with low pathogenicity were resistant to chicken sera, and each was intermediate resistant, slow-resistant and sensitive to chicken sera, respectively. The high prevalence of resistance to chicken sera in high pathogenic isolates suggested that the resistance to chicken sera seemed to be important for the pathogenesis of colibacillosis.In high pathogenic E.coli isolates, 64.0% (130/203) strains showed mannose sensitive hemagglutination (MSHA) and 6.9% (14/203) strains caused mannose resistant hemagglutination (MRHA) of chicken erythrocytes if those isolates were grown at 37℃. 77.5% (117/151) strains showed MSHA and 3.3% (5/151) strains caused MRHA of guinea pigs erythrocytes if those isolates were grown at 37℃. In intermediate pathogenic E.coli isolates, 52.9% (18/34) strains showed MSHA and 5.9% (2/34) strains caused MRHA of chicken erythrocytes if those isolates were grown at 37℃ while 70.0% (14/20) strains showed MSHA of guinea pigs erythrocytes if those isolates were grown at 37℃. 3 of 6 isolates with low pathogenicity exhibited MSHA of chicken erythrocytes while the other one showed MRHA if those isolates were grown at 37℃. 2 of 4 isolates with low pathogenicity exhibited MSHA and MRHA of guinea pigs erythrocytes respectively if those isolates were grown at 37℃.In high pathogenic E.coli isolates. 51.2% (104/203) strains showed MSHA of chicken erythrocytes if those isolates were grown at 26℃, while 41.2% (14/34) of those intermediate pathogenic E.coli isolates exhibited MSHA of chicken erythrocytes when cultured in the same condition. One of 6 isolates with low pathogenicity showed MSHA of chicken erythrocytes if these isolates were grown at 26℃.To get more insights in the distribution and the occurrence of the virulence-associated genes of avian pathogenic E.coli. we examined all 243 E. colistrains for the presences of the following genes: aer, tsh, iro, irp2,fyuA, ler and eaeA. The genes aer, tsh and iro code for aerobactin, temperature-sensitive hemagglutinin and iro system, respectively. Irp2 and fyuA (HPI) were regularly detected in combination. In high pathogenic E.coli isolates, aer, tsh, iro and HPI were presented in 70.9% (144/203), 73.9% (150/203), 90.6% (184/203) and 71.9% (146/203), no ler or eaeA genes were detected in these isolates. 47.1% (16/34), 47.1% (16/34), 52.9% (18/...
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