Colibacillosis, an important disease of poultry caused by avian pathogenic Escherichia coli (APEC), is responsible for great economic losses in the poultry industry around the world. The aims of the present study were to investigate the prevalence of the increased serum survival (iss) gene and to compare the clonal distribution and relatedness of the isolates among septicemic and cecal (commensal) E. coli isolates from poultry in Iran using a random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR). A total of 97 (41 septicemic and 56 cecal isolates) were screened for the presence of the iss gene. Then, 66 isolates were randomly chosen for clonal comparison, including 44 iss-positive and 22 iss-negative isolates. The prevalence of the iss gene was 90.3% (37/41) in the septicemic and 64.3% (36/56) among cecal isolates, a significant difference (p = 0.004). The 66 isolates examined by RAPD-PCR were assigned to 4 main clusters, of which 2 were predominant (B and D, 65.6%). As 3 of the 4 clusters, including the 2 largest clusters, did not differ in the relative distribution of iss-positive isolates and as all 4 clusters did not differ in the source of isolates (tissue as compared to cecum), it appears that potentially pathogenic strains can be commonly distributed among the intestinal bacterial flora of birds. The fingerprinting analysis also yielded a high variety of RAPD profiles, indicating a substantial diversity among avian E. coli strains.

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