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Background and Aim:  Avian colibacillosis, which is caused by avian pathogenic  Escherichia coli  (APEC), is a major bacterial disease that affects birds of all ages worldwide, causing significant economic losses. APEC manifests in several clinical forms, including cellulitis, and its high pathogenicity is attributed to harboring numerous virulence-associated genes (VGs). This study evaluated the pathogenicity of the cellulitis-derived  E. coli  (O78) strain through molecular identification of genes coding for seven virulence factors and by conducting an  in vivo  assessment of capability for cellulitis induction in broiler chickens.    Materials and Methods:  This study was performed using a previously isolated and identified cellulitis-derived  E. coli  (O78), which was screened for seven VGs using molecular detection and identification through polymerase chain reaction followed by nucleotide sequencing and phylogenetic analysis. Experimental infection by subcutaneous (SC) inoculation in broilers and its pathogenicity was confirmed  in vivo  by cellulitis induction. The impact of cellulitis on broiler performance was assessed.    Results:  Molecular genotyping proved that the isolate harbored five virulence genes ( iro N,  iut A,  tsh ,  iss , and  pap C) and was negative for  stx 1 and  hly  genes. The amplified products for  iro N,  iss , and  iut A were subjected to sequencing and phylogenetic analysis, and the results indicate the highest similarity and matching with  E. coli  submitted to the National Center for Biotechnology Information GenBank. SC inoculation of bacteria in broiler chickens resulted in cellulitis, as indicated by thick red edematous skin with yellowish-white material in the SC tissue at the inoculation site, and the abdominal muscle showed redness and increased vacuolization. Histopathological examination revealed moderate-to-severe caseous inflammatory reaction with a marked accumulation of heterophils and mononuclear cells in the SC fatty tissue. The average feed intake, body weight gain (BWG), and feed conversion ratio (FCR) were lower in infected chickens in comparison with those of the control non-infected chickens.    Conclusion:  This study proves that molecular techniques are accurate for pathogenicity determination in virulent bacteria, with the advantages of being rapid, time-saving, and economical. Cellulitis is associated with economic losses that are represented by a lower BWG and FCR.

Molecular identification, genotyping of virulence-associated genes, and pathogenicity of cellulitis-derived Escherichia coli