Unique chromosomal regions associated with virulence of an avian pathogenic Escherichia coli strain

The avian pathogenic Escherichia coli strain X7122 (serotype O78:K80:H9) causes airsacculitis and coli-septicemia in chickens. To identify genes associated with avian disease, a genomic subtraction technique was performed between strain x7122 and the E. coli K-12 strain X289. The DNA isolated using this method was found only in strain X7122 and was used to identify cosmid clones carrying unique DNA from a library of X7122 that were then used to map the position of unique DNA on the E. coli chromosome. A total of 12 unique regions were found, 5 of which correspond to previously identified positions for unique DNA sequence in E. coli strains. To assess the role each unique region plays in virulence, mutants of X7122 were constructed in which a segment of unique DNA was replaced with E. coli K-12 DNA by cotransduction of linked transposon insertions in DNA flanking the unique sequence. The resulting replacement mutants were assessed for inability to colonize the air sac and cause septicemia in 2-week-old white Leghorn chickens. Two mutants were found to be avirulent when injected into the right caudal air sac of 2- week-hid chickens. One avirulent mutant, designated X7145, carries a replacement of the rfb locus at 44 min, generating a rough phenotype. The second mutant is designated X7146, and carries a replacement at position 0.0 min on the genetic map. Both mutants could be complemented to partial virulence by cosmids carrying sequences unique to X7122.