Increased tracheal colonization in chickens without impairing pathogenic properties of avian pathogenic Escherichia coli MT78 with a fimH deletion

Several studies suggest that the expression of F1 fimbriae could be involve d in the virulence of Escherichia coli for chickens. Fl fimbriae display mu ltivalent properties such as adhesion to epithelia or interaction with the immune system chat imply specific interactions between the adhesin FimH and different cell receptors. We constructed a Delta fimH mutant of the avian pathogenic E. coli MT78 and evaluated its in vivo colonization and pathogen icity, as compared to that of the parent strain. The generated mutant PA68 was unable to adhere in vitro to chicken epithelial pharyngeal or tracheal cells; mutant bacteria were mostly afimbriated although a minority of them displayed altered piliation phenotypes. Two inoculation routes were used to compare the ability of MT78 and PA68 to colonize the respiratory tract and to induce colibacillosis in chickens. In the first model, 2-wk-old axenic chickens were inoculated intratracheally with one or both E. coli strains, after primary infection with infectious bronchitis virus. In the second mod el, 3-wk-old specific-pathogen-free chickens were inoculated via the caudal thoracic air sac. After intratracheal inoculation, the Delta fimH mutant w as found to be a better colonizer than MT78 in the trachea of inoculated ch ickens. Furthermore, when both strains were inoculated simultaneously, the Delta fimH mutant constituted 98% of the bacterial population in the trache a at day 7 postinoculation. Irrespective to the inoculation route, MT78 and PA68 showed similar abilities to induce macroscopic lesions in chickens, t o provoke bacteremia, and to colonize the internal organs. However, 4 days after intra-air sac inoculation, bacterial counts of the mutant were lower in the spleen and liver than those of MT78. Our results show that FimH is n ot required for colonization of the trachea of axenic chickens by E. roll a nd that it is not a major determinant of bacterial pathogenicity On the con trary, the lack of expression of FimH seems to favor the in vivo colonizati on of the trachea of chickens by E. coli.