Characterization and Functional Analysis of AatB, a Novel Autotransporter Adhesin and Virulence Factor of Avian Pathogenic Escherichia coli

Abstract

ABSTRACTAutotransporter (AT) proteins constitute a large family of extracellular proteins that contribute to bacterial virulence. A novel AT adhesin gene,aatB, was identified in avian pathogenicEscherichia coli(APEC) DE205B via genomic analyses. The open reading frame ofaatBwas 1,017 bp, encoding a putative 36.3-kDa protein which contained structural motifs characteristic for AT proteins: a signal peptide, a passenger domain, and a translocator domain. The predicted three-dimensional structure of AatB consisted of two distinct domains, the C-terminal β-barrel translocator domain and an N-terminal passenger domain. The prevalence analyses ofaatBin APEC indicated thataatBwas detected in 26.4% (72/273) of APEC strains and was strongly associated with phylogenetic groups D and B2. Quantitative real-time reverse transcription-PCR analyses revealed that AatB expression was increased during infectionin vitroandin vivo. Moreover, AatB could elicit antibodies in infected ducks, suggesting that AatB is involved in APEC pathogenicity. Thus, APEC DE205B strains with a mutatedaatBgene and mutated strains complemented with theaatBgene were constructed. Inactivation ofaatBresulted in a reduced capacity to adhere to DF-1 cells, defective virulence capacityin vivo, and decreased colonization capacity in lung during systemic infection compared with the capacities of the wild-type strain. Furthermore, these capacities were restored in the complementation strains. These results indicated that AatB makes a significant contribution to APEC virulence through bacterial adherence to host tissuesin vivoandin vitro. In addition, biofilm formation assays with strain AAEC189 expressing AatB indicated that AatB mediates biofilm formation.