Abstract
Avian pathogenic Escherichia coli (APEC), a pathotype of extraintestinal pathogenic E. coli, causes one of the most serious infectious diseases of poultry and shares some common virulence genes with neonatal meningitis-associated E. coli. TonB-dependent receptors (TBDRs) are ubiquitous outer membrane β-barrel proteins; they play an important role in the recognition of siderophores during iron uptake. Here, in the APEC strain DE205B, we investigated the role of four putative TBDRs—ireA, 0007, 0008, and 2235—in iron uptake. Glutathione-S-transferase pulldown assays indicated that the proteins encoded by these genes directly interact with TonB. Moreover, the expression levels of all four genes were significantly upregulated under iron-depleted conditions compared with iron-rich conditions. The expression levels of several iron uptake-related genes were significantly increased in the ireA, 0007, 0008, and 2235 deletion strains, with the upregulation being the most prominent in the ireA deletion mutant. Furthermore, iron uptake by the ireA deletion strain was significantly increased compared to that by the wild-type strain. Moreover, a tonB mutant strain was constructed to study the effect of tonB deletion on the TBDRs. We found that regardless of the presence of tonB, the expression levels of the genes encoding the four TBDRs were regulated by fur. In conclusion, our findings indicated that ireA, 0007, 0008, and 2235 indeed encode TBDRs, with ireA having the most important role in iron uptake. These results should help future studies explore the mechanisms underlying the TonB-dependent iron uptake pathway.
Highlights
Avian pathogenic Escherichia coli (APEC), a pathotype of extraintestinal pathogenic E. coli (ExPEC), causes serious infectious diseases in poultry [1, 2]
Iron depletion upregulated the expression levels of the genes encoding the putative TonB-dependent receptors (TBDRs) The expression levels of all four genes were significantly upregulated under iron-depleted conditions compared with iron-rich conditions; the expression levels of 0007, 0008 and 2235 were upregulated 1.76 times (P < 0.001), 1.43 times (P < 0.05), and 1.71 times (P < 0.01), respectively
The obtained results indicated that the growth rates of the mutant strains (DE205BΔireA, Iron uptake by DE205BΔireA increased and that by DE205BΔtonB decreased We found that at the mid-log phase of growth, Fe uptake by DE205BΔireA (Figure 5A) increased (P < 0.05) compared to that by the WT strain, whereas Fe uptake by DE205BΔtonB (Figure 5B) decreased (P < 0.05)
Summary
Avian pathogenic Escherichia coli (APEC), a pathotype of extraintestinal pathogenic E. coli (ExPEC), causes serious infectious diseases in poultry [1, 2]. Bacteria either excessively reduce the external pH or dissolve iron oxide to meet their iron requirements by reducing ferric iron to a relatively soluble ferrous form Another common strategy is to synthesize and secrete iron chelators, such as siderophores, as intracellular iron (Fe2+) is rarely found in natural conditions, and Fe2+ can be readily oxidized to Fe3+ in the presence of oxygen and water [15]. Siderophore-mediated ferric uptake requires specific outer membrane (OM) receptors, such as FhuA, FecA, and FepA, which reportedly exist in E. coli [16]. These OM receptors share the same structural properties and acquire energy by coupling with TonB proteins located in the inner membrane, and they are referred to as TonB-dependent receptors (TBDRs) [17]
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