Contribution of Active Iron Uptake to Acinetobacter baumannii Pathogenicity

Abstract

Acinetobacter baumannii is an important nosocomial pathogen. Mechanisms that allow A. baumannii to cause human infection are still poorly understood. Iron is an essential nutrient for bacterial growth in vivo, and the multiplicity of iron uptake systems in A. baumannii suggests that iron acquisition contributes to the ability of A. baumannii to cause infection. In Gram-negative bacteria, active transport of ferrisiderophores and heme relies on the conserved TonB-ExbB-ExbD energytransducing complex, while active uptake of ferrous iron is mediated by the Feo system. The A. baumannii genome invariably contains three tonB genes (tonB1, tonB2, and tonB3), whose role in iron uptake is poorly understood. Here, we generated A. baumannii mutants with knockout mutations in the feo and/or tonB gene. We report that tonB3 is essential for A. baumannii growth under iron-limiting conditions, whereas tonB1, tonB2, and feoB appear to be dispensable for ferric iron uptake. tonB3 deletion resulted in reduced intracellular iron content despite siderophore overproduction, supporting a key role of TonB3 in iron uptake. In contrast to the case for tonB1 and tonB2, the promoters of tonB3 and feo contain functional Fur boxes and are upregulated in iron-poor media. Both TonB3 and Feo systems are required for growth in complement-free human serum and contribute to resistance to the bactericidal activity of normal human serum, but only TonB3 appears to be essential for virulence in insect and mouse models of infection. Our findings highlight a central role of the TonB3 system for A. baumannii pathogenicity. Hence, TonB3 represents a promising target for novel antibacterial therapies and for the generation of attenuated vaccine strains.

The UDP‐GalNAcA biosynthesis genes gna‐gne2 are required to maintain cell envelope integrity and in vivo fitness in multi‐drug resistant Acinetobacter baumannii

“…Until recently, just a subset of virulence factors had been identified, including capsule, lipooligosaccharide, metal acquisition systems (iron and zinc), secretion systems (Type I, II and VI), the phenotypic switch and a lytic transglycosylase (Wong et al, 2017;Chin et al, 2018;Crépin et al, 2018;Harding et al, 2018;Waack et al, 2018;Runci et al, 2019). However, the regulation of these factors and their specific roles in the pathobiology of A. baumannii infections are yet to be elucidated.…”

New Shuttle Vectors for Real-Time Gene Expression Analysis in Multidrug-Resistant Acinetobacter Species: In Vitro and In Vivo Responses to Environmental Stressors

“…Since Fur-dependent regulation of PbasA has previously been demonstrated (19,20,43), we initially wondered if pLPV plasmids could directly be employed for the genetic screening of Fur-repressible promoters by a Fur titration assay (FURTA) (44). For this purpose, E. coli H1717 carrying either pLPV1Z::PbasA or pLPV3Z::PbasA was grown on MacConkey agar plates supplemented with increasing Fe 2ϩ concentrations, and ␤-galactosidase expression directed by the chromosomal fhuA::lacZ fusion was recorded after overnight incubation at 37°C (44).…”

“…Accordingly, growth of A. baumannii under conditions of increasing iron deficiency (M9-S medium supplemented with different concentrations of either FeCl 3 or DIP) showed progressive upregulation of the PbasA promoter with decreasing iron availability. The lacZ-based pLPV2Z::PbasA fusion provided a more robust and linear response to iron starvation than pLPV1Z::PbasA and pLPV3Z::PbasA and overall higher ␤-galactosidase levels than with the low-copy-number pMP220::PbasA construct, which was previously employed by our group to probe the iron starvation response in A. baumannii (19,20) (see Fig. S5 in the supplemental material).…”

“…Two promoter-probe vectors have occasionally been used for gene expression analysis in Acinetobacter spp., namely, the psychrophilic species-restricted vector pBAP1 (17) and the low-copy-number broad-host-range lacZ-based pMP220 plasmid (18). Although pMP220 made it possible to investigate iron and ethanol regulation in A. baumannii (14,19,20), it suffers from major limitations due to complexity of manipulation, an inadequate resistance marker (tetA) for selection in MDR Acinetobacter strains, and poor signal output. A new set of Escherichia coli-Acinetobacter species shuttle vectors has recently been generated, carrying suitable zeocin (Zeo; ble) and/or gentamicin (Gm; aacC1) resistance cassettes, and used as gene cloning vehicles in MDR, XDR, and PDR Acinetobacter spp.…”

Protective potentials of a siderophore receptor against Acinetobacter baumannii infections

“…Using specific antibodies is considered as a strategy to prevent the A. baumannii infections for high risk persons [39,40]. Iron uptake system, as an important virulence factor of A. baumannii, has been studied in terms of functionality, pathogenicity, and immunogenicity [11, [41][42][43][44][45][46][47]. WP_000413999, a siderophore receptor from cluster I, is strongly expressed in the early stages of bacteremia [26].…”