Functional Features of TonB Energy Transduction Systems of Acinetobacter baumannii

Abstract

Acinetobacter baumannii is an opportunistic pathogen that causes severe nosocomial infections. Strain ATCC 19606 T utilizes the siderophore acinetobactin to acquire iron under iron-limiting conditions encountered in the host. Accordingly, the genome of this strain has three tonB genes encoding proteins for energy transduction functions needed for the active transport of nutrients, including iron, through the outer membrane. Phylogenetic analysis indicates that these tonB genes, which are present in the genomes of all sequenced A. baumannii strains, were acquired from different sources. Two of these genes occur as components of tonB-exbB-exbD operons and one as a monocistronic copy; all are actively transcribed in ATCC 19606T . The abilities of components of these TonB systems to complement the growth defect of Escherichia coli W3110 mutants KP1344 (tonB) and RA1051 (exbBD) under iron-chelated conditions further support the roles of these TonB systems in iron acquisition. Mutagenesis analysis of ATCC 19606 T tonB 1 (subscripted numbers represent different copies of genes or proteins) and tonB 2 supports this hypothesis: their inactivation results in growth defects in iron-chelated media, without affecting acinetobactin biosynthesis or the production of the acinetobactin outer membrane receptor protein BauA. In vivo assays using Galleria mellonella show that each TonB protein is involved in, but not essential for, bacterial virulence in this infection model. Furthermore, we observed that TonB 2 plays a role in the ability of bacteria to bind to fibronectin and to adhere to A549 cells by uncharacterized mechanisms. Taken together, these results indicate that A. baumannii ATCC 19606 T produces three independent TonB proteins, which appear to provide the energy-transducing functions needed for iron acquisition and cellular processes that play a role in the virulence of this pathogen.

Structure and Function of the PiuA and PirA Siderophore-Drug Receptors from Pseudomonas aeruginosa and Acinetobacter baumannii

“…A similar growth impairment under iron deficiency was reported by Zimbler et al for tonB1 and tonB2 deletion mutants of strain ATCC 19606 (42). Since the authors of the study could not generate a tonB3 deletion mutant by homologous recombination, the growth phenotype could not be assessed, and the authors concluded the TonB3-ExbB3-ExbD3 system was essential in A. baumannii.…”

“…Interestingly, the tonB3 gene, but not the tonB1 and tonB2 genes (see Fig. S2 in the supplemental material), has recently been shown to be induced under iron-limiting conditions (42). The TonB3-ExbB3-ExbD3 system could therefore be a likely candidate for providing energy to PiuA and PirA siderophore receptors in A. baumannii.…”

“…Whether this was due to the experimental conditions or to differences in strain backgrounds is unclear. Supplementation with iron could restore the growth defects in their tonB1 and tonB2 mutants (42), as well as in our spontaneous tonB3 and exbD3 mutants of strain 307-0294 (Fig. 5).…”

“…S2 in the supplemental material). A. baumannii harbors three distinct chromosomally encoded TonB systems, among which TonB3-ExbB3-ExbD3 was recently shown to be iron regulated (42). Spontaneous resistant clones able to grow on Ͼ4 mg/liter of BAL30072 carried frameshift mutations in tonB3 or exbD3 (see Fig.…”

Acinetobacter baumannii Response to Host-Mediated Zinc Limitation Requires the Transcriptional Regulator Zur

“…In this regard, we identified a TonB system that is also Zur regulated. This could reflect a specialized system in A. baumannii, an organism that encodes several predicted TonB systems (53,54). Furthermore, other regulators, transporters, virulence factors, and secreted proteins have been identified as being Zur or Zn regulated in various bacterial species (39-45, 48, 50).…”