BackgroundIron acquisition systems are important virulence factors in pathogenic bacteria. To identify these systems in Acinetobacter baumannii, the transcriptomic response of the completely sequenced strain ATCC 17978 under iron limiting conditions was investigated using a genomic microarray that contained probes for all annotated open reading frames.ResultsUnder low iron conditions, transcription levels were more than 2-fold up-regulated for 463 genes, including 95 genes that were up-regulated more than 4-fold. Of particular significance, three siderophore biosynthesis gene clusters, including one novel cluster, were highly up-regulated. Binding sites for the ferric uptake regulator were identified in the promoter regions of many up-regulated genes, suggesting a prominent role for this regulator in the Acinetobacter iron acquisition response. Down-regulation under iron limitation was less dramatic as the transcription of only 202 genes varied more than 2-fold. Various genes involved in motility featured prominently amongst the genes down-regulated when iron was less readily available. Motility assays confirmed that these transcriptional changes are manifested at the phenotypic level. The siderophore biosynthesis gene clusters were further investigated by means of comparative genomic analysis of 10 sequenced Acinetobacter isolates. These analyses revealed important roles for mobile genetic elements in shaping the siderophore meditated iron acquisition mechanisms between different Acinetobacter strains.ConclusionsA. baumannii grown under iron limited conditions resulted in major transcriptional changes of not only many iron acquisition related genes, but also genes involved in other processes such as motility. Overall, this study showed that A. baumannii is well adaptable to growth in an environment which has limiting iron availability.

Significance Drug resistance is an increasing problem in clinical settings with some bacterial pathogens now resistant to virtually all available drugs. Chlorhexidine is a commonly used antiseptic and disinfectant in hospital environments, and there is increasing resistance to chlorhexidine seen in some pathogenic bacteria, such as Acinetobacter baumannii . This paper examines the global gene expression of A. baumannii in response to chlorhexidine exposure and identifies a gene that we demonstrate to mediate chlorhexidine resistance. Biochemical investigation reveals that this gene encodes a previously uncharacterized type of drug efflux pump that actively transports chlorhexidine out of the cell.

Pseudomonas aeruginosa is a ubiquitous environmental bacterium and a clinically significant opportunistic human pathogen. Central to the ability of P. aeruginosa to colonise both environmental and host niches is the acquisition of zinc. Here we show that P. aeruginosa PAO1 acquires zinc via an ATP-binding cassette (ABC) permease in which ZnuA is the high affinity, zinc-specific binding protein. Zinc uptake in Gram-negative organisms predominantly occurs via an ABC permease, and consistent with this expectation a P. aeruginosa ΔznuA mutant strain showed an ~60% reduction in cellular zinc accumulation, while other metal ions were essentially unaffected. Despite the major reduction in zinc accumulation, minimal phenotypic differences were observed between the wild-type and ΔznuA mutant strains. However, the effect of zinc limitation on the transcriptome of P. aeruginosa PAO1 revealed significant changes in gene expression that enable adaptation to low-zinc conditions. Genes significantly up-regulated included non-zinc-requiring paralogs of zinc-dependent proteins and a number of novel import pathways associated with zinc acquisition. Collectively, this study provides new insight into the acquisition of zinc by P. aeruginosa PAO1, revealing a hitherto unrecognized complexity in zinc homeostasis that enables the bacterium to survive under zinc limitation.

Streptococcus pneumoniae is a globally significant human pathogen responsible for nearly 1 million deaths annually. Central to the ability of S. pneumoniae to colonize and mediate disease in humans is the acquisition of zinc from the host environment. Zinc uptake in S. pneumoniae occurs via the ATP-binding cassette transporter AdcCB, and, unusually, two zinc-binding proteins, AdcA and AdcAII. Studies have suggested that these two proteins are functionally redundant, although AdcA has remained uncharacterized by biochemical methods. Here we show that AdcA is a zinc-specific substrate-binding protein (SBP). By contrast with other zinc-binding SBPs, AdcA has two zinc-binding domains: a canonical amino-terminal cluster A-I zinc-binding domain and a carboxy-terminal zinc-binding domain, which has homology to the zinc-chaperone ZinT from Gram-negative organisms. Intriguingly, this latter feature is absent from AdcAII and suggests that the two zinc-binding SBPs of S. pneumoniae employ different modalities in zinc recruitment. We further show that AdcAII is reliant upon the polyhistidine triad proteins for zinc in vitro and in vivo. Collectively, our studies suggest that, despite the overlapping roles of the two SBPs in zinc acquisition, they may have unique mechanisms in zinc homeostasis and act in a complementary manner during host colonization.

Acinetobacter baumannii continues to be a major health problem especially in hospital settings. Herein, features that may play a role in persistence and disease potential were investigated in a collection of clinical A. baumannii strains from Australia. Twitching motility was found to be a common trait in A. baumannii international clone I strains and in abundant biofilm formers, whereas swarming motility was only observed in isolates not classified within the international clone lineages. Bioinformatic analysis of the type IV fimbriae revealed a correlation between PilA sequence homology and motility. A high level of variability in adherence to both abiotic surfaces and epithelial cells was found. We report for the first time the motility characteristics of a large number of A. baumannii isolates and present a direct comparison of A. baumannii binding to nasopharyngeal and lung epithelial cells.