Light modulates iron acquisition in Acinetobacter baumannii

TUTTOBENE MARISEL; M. A. MUSSI

Córdoba

Congreso; XIII Encuentro Latinoamericano de Fotoquímica y Fotobiología (XIII ELAFOT); 2017

Introduction: In our previous studies we have shown that light exhibits a global effect in the physiology of the human pathogen Acinetobacter baumannii. In particular, we have presented evidence that light modulates processes associated with its pathogenicity such as the formation of biofilms, motility and virulence against eukaryotic hosts such as Candida albicans [1], and even susceptibility to certain antibiotics [2]. Recently, we identified new traits whose expression are modulated by light in this pathogen such as the PAA catabolic pathway, trehalose biosynthesis, acetoin degradation, and also clusters related to lipid metabolism, bacterial competition such T6SS and tolerance to antibiotics, and determined that these responses depended on the BLUF photoreceptor BlsA [3]. Objective: To identify possible BlsA partners and get insights into the mechanism of light signal transduction, we performed pull down experiments. Methods: Culture extracts from A. baumannii ATCC 17978 cells grown at 24°C in the dark were used to perform pull down assays with His-tagged column immobilized BlsA as bait. Results: One of the proteins that emerged as a potential partner of BlsA from pull down experiments is known as Fur, for ferric uptake regulator. Free iron is a limited micronutrient in hosts where it is typically tightly bound within a range of biomolecules, such as heme. As such, iron acquisition systems are important factors for the virulence of pathogenic organisms [4]. Expression of most proteins required for bacterial iron acquisition systems, including siderophore biosynthetic enzymes, are frequently regulated at the transcriptional level by Fur [5]. We therefore decided to evaluate the ability of bacteria to grow in low-iron medium under blue light or in the dark, to determine whether iron acquisition could be modulated by light and study dependance on BlsA. Therefore, growth of A. baumannii ATCC 17978 and ATCC 19606 were investigated under varying iron concentrations without shaking under blue light or in the dark at 24°C. Reduction of available iron in LB Difco medium was achieved by supplementation of the media with 2,2?-dipyridyl (DIP), a synthetic iron chelator. In the case of strain ATCC 19606, higher bacterial growth was observed in the dark respect to illuminated conditions. This difference in bacterial growth was lost in the isogenic blsA mutants strain, behaving as in light conditions, and the wild type phenotype was rescued by the complementing strain expressing the blsA gene from a plasmid. On the contrary, we did not observe significant differences in the growth of strain ATCC 17978 and isogenic blsA mutants under such conditions. Conclusions: We presented evidence that Fur would interact with BlsA by pull down experiments, and showed that iron acquisition is modulated by light at 24ºC, a process that depends on the presence of BlsA for enhanced growth under iron limiting conditions in the dark. We therefore propose a model that postulates that BlsA binds to Fur in darkness, consequently sequestering the repressor and allowing the proteins required for the bacterial iron acquisition system to be expressed, under iron-limiting conditions. It should be noted that ATCC 17978 harbors additional systems for iron acquisition rather than the only one present in ATCC 19606, the acinetobactin acquisition system, and this could explain the absence of phenotype observed in this strain.