Trehalose, a temperature‐ and salt‐induced solute with implications in pathobiology of Acinetobacter baumannii

Abstract

Acinetobacter baumannii is an opportunistic human pathogen that has become a global threat to healthcare institutions worldwide. A major factor contributing to success of this bacterium is its outstanding ability to survive on dry surfaces. The molecular basis for desiccation resistance is not completely understood. This study focused on growth under osmotic stress and aimed to identify the pool of compatible solutes synthesized in response to these low water activity conditions. A. baumannii produced mannitol as compatible solute, but in contrast to Acinetobacter baylyi, also trehalose was accumulated in response to increasing NaCl concentrations. The genome of A. baumannii encodes a trehalose-6-phosphate phosphatase (OtsB) and a trehalose-6-phosphate synthase (OtsA). Deletion of otsB abolished trehalose formation, demonstrating that otsB is essential for trehalose biosynthesis. Growth of the mutant was neither impaired at low salt nor at 500 mM NaCl, but it did not grow at high temperatures, indicating a dual function of trehalose in osmo- and thermoprotection. This led us to analyse temperature dependence of trehalose formation. Indeed, expression of otsB was not only induced by high osmolarity but also by high temperature. Concurrently, trehalose was accumulated in cells grown at high temperature. Taken together, these data point to an important role of trehalose in A. baumannii beyond osmoprotection.

Salt induction and activation of MtlD, the key enzyme in the synthesis of the compatible solute mannitol in Acinetobacter baumannii

“…However, at high salt the Δ mtlD mutant showed almost no growth (Figure ). The wild‐type growth phenotype at high salt concentrations could be completely restored by addition of 1 mmol/L glycine betaine to the medium (Figure ), which is known to serve as a compatible solute in many bacteria including Acinetobacter species, indicating that mannitol is an essential compatible solute that cannot be substituted by the other endogenous solutes glutamate or trehalose (Sand et al., ; Zeidler et al., ). To prove that MtlD is indeed essential for mannitol biosynthesis, we analyzed the intracellular mannitol pool at a lower salt concentration, where growth of the Δ mtlD mutant was still possible.…”

The role of compatible solutes in desiccation resistance of Acinetobacter baumannii

“…() (Figure ). Without additional NaCl, the growth rate of the mutant was 0.60 ± 0.04 hr −1 , which is nearly identical to the wild type (92%) (Zeidler et al., ). In the mineral medium supplemented with 200, 300, or 400 mM NaCl, growth rates were 0.50 ± 0.04 hr −1 (86% of the wild type), 0.34 ± 0.08 (67%), and 0.06 ± 0.01 (19%), respectively, and at 500 mM NaCl no growth was observed.…”

“…A double deletion mutant of the genes mtlD (HMPREF0010_00722, encoding a mannitol dehydrogenase) and otsB (HMPREF0010_01306, encoding a trehalose‐phosphate‐phosphatase) in A. baumannii ATCC 19606 T was created. A markerless Δ otsB mutant described before (Zeidler et al., ) was used to additionally delete mtlD . This was done by double homologous recombination as described by Zeidler et al.…”

“…For the protein content of freeze‐dried cells, the mean value obtained by Zeidler et al. () was applied.…”

“…In response to salt stress, A. baumannii accumulates glutamate, mannitol, and trehalose or, if present, takes up glycine betaine from the environment (Zeidler et al., ), but so far nothing is known about a possible involvement of these solutes in desiccation tolerance. In particular, trehalose is a very potent protector against desiccation used, amongst others, by anhydrobiotes (Crowe, Oliver, & Tablin, ), and the unusual solute mannitol, which is a radical scavenger, could be involved in protection against oxidative stress that occurs upon rehydration (Efiuvwevwere, Gorris, Smid, & Kets, ).…”