CsrA Coordinates Compatible Solute Synthesis in Acinetobacter baumannii and Facilitates Growth in Human Urine.

CsrA is a global regulator widespread in bacteria and known to be involved in different physiological processes, including pathogenicity. Deletion of csrA of Acinetobacter baumannii strain ATCC 19606 resulted in a mutant that was unable to utilize a broad range of carbon and energy sources, including amino acids. This defect in amino acid metabolism was most likely responsible for the growth inhibition of the ΔcsrA mutant in human urine, where amino acids are the most abundant carbon source for A. baumannii. Recent studies revealed that deletion of csrA in the A. baumannii strains AB09-003 and ATCC 17961 resulted in an increase in hyperosmotic stress resistance. However, the molecular basis for this observation remained unknown. This study aimed to investigate the role of CsrA in compatible solute synthesis. We observed striking differences in the ability of different A. baumannii strains to cope with hyperosmotic stress. Strains AB09-003 and ATCC 17961 were strongly impaired in hyperosmotic stress resistance in comparison to strain ATCC 19606. These differences were abolished by deletion of csrA and are in line with the ability to synthesize compatible solutes. In the salt-sensitive strains AB09-003 and ATCC 17961, compatible solute synthesis was repressed by CsrA. This impairment is mediated via CsrA and could be overcome by deletion of csrA from the genome. IMPORTANCE The opportunistic human pathogen Acinetobacter baumannii has become one of the leading causes of nosocomial infections around the world due to the increasing prevalence of multidrug-resistant strains and their optimal adaptation to clinical environments and the human host. Recently, it was found that CsrA, a global mRNA binding posttranscriptional regulator, plays a role in osmotic stress adaptation, virulence, and growth on amino acids of A. baumannii AB09-003 and ATCC 17961. Here, we report that this is also the case for A. baumannii ATCC 19606. However, we observed significant differences in the ΔcsrA mutants with respect to osmostress resistance, such as the AB09-003 and 17961 mutants being enhanced in osmostress resistance whereas the ATCC 19606 mutant was not. This suggests that the role of CsrA in osmotic stress adaptation is strain specific. Furthermore, we provide clear evidence that CsrA is essential for growth in human urine and at high temperatures.

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

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.