Desiccation-tolerant Acinetobacter as a robust chassis candidate for gas-phase bioprocesses

Gas-phase bioprocesses that immobilize microbial cells on solid carriers enable efficient conversion of volatile or poorly water-soluble substrates. Acinetobacter sp. Tol 5 is a highly adhesive bacterium capable of utilizing various hydrocarbons, making it a promising chassis candidate for gas-phase bioprocesses. However, Gas-phase bioprocesses expose cells to fluctuating humidity and transient desiccation that can compromise viability and catalytic performance. In Gram-negative bacteria, especially in pathogens, desiccation is known to impose multifactorial stress, such as loss of cellular water and energetics, damage to DNA and proteins, oxidative stress, and disruption of the cell envelope. In contrast, for chassis strains used in or being considered for gas-phase bioprocesses, their desiccation tolerance including robustness of biocatalytic activity following humidity fluctuations and responses to desiccation stress remain incompletely defined. Here, we evaluated the viability, energy status, and gas-phase toluene degradation of Tol 5 as a chassis for gas-phase processes after controlled desiccation (8%, 52%, and > 95% RH), in comparison with Acinetobacter baylyi ADP1, Pseudomonas putida, and Escherichia coli. To minimize adhesion-related bias in post-desiccation measurements, we used an ataA-deficient Tol 5 mutant (Tol 5 ΔataA) for the assays. Acinetobacter strains maintained high viability during desiccation for 16 days, whereas P. putida and E. coli showed significant loss of viability at 8% RH and 52% RH. Intracellular ATP measurements further indicated that desiccation reduced intracellular ATP in all strains, but E. coli rapidly exhausted ATP at 8% and 52% RH, whereas Acinetobacter strains and P. putida retained intracellular ATP. In a gas-phase toluene degradation assay, immobilized Tol 5 retained higher toluene-degrading activity after desiccation than P. putida mt-2. Transcriptome profiling revealed a multilayered Tol 5 response involving DNA and RNA maintenance, cell envelope trafficking and remodeling, redox-responsive functions, and broad repression of growth-associated metabolism. Our results highlight Acinetobacter, particularly Tol 5, as a promising chassis candidate for gas-phase bioprocesses and suggest potential mechanistic targets for stabilizing bacterial cells under low-water-activity conditions.