## 3 Mechanisms Protecting _A. baumannii_ against Desiccation  
Desiccation, as a common environmental stressor, poses challenges to bacterial cells. Water molecules, as the only nonvolatile solvent in cells, are critical in reaction mechanisms; they also confer stability to lipids, DNA, and proteins, as well as contributing structural order. Loss of membrane integrity during desiccation disrupts the respiratory chain leading to the accumulation of superoxide ions. Furthermore, the malfunction of transport proteins and destabilization of proteins with iron-sulfur clusters cause an increase in the level of intracellular iron. Superoxide radicals can participate in Fenton and Haber-Weiss reactions with ferrous or ferric ions leading to the production of highly toxic hydroxyl radicals [79]. Therefore, oxidative damage of DNA, lipids, and proteins is one of the effects of water loss. _Acinetobacter_ spp., compared with other Gram-negative rods, are more resistant to dry conditions [80]. _A. baumannii_ uniquely survives on inanimate objects and fingertips for extended periods, which explains its potency in cross-infection breakouts [81]. The main structures that facilitate bacteria to enhance water retention are CPS and LPS/LOS [36]. CPS were shown to contribute to desiccation tolerance in _A. baumannii_[51]. However, other studies demonstrated that there is no simple correlation between the capsule thickness and survival rate under desiccation [49]. Therefore, it seems that the type and structure of CPS or other mechanisms, including hepta-acetylated lipid A, must contribute to the outstanding desiccation tolerance of _A. baumannii_. [PERSON] et al. demonstrated that the _A. baumannii_ mutant lacking the LpxM acylsynthetaze produces penta-acylated lipid A, instead of hepta-acylated lipid A, and it exhibits decreased desiccation tolerance, probably due to increased membrane fluidity [59]. The accumulation of organic osmolytes, generally designated as compatible solutes, is a prerequisite for the adaptation of bacteria to osmotic stress imposed by water loss. A crucial role in desiccation resistance in various microorganisms is played by the non-reducing disaccharide, trehalose. Trehalose acts as an osmolyte, chemical chaperone, and metabolite that can directly or indirectly stabilize proteins and membranes [82; 83; 84; 85]. It seems that endogenous trehalose is not involved in desiccation tolerance in _A. baumannii_, but exogenous trehalose was found to efficiently protect _A. baumannii_ on dry surfaces [68]. In response to osmotic stress, _A. baumannii_ also accumulates mannitol and glutamate; however, their contribution to desiccation resistance remains mostly unexplored [81]. To counteract the effects of oxidative stress, the expression of anti-oxidant enzymes, such as catalases KatE and KatG, superoxide dismutase, and glutathione peroxidase, is induced in desiccation-stressed _A. baumannii_[49; 86; 87]. A recent study by [PERSON] et al. proved that the global BfmR regulator contributes to that desiccation tolerance [49]. [PERSON] et al. found that desiccation stress affects the composition of the OM. The overproduction of OMPs (Omp25, DcaP-like, and CarO) was observed, indicating a shift in membrane permeability. This study also revealed that several genes encoding proteins involved in transcription and translation, including RNA polymerase subunits RpoA and RpoC, ribosome-associated proteins, and the elongation factor Tu, are downregulated in _A. baumannii_ during desiccation. Proteins whose expression was upregulated included ribosomal recycling factor (RRF), integration host factor (IHF), and the histone-like protein HU. RRF facilitates disassembly of the ribosome at the end of translation. IHF and HU are involved in transcription regulation, and they are essential for maintaining DNA supercoiling and compaction. Consistent with this finding, the presence of an electron-dense region inside desiccation-stressed _A. baumannii_ cells was detected [87]. All these observations led to the conclusion that _A.