Water Uptake of\nAirborne Cells of P.\nsyringae Measured with a Hygroscopicity Tandem Differential\nMobility Analyzer

Airborne microorganisms impact cloud formation and are\ninvolved\nin disease spreading. The ability of airborne cells to survive and\nexpress genes may be limited by reduced water availability in the\natmosphere and depend on the ability of the cells to attract water\nvapor at subsaturated conditions, i.e., their hygroscopicity. We assessed\nhygroscopic properties of the plant pathogen Pseudomonas\nsyringae, known to participate in cloud formation.\nWe used a hygroscopicity tandem differential mobility analyzer to\nexamine both hydration and dehydration behavior in the relative humidity\n(RH) range 5–90%. The cells were aerosolized either from Milli-Q\nwater or from a 35 g L<sup>–1</sup> NaCl solution, resulting\nin pure cells or cells associated with NaCl. Pure cells exhibited\nno deliquescence/efflorescence and a small gradual water uptake reaching\na maximum growth factor (GF) of 1.09 ± 0.01 at 90% RH. For cells\nassociated with NaCl, we observed deliquescence and a much larger\nmaximum GF of 1.74 ± 0.03 at 90% RH. Deliquescence RH was comparable\nto that of pure NaCl, highlighting the major role of the salt associated\nwith the cells. It remains to be investigated how the observed hygroscopic\nproperties relate to survival, metabolic, and ice-nucleation activities\nof airborne P. syringae.