AIR POLLUTION AND OTHER ENVIRONMENTAL STRESSES: GASEOUS NO_2 EXPOSURE LEADS TO SPECIFIC ALTERATIONS OF PSEUDOMONAS FL UORESCENS

摘要

In the environment, microorganisms are subjected to a wide range of stresses. These stresses can be of natural origin, like temperature variations and ultraviolet exposure, but can also originate from humans like air pollution. The effects of air pollution on humans are more and more studied and reveal increasing concerns for human health, including augmentations of respiratory infections. However, the microbial responses to atmospheric pollution are still largely unknown. In a similar fashion, few studies investigate the effects of UV radiation on microorganisms. As NO_2 is an air pollutant causing nitrosative stress in biological organisms by reacting with various biological molecules, solar UV radiations are also an important environmental source of cell damage. UVB can directly damage DNA and cause erythema, but only represent 6% of the total UV reaching the earth surface. The 94% others are UVA, that cause oxidative stress in the cells. Since oxidative and nitrosative stresses are interlinked, the exposition of airborne bacteria to these two stresses could have synergistic consequences. In this study, the airborne Pseudomonas fluorescens strain MFAF76a was exposed successively to gaseous NO_2 and UV light to assess whether these two environmental stresses have synergistic effects on bacterial physiology. Bacterial growth was assessed by optical density and membrane permeability by flow cytometry. Exposures to successively gaseous NO_2 and UVB light lead to a non-synergistic decrease of bacterial viability. Furthermore, only NO_2 seems to damage the membrane and induces membrane permeabilization. Lipidomic analysis reveals similarities between the lipidic profile of bacteria in their exponential growth phase or for the exposed ones to NO_2 during their stationary growth phase. Furthermore, lipidic alterations show that mechanisms induced by NO_2 differ from those implemented by temperature. In conclusion, this study reveals that bacterial alterations caused by NO_2 are specific, with a strong emphasis on membrane damage.