Bacteria are frequently exposed to endogenous and exogenous reactive oxygen and nitrogen species which can damage various biomolecules such as DNA, lipids, and proteins. High concentrations of these molecules can induce oxidative and nitrosative stresses in the cell. Reactive oxygen and nitrogen species are notably used as a tool by prokaryotes and eukaryotes to eradicate concurrent species or to protect themselves against pathogens. The main example is mammalian macrophages that liberate high quantities of reactive species to kill internalized bacterial pathogens. As a result, resistance to these stresses is determinant for the survival of bacteria, both in the environment and in a host. The first bacterial component in contact with exogenous molecules is the envelope. In Gram-negative bacteria, this envelope is composed of two membranes and a layer of peptidoglycan lodged between them. Several mechanisms protecting against oxidative and nitrosative stresses are present in the envelope, highlighting the importance for the cell to deal with reactive species in this compartment. This review aims to provide a comprehensive view of the challenges posed by oxidative and nitrosative stresses to the Gram-negative bacterial envelope and the mechanisms put in place in this compartment to prevent and repair the damages they can cause.

Anthropogenic atmospheric pollution and immune response regularly expose bacteria to toxic nitrogen oxides such as NO• and NO2. These reactive molecules can damage a wide variety of biomolecules such as DNA, proteins and lipids. Several components of the bacterial envelope are susceptible to be damaged by reactive nitrogen species. Furthermore, the hydrophobic core of the membranes favors the reactivity of nitrogen oxides with other molecules, making membranes an important factor in the chemistry of nitrosative stress. Since bacteria are often exposed to endogenous or exogenous nitrogen oxides, they have acquired protection mechanisms against the deleterious effects of these molecules. By exposing bacteria to gaseous NO2, this work aims to analyze the physiological effects of NO2 on the cell envelope of the airborne bacterium Pseudomonas fluorescens MFAF76a and its potential adaptive responses. Electron microscopy showed that exposure to NO2 leads to morphological alterations of the cell envelope. Furthermore, the proteomic profiling data revealed that these cell envelope alterations might be partly explained by modifications of the synthesis pathways of multiple cell envelope components, such as peptidoglycan, lipid A, and phospholipids. Together these results provide important insights into the potential adaptive responses to NO2 exposure in P. fluorescens MFAF76a needing further investigations.

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 NO2 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 NO2 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 NO2 and UVB light lead to a non-synergistic decrease of bacterial viability. Furthermore, only NO2 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 NO2 during their stationary growth phase. Furthermore, lipidic alterations show that mechanisms induced by NO2 differ from those implemented by temperature. In conclusion, this study reveals that bacterial alterations caused by NO2 are specific, with a strong emphasis on membrane damage.

Bacteria are often exposed to nitrosative stress from their environment, from atmospheric pollution or from the defense mechanisms of other organisms. Reactive nitrogen species (RNS), which mediate nitrosative stress, are notably involved in the mammalian immune response through the production of nitric oxide (NO) by the inducible NO synthase iNOS. RNS are highly reactive and can alter various biomolecules such as lipids, proteins and DNA, making them toxic for biological organisms. Resistance to RNS is therefore important for the survival of bacteria in various environments, and notably to successfully infect their host. The fuel combustion processes used in industries and transports are responsible for the emission of important quantities of two major RNS, NO and the more toxic nitrogen dioxide (NO2). Human exposure to NO2 is notably linked to increases in lung infections. While the response of bacteria to NO in liquid medium is well-studied, few data are available on their exposure to gaseous NO and NO2. This study showed that NO2 is much more toxic than NO at similar concentrations for the airborne bacterial strain Pseudomonas fluorescens MFAF76a. The response to NO2 involves a wide array of effectors, while the response to NO seemingly focuses on the Hmp flavohemoprotein. Results showed that NO2 induces the production of other RNS, unlike NO, which could explain the differences between the effects of these two molecules.