# Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants
## 4 Bacteria and Salinity Stress  
Microorganisms, as well as plants, have developed diverse mechanisms to adapt to salinity stress. They can be characterized as halotolerant and halophilic depending on the attitude towards salinity. The first can grow in the case of high concentrations of NaCl in the medium or without it; thus, they do not \"need\" salt, they just tolerate it. In contrast, halophiles are widely spread around the world in salt-rich ecosystems and habitats, representing natural communities. They require Na+ for their metabolisms and development, so they need it to live [47]. In that sense, the halophiles can be divided into three categories according to their optimal NaCl concentration in the medium: slight halophiles, 1-3% of sodium chloride; moderate halophiles, 3-15% of sodium chloride; extreme halophiles, who love to grow at 15% to 30% of NaCl. Physical or chemical properties are involved in the bacterial perception and tolerance to the abovementioned concentrations, such as temperature, pH value, and nutrients [48]. In the present study, it can be summarized that the most common bacteria that tolerate elevated concentrations of NaCl (1-15%) are _Ochrobartum intermedium_, _Bacillus subtilis_, _Pseudomonas fluorescence_, _Kocuria rhizophilia_, consortium, _Bacillus amyliquefaciens_, _Bacillus firms_, _Azotobacter droocccum_, _Ps. stutzeri_, _Az. brasiliense_, _Az. lipoferum_, _Pseudomonas putida_, _Curtobacterium flaccumfaciens_, _Arthrobacter_ sp., and _B. safensis_ (Table 1). These strains were found to improve plant growth and enhance the tolerance to sodium chloride in maize, wheat, barley, rice, soybean, sunflower, and tomato.  
Soil salinity affects the structure, activities, and biodiversity of rhizosphere bacterial communities [49], which is not a well-explored area. [PERSON] and collaborators found that the EC of soils was the most influential driving force of bacterial community composition (20.83%), while the second most important factor was sodium content (14.17%), suggesting a clear separation of bacterial communities in accordance with the EC. The dominant bacterial groups were _Plantomycetes_, _Proteobacteria_, and _Bacteroidetes_ among different salty soils. As the salt concentration increased, the indicators changed from _Plantomycetes_ and _Bacteroidetes_ to _Proteobacteria_ and _Frimicutes_[49]. One factor that indirectly affects the structure of bacterial communities in the rhizosphere is the quality and quantity of root exudates being influenced by the salinity [50]. In addition, halotolerant bacteria have built mechanisms to survive in salinity-rich environments. The main mechanisms are related to cell-wall construction and the capacity to pump ions out of the cell, as follows: (i) generation of extracellular polymeric substances (EPSs) that support biofilm formation, limiting the salt import into the cell [14; 37; 51]; (ii) soluble salt production through protein and enzyme adaptation [5]; (iii) optimizing the intracellular concentration of Na+ ions using the Na+/H+ antiporter of the cell membrane [51; 52; 53]--the vital function of this \"pump\" is to maintain the elevated values of the K+/Na+ ratio in the cytosol compared to the low value in the surrounded habitat; (iv) avoiding the entry of high salt concentrations through the cell membrane because of the specific structure of the cell wall (structural and integral proteins, lipid composition, polysaccharides, between others) [15; 52; 53]; (v) endogenous synthesis of amino acids and solutes [54; 37; 51].