# Halophytes as Medicinal Plants against Human Infectious Diseases
## 1 Introduction  
Halophytes are ecologically, physiologically and biochemically specialized plants that are able to grow and reproduce under salinities >200 mM NaCl. Although representing only 1% of terrestrial angiosperms, approximately 3000 species in the world are considered as halophytes, corresponding to 550 genera and 120 families [1]. Halophytes have an extremely broad distribution, occurring over a wide range in mostly costal and wetlands habitats [2] that are directly or indirectly influenced by ocean waters, such as salt marshes/magnroves and sand dunes, and the significant fraction of once arable soils are now threatened by salinization as a consequence of irrigation practices or climate change [3].  
A first line of physiological adaptation involves salt-induced signaling pathways of osmotic adjustment that trigger the removal of Na\({}^{+}\) and Cl\({}^{-}\) from sap and sequestration in intracellular compartments known as vacuoles [4; 5], whereas small organic molecules, such as soluble carbohydrates, polyols, amino acids and betases, accumulate in the cytoplasm to further ensure osmotic balance [4; 6; 7]. Insufficient adjustment leads to osmotic and oxidative stress, ion toxicity and nutrient deficiency [4]. High concentrations of NaCl in soil reduce water availability to the roots, reduce the water potential of leaves and, ultimately, limit nutrient uptake [8]. Net photosynthesis is reduced because of stomatal closure, and the balance between the production and the scavenging of reactive oxidative species (ROS) is disrupted [9; 10]. Energy is diverted from biosynthetic pathways associated with growth to the expression of enzymes and phytohormones involved in stress responses and homeostasis [11] with an overall reduction of productivity.  
The multifactorial adaptive responses of halophytes involve a complex network of biochemical mechanisms and a plethora of bioactive molecules, such as phenolic compounds(e.g., phenolic acids, flavonoids and tannins), polysaccharides, glycosides and related compounds, lipids (e.g., fats and fatty acids, phytosterols and tocopherols, essential oils, acetylenic lipids, carotenoids) and alkaloids [12]. The ethnobotanical literature provides evidence that, because of their phytochemical richness, halophytic medicinal plants have long been used to treat various infectious diseases, particularly in developing countries where traditional medicine remains the first approach to minor ailments [12; 13]. In recent decades, little has been investigated about the utilization of halophytes for medicinal purposes as a way to validate their use in traditional medicine or as a source of pharmacological compounds and only recently, with the building up of antibiotic resistance and the perspective of a post-antibiotic era, has the screening of halophytes for antimicrobial compounds regained interest [12; 14; 15; 16; 17]. Furthermore, the high nutritional/nutraceuticalial/pharmacological value of some halophytic species may represent added value to the emerging activity of saline agriculture [12; 14; 18; 19].  
The present review intends to provide a critical perspective on the empirical use of halophytes in traditional medicine and on the recently produced scientific evidence of the antiviral, antibacterial and antifungal activities underlying ethnopharmacological applications, highlighting the importance of halophytes as a valuable and still unexplored source of antimicrobial compounds for clinical applications.