Vaccinology is one of the most important cornerstones in modern medicine, providing better quality of life. The human immune system is composed of innate and adaptive immune processes that interplay when infection occurs. Innate immunity relies on pathogen-associated molecular patterns which are recognized by pathogen recognition receptors localized in antigen presenting cells. After antigen processing and presentation, CD4+ T cell polarization occurs, further leading to B cell and CD8+ activation and humoral and cell-mediated adaptive immune responses. Liposomes are being employed as vaccine technologies and their design is of importance to ensure proper immune responses. Physicochemical parameters like liposome size, charge, lamellarity and bilayer fluidity must be completely understood to ensure optimal vaccine stability and efficacy. Liposomal vaccines can be developed to target specific immune cell types for the induction of certain immune responses. In this review, we will present promising liposomal vaccine approaches for the treatment of important viral, bacterial, fungal and parasitic infections (including tuberculosis, TB). Cationic liposomes are the most studied liposome types due to their enhanced interaction with the negatively charged immune cells. Thus, a special section on the cationic lipid dimethyldioctadecylammonium and TB is also presented.

Different aspects of bacterial and fungal siderophore biotechnological applications will be discussed. Areas of application presented include, but are not limited to agriculture, medicine, pharmacology, bioremediation, biodegradation and food industry. In agriculture-related applications, siderophores could be employed to enhance plant growth due to their uptake by rhizobia. Siderophores hindered the presence of plant pathogens in biocontrol strategies. Bioremediation studies on siderophores discuss mostly the mobilization of heavy metals and radionuclides; the emulsifying effects of siderophoreproducing microorganisms in oil-contaminated environments are also presented. The different applications found in literature based in medicine and pharmacological approaches range from iron overload to drug delivery systems and, more recently, vaccines. Additional research should be done in siderophore production and their metabolic relevance to have a deeper understanding for future biotechnological advances.

Siderophores are iron-chelating molecules produced by microbes when intracellular iron concentrations are low. Low iron triggers a cascade of gene activation, allowing the cell to survive due to the synthesis of important proteins involved in siderophore synthesis and transport. Generally, siderophores are classified by their functional groups as catecholates, hydroxamates and hydroxycarboxylates. Although other chemical structural modifications and functional groups can be found. The functional groups participate in the iron-chelating process when the ferri-siderophore complex is formed. Classified as acidophiles, alkaliphiles, halophiles, thermophiles, psychrophiles, piezophiles, extremophiles have particular iron requirements depending on the environmental conditions in where they grow. Most of the work done in siderophore production by extremophiles is based in siderophore concentration and/or genomic studies determining the presence of siderophore synthesis and transport genes. Siderophores produced by extremophiles are not well known and more work needs to be done to elucidate chemical structures and their role in microorganism survival and metal cycling in extreme environments.

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