Antimicrobial Lipids from Plants and Marine Organisms: An Overview of the Current State-of-the-Art and Future Prospects

In the actual post-antibiotic era, novel ways of rethinking antimicrobial research approaches are more urgent than ever. Natural compounds with antimicrobial activity such as fatty acids and monoacylglycerols have been investigated for decades. Additionally, the interest in other lipid classes as antimicrobial agents is rising. This review provides an overview on the research about plant and marine lipids with potential antimicrobial activity, the methods for obtaining and analyzing these compounds, with emphasis on lipidomics, and future perspectives for bioprospection and applications for antimicrobial lipids. Lipid extracts or lipids isolated from higher plants, algae or marine invertebrates are promising molecules to inactivate a wide spectrum of microorganisms. These lipids include a variety of chemical structures. Present and future challenges in the research of antimicrobial lipids from natural origin are related to the investment and optimization of the analytical workflow based on lipidomics tools, complementary to the bioassay-guided fractionation, to identify the active compound(s). Also, further work is needed regarding the study of their mechanism of action, the structure–activity relationship, the synergistic effect with conventional antibiotics, and the eventual development of resistance to lipids, which, as far as is known, is unlikely.


Introduction
The consumption of antibiotics in the world population is alarming. In 2016, the top five World Health Organization (WHO)'s major antibiotic consuming countries were Brazil, Turkey, Iran, Russia, and France, by decreasing order [1]. In Brazil, more than 2000 metric tons of antibiotics were consumed annually, followed by ca. 1000 metric tons in Turkey and Iran [1].
Both misuse and overuse of antibiotics has led to the development of antimicrobial resistance (AMR) in microorganisms, which has been a global problem and a growing threat for many years. Antimicrobial resistant microbes are found in people, animals, food, and the environment (hospital or other health care facilities, water, soil and air). Because of AMR, several disease conditions are becoming harder to treat, as tuberculosis, pneumonia, blood poisoning, gonorrhea, and foodborne diseases [2]. AMR leads to higher medical costs, prolonged hospital stays, and increased mortality,

Antimicrobial Lipids from Plants
Over the evolution, higher plants have developed several resilience strategies that allow them to resist or escape external attacks (e.g., microorganisms, pathogens, and predators). Their innate immune system had to be equipped with highly complex mechanisms of resistance and survival. The defense mechanisms of plants are unique and consist of both physical barriers and production of secondary metabolites. Plant secondary metabolites are formed in particular biosynthetic pathways by means of substrate-specific enzymes. The precursors of these secondary metabolites stem from primary metabolites, such as amino acids, FA, sugars, or acetyl-CoA. Some of the secondary metabolites serve as constitutive chemical barriers against the microbial attack (phytoanticipins) while others serve as inducible antimicrobials (phytoalexins) [44].

Aim of the Study
This review aims to provide a critical overview on the research about plant and marine lipids with potential antimicrobial activity, the methods for obtaining and analyzing these compounds with emphasis on lipidomics, and future perspectives for bioprospecting and applying these antimicrobial lipids.

Antimicrobial Lipids from Plants
Over the evolution, higher plants have developed several resilience strategies that allow them to resist or escape external attacks (e.g., microorganisms, pathogens, and predators). Their innate immune system had to be equipped with highly complex mechanisms of resistance and survival. The defense mechanisms of plants are unique and consist of both physical barriers and production of secondary metabolites. Plant secondary metabolites are formed in particular biosynthetic pathways by means of substrate-specific enzymes. The precursors of these secondary metabolites stem from primary metabolites, such as amino acids, FA, sugars, or acetyl-CoA. Some of the secondary metabolites serve as constitutive chemical barriers against the microbial attack (phytoanticipins) while others serve as inducible antimicrobials (phytoalexins) [44].
Oxylipins are a large family of plants' secondary metabolites derived from PUFA that make part of their immune system and play key roles as antimicrobial agents. They are formed through enzymatic or radical oxidation of FA 18:2 and 18:3, in order to protect plants against pests and pathogens [45]. The enzymatic biosynthesis of these molecules is triggered by an alpha-dioxygenase (DOX) and by lipoxygenases (LOX) [46,47] that lead to the formation of the different types of molecules, including hydroxy-, hydroperoxy-, divinyl-, oxo-, and keto-derivatives of FA. Oxylipins are formed during abiotic and biotic stresses [48]. They are plant signaling molecules that can induce cell death and have an effect on the growth of eukaryotic microbes [45]. A deeper knowledge on plant response to stresses at molecular, physiological and metabolic levels will allow the development of new plant-derived antimicrobial molecules for use in the clinical field and as biopesticides [48].
The search for novel antimicrobials has led to exploring also amide derivatives of FA because they are natural self-defense agents in plants. FA amides are bioactive lipids [49] formed by the amidation of long chain saturated and unsaturated FA (UFA) [50]. They have higher antimicrobial activity against yeasts and bacteria than unmodified FA [51]. Amide derivatives of FA possess a broad bioactivity against different pathologic conditions (bacterial and parasitic infections, cancer, inflammation, etc.,) and their mechanisms of action imply protein synthesis inhibition and membrane leakage [52]. Also, microorganisms inside healthy plant tissues are unique to explore novel bioactive compounds. The FA and their amides from plant's endogenous microorganisms have been scarcely reported despite being bioactive in a variety of processes and should be more explored as new therapeutic agents [52].
Lipids represent up to 7% of the dry weight of the leaves of higher plants and are important constituents of cell membranes, chloroplasts, and mitochondria [53]. Besides their structural function as main constituents of cell membranes, they have functional roles in plants (intracellular mediators, extracellular signalers, inter-species communication, and plant defense) and also serve as energy reserves (namely in seeds during germination) [21]. Palmitic acid (16:0) is the major saturated FA in leaf lipids. On the other hand, chloroplast membranes can have up to 90% α-18:3 FA in some lamellae [21]. FA exist in plants mainly linked to more complex molecules, as acylglycerols, esterified to a glycerol backbone in the form of triacylglycerols, sterol esters, MAG and diacylglycerols, phospholipids, or glycolipids. Several lipid classes, besides FA and MAG, have been identified in a diverse group of higher plants and tested for their antimicrobial activity, as will be detailed in the next sub-sections. Figure 2 illustrates the chemical structures of the different lipid classes with antimicrobial activity isolated from natural sources.

Extraction and Isolation of Plant Lipids
Studies that extract or isolate lipids from plants to test their antimicrobial activity are scarce (Table 1). The biomass used to extract lipids includes leaves, fruits, seeds, stems, rhizomes, shoots, stem barks, and heartwoods (Table 1). Lipid extraction from plants is usually carried out with organic solvents of different polarities (mainly n-hexane, CHCl 3 , CH 2 Cl 2 , EtOAc, EtOH, BuOH, MeOH, and their mixtures) ( Table 1). Liquid/liquid extractions or Soxhlet extraction are commonly performed to obtain total lipid extracts [54][55][56][57][58]. Instead of analyzing one lipid class or one lipid molecule, some studies have analyzed the total lipid extracts that were obtained by sequential extraction.       [54] Artemisia incisa Pamp.
Asteraceae Pakistan Roots MeOH and recovered after elution on a SiO 2 column with CH 2 Cl 2 /MeOH (9:1, v/v) following previous elution with n-hexane/EtOAc (5:4, v/v) Artemceramide-B [73]  In order to obtain a class of lipids or a particular lipid, the total lipid extract can be fractionated into different groups of lipids, depending on the polarity of the compounds, by thin-layer chromatography (TLC) or by column chromatography. Thus, for example, to recover the neutral lipids (e.g., sterol esters and triacylglycerols) by column chromatography, the extract can be eluted with CHCl 3 , followed by acetone to elute the glycolipids and, finally, with MeOH to elute the phospholipids, as mentioned for the leaves, stems and fruit of Zygophyllum oxianum [59]. The majority of the studies on antimicrobial plant lipids obtained and analyzed FA and their derivatives. FA have been isolated from a series of plant parts by using MeOH/benzene/sulfuric acid, 85% ethanol or supercritical fluid extraction (SFE) with CO 2 and analyzed as FAME [56,[60][61][62][63][64].
Mixtures of FA and FAME were obtained, but it was not clear if these FA were found in the free or esterified forms, since the derivatization methods (methylation) used in these studies convert free and esterified FA to FAME. However, because of their high abundance, presumably, the referred FA were esterified to other lipids. Several oxylipins were retrieved from roots, stems, and leaves of Brazilian joyweed (Alternanthera brasiliana) by extracting with EtOH and EtOAc [65]. Acetylenic FA were isolated from the roots of Pentagonia gigantifolia with 95% ethanol [38].
The fractionation of the extracts has been usually carried out by column chromatography and the purification of the compounds can be achieved by semi-preparative HPLC. dichotomus EtOAc extract at 0.5 g/mL (7 mm against E. coli), at 65 mg/mL (8.33 mm against S. aureus), and at 0.25 g/mL (7 mm against Enterobacter sp.
Antibiotics 2020, 9, 441 26 of 86 Different analytical platforms have been used to identify and characterize the structure of lipids in plant extracts. Generally, in natural products research, several complementary methods are used, such as 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy, gas chromatography (GC) coupled to a flame ionization detector (GC-FID) or to a mass spectrometer (GC-MS), as well as MS with electrospray ionization (ESI-MS). Liquid chromatography-MS (LC-MS) and LC-MS/MS has not been much used on antimicrobial plant lipids' research, only for sphingolipids analysis [74] and for linoleate oxylipins' structural characterization [65]. Besides these common techniques, two-dimensional NMR techniques (2-D NMR such as correlation spectroscopy-COSY, nuclear Overhauser effect spectroscopy-NOESY, heteronuclear single quantum coherence-HSQC, and heteronuclear multiple bond correlation-HMBC) have been used for the identification of artemceramide-D [73] and glyceroglycolipids [72]. Other methods are regularly used for the analysis of lipid extracts or their fractionation, such as TLC [57,69,74,77], column chromatography as mentioned above, or paper chromatography, but the information is very limited [69]. Analysis of FA is mostly performed by GC-FID or GC-MS, after derivatization. Total lipid extracts or lipid fractions are subjected to derivatization techniques using acid or alkaline hydrolysis or transmethylation to obtain FAME.
To identify and/or quantify mixtures of compounds, simpler techniques can be applied as biochemical assays using colorimetric tests, as for instance, for sterols' and steryl glycosides' identification and quantification [68,69,72]. The data obtained for compounds' identification in these studies on antimicrobial plant lipids are normally compared with data reported in the literature, especially for spectroscopic data [72,83].

Susceptibility Testing, Inhibitory, and Microbicidal Activities of Plant Lipids
Several microbial strains were used in plant lipid studies, comprising Gram-positive bacteria, Gram-negative bacteria, acid-fast bacteria, yeasts, filamentous fungi, parasitic protozoa, and some MDR strains and/or hospital isolated strains, such as MRSA ( Table 2).
The lowest MIC against S. aureus were observed for the mixture of lipid classes from the aerial parts of Hedyotis pilulifera (1.25 µg/mL) [83], the artemceramide-B from the roots of A. incisa (0.0313 mg/mL) [73], the linoleate oxylipins isolated from Alternanthera brasiliana (50 µg/mL) [65], and the FAME extracted from the shoots of Salicornia brachiata (60 µg/mL, the same MIC also verified for a MRSA strain) [61]. In the case of artemceramide-B, its high inhibitory potential against S. aureus was assigned to this polar lipid bearing four hydroxyl groups and an amide linkage between two long aliphatic chains [73]. In the case of the linoleate oxylipins from A. brasiliana plant tissues, five isolated oxylipins were also found to be synthesized by endophytic Bacillus strains isolated from this plant. So, it was speculated that the antimicrobial activity of the oxylipins from this plant could be derived from the endophytic bacteria, supposing an ecological crosstalk between this plant and its endogenous microbiome [65]. Also, the LC-MS/MS approach was crucial to identify these antimicrobial compounds both in the plants and in the bacteria, shedding some light on the plant-bacteria interplay [65].
The FA from the extract of Cassia tora's leaves and stems exhibited MIC between 200 and 1000 µg/mL against MRSA [55]. The minimum bactericidal concentrations (MBC) against MRSA were determined on FAME from the leaves of Excoecaria agallocha (0.25 mg against S. aureus) [62], for MRSA the leaves of Sesuvium portulacastrum (1.0 mg/mL) [79], and the hexadecanoate methyl and hexadecanoic acid <n-> obtained from the stem bark of Scaphium macropodum (3.13 mg/mL against S. aureus) [82].
Other studies have tested the antimicrobial activity of lipids against other pathogenic microorganisms of great relevance for the clinical area and included in WHO's guidelines, such as those of the "ESKAPE" group. The lipid extracts with greater inhibiting capacity over Escherichia coli were the FA and their derivatives from n-hexane and CHCl 3 extracts of the heartwood of A. adianthifolia (1 µg) [79], and the acyl steryl glycosides obtained from roots of B. portulacoides (50 µg/mL) [70]. Also with low MIC values, the FAME extracted from the shoots of S. brachiata (0.5 mg/mL) [61] and the FAME and steroids from Trigonella foenum-graecum seeds (100 µg/mL) [63]. A MBC range between 1.0 and 2.0 mg/mL was verified for FAME extracts from the leaves of different plants [60][61][62].
Some lipid extracts were found to have low MIC against P. aeruginosa, as the FA and their derivatives obtained from the n-hexane and CHCl 3 extracts of the heartwood of A. adianthifolia (50 µg) [79], FAME and steroids from fenugreek seeds (T. foenum-graecum, 100 µg/mL) [63], and the SQDG extracted from the neem leaves (A. indica, 128 µg/mL) [71]. MBC toward P. aeruginosa between 0.1 and 2.0 mg/mL were verified for the extracts of FAME from leaves of different plants [60][61][62], similarly to the findings for the E. coli strains.
Studies with Mycobacterium sp. demonstrated antimicrobial activity of extracts of fenugreek seeds that contained a mixture of FAME and steroids, having a MIC of 100 µg/mL against M. tuberculosis [63]. Activity against Mycobacterium smegmatis was also verified by extracts containing hexadecanoate methyl and hexadecanoic acid <n-> FA from the stem bark of Malva nut, S. macropodum (3.13 mg/mL) and a MBC of 6.25 mg/mL [82]. Finally, the triterpenoids oleanolic acid and rotungenic acid were found to have activity against M. smegmatis with a MIC of 2.5 µg/mL and 1.25 µg/mL, respectively [83].
The fenugreek seed extracts from which conjugated linoleic acid methyl ester, saturated FAME, and steroids were identified, showed an inhibitory effect against Plasmodium falciparum with a MIC of 0.29 µg/mL [63].
The glycolipid SQDG isolated from neem has a broad-spectrum of activity against MDR bacterial strains [71] and anti-helminthic activity [72], which proves to be a promising natural antimicrobial agent. This class of compounds isolated from neem has demonstrated antiviral activity (herpes simplex viruses, HSV-1 and HSV-2) [71], significant DNA binding properties [84], and anti-leukemic activity [85]. However, it is difficult to isolate a single compound or a class of compounds from complex matrices as plants. In most studies, the antimicrobial effects may be due to a synergistic effect between several natural antimicrobial compounds, and not just to the referred lipids. As such, more studies must be done to understand which lipids can effectively be responsible by the inhibitory or microbicidal effect and the structure-activity relationship.

Antimicrobial Lipids from Marine Organisms
The most studied antimicrobial compounds of marine origin are peptides and alkaloids [86][87][88], contrarily to lipids. However, lipids are ubiquitously distributed in the different marine phyla, being quite abundant in some of them. Besides, several lipid classes from marine organisms have been recognized by their biological activity with a high potential to discover new antimicrobial compounds.

Marine Algae
Algal biomass is mainly composed by minerals, sugars, proteins, and lipids, that represent 1-15%, depending on the algal species and its habitat. Lipids found in the macroalgae from the three phyla, Rhodophyta, Chlorophyta, and Ochrophyta (Table 3), have demonstrated antimicrobial activity against Gram-positive and Gram-negative bacteria, yeasts, and fungi [29,[89][90][91][92] (Table 4). Most of these antimicrobial lipids were isolated from Rhodophyta and Chlorophyta. While the former shows a high diversity of algal species as source of antimicrobial lipids, studies in Chlorophyta were focused on species belonging to the order Bryopsidales. Table 3. Algae lipids and lipid-rich extracts with antimicrobial potential, their origin and extraction method.

Scientific Name Collection Site Extracting Solvent(s)/Method Isolated Lipids or Lipid Classes
Methods for Compounds Identification Ref.

Macroalgae-Chlorophyta
Caulerpa racemosa Qionghai, Hainan, China EtOH (95%). Extract partitioned with EtOAc and n-BuOH                    Several studies have tested the antimicrobial activity of macroalgal extracts obtained with different solvents. Shanab (2007) compared extracts from three macroalgae species (Sargassum dentifolium, Laurencia papillosa, and Jania corniculata) using two solvents, EtOH and CH 2 Cl 2 [93]. Both extracts exhibited similar antimicrobial activity against all microorganisms tested (bacteria and yeasts), except against the mold Aspergillus flavus [93]. Several extracts from Gracilaria gracilis were studied to identify potential bioactive compounds [94]. CHCl 3 and Et 2 O extracts (apolar solvents) presented lower extraction yields (% of dry algal biomass) than extracts from more polar solvents (EtOH, MeOH, and acetone). Less polar solvents isolated minor lipid classes (e.g., neutral and medium polar lipids) and showed lower amounts of soluble carbohydrates and total phenols than polar solvents. However, the diameter of the inhibition zones against B. subtilis were slightly lower in these extracts than in the extracts obtained with polar solvents (rich in soluble carbohydrates and phenolic compounds) [94]. These results suggest that although less polar solvents have lower yields, they contain compounds with interesting antibacterial features.
The lipids or lipid mixtures, their extracting solvent(s), and the methods used for their characterization in algae species are summarized in Table 3. The results of the antimicrobial assays with lipids and lipid-rich extracts from these algae are summarized in Table 4. Chemical structures of lipids isolated from algae are represented in Figure 2.

Fatty Acids
FA have been commonly reported in algae as antimicrobial agents. However, most studies tested total extracts instead of lipid-rich fractions and the antimicrobial activity was usually assigned to the most abundant FA in the extract [96,105,116]. The lack of target analyses limits the interpretation of the findings because FA are usually low abundant in total lipid extracts in the free form, being mainly esterified to other lipids, such as polar lipids, sterols, and triacylglycerols.
Although several studies have suggested that the FA chain length and double bond position influence their antimicrobial activity, there is no unanimity on the correlation between PUFA and antimicrobial activity [127].
Glycolipids have been identified as biogenic compounds possessing a variety of bioactivities, such as antioxidant, antiviral, or antitumor [89,95,103,122,[130][131][132]. The biological activity of glycolipids has been associated to the length of their fatty acyl chains, the number and position of the double bonds, the structure of the sugar moiety, and its anomeric configuration [131]. Gerasimenko et al. (2014) characterized the FA profiles and the antimicrobial activity of glycolipid fractions from S. pallidum throughout the year but could not assign any relationship between the amount of UFA and antimicrobial activity of DGDG, MGDG, and SGDG fractions [118]. However, a dependent effect of SFA level was observed in the antimicrobial activity of SQDG and MGDG fractions [118].
Glycolipids from the red alga Chondria armata were recognized by their antibacterial and antifungal activities against several pathogens, such as C. albicans and C. neoformans or Klebsiella sp. [104]. The main bioactive lipid from the glycolipid fraction was identified as 1-eicosapentanoyl-2-palmitoyl-3-Ogalactopyranosyl-glycerol, MGDG(20:5/16:0) [104]. The isolated fractions of MGDG and SQDG obtained from the lipophilic fractions of the brown seaweed Laminaria cichorioides inhibited the growth of the yeast Safale and C. albicans, the fungi A. niger and F. oxysporum, and the bacteria S. aureus and E. coli [91].
Several studies demonstrated higher antimicrobial activity in lipid-rich fractions than in total lipid extracts [91,118]. In the brown seaweed S. pallidum, the isolated fractions of glycolipids and free FA demonstrated to be more efficient in inhibiting bacterial growth than the total lipid extract [118]. The antimicrobial activity efficiency may be conditioned by seasonal variation, that must be related with shifts in the lipid profiles promoted by alterations in growth-related environmental conditions [118]. Nevertheless, a seasonal variation was not always verified, like in the green seaweed U. rigida that demonstrated a uniform antimicrobial activity throughout the year [27].

Other Lipids
The exploration of algal lipidomes has revealed a panoply of compounds with antimicrobial activity. Two compounds with antiviral properties against Semeliki forest and Japanese encephalitis viruses were isolated from U. fasciata's lipophilic fractions, a sphingosine (N-palmitoyl-2-amino-1,3,4,5tetrahydroxyoctadecane) and a ceramide (erythro-sphinga-4,8-dienine-N-palmitate), respectively [101,102]. Two bromoditerpenes, with antibacterial and antimalarial activity against S. aureus and the chloroquine-resistant P. falciparum, respectively, were isolated from the red alga Sphaerococcus coronopifolius [114]. Four halogenated sesquiterpenes isolated from the surface of the red macroalga Laurencia spp. demonstrated antibacterial activities toward six Gram-negative bacteria such as V. parahaemolyticus, Chromobacterium violaceum, or Erwinia sp. Neophytadiene and phytol were identified in the microalga D. salina as putative antimicrobial compounds against E. coli, S. aureus, C. albicans, and A. niger [123]. Several terpenes isolated from the brown seaweeds Sargassum fusiforme and S. vulgare were identified as putative antimicrobial molecules against clinical bacteria [117]. Terpenes were also suggested as antibacterial compounds against MRSA and E. coli in the red seaweeds Caulerpa racemosa and Caulerpa lentillifera [96].
Clerosterol was the main compound identified in a bioactive fraction from the green seaweed Codium amplivesiculatum against Gram-positive (S. aureus) and Gram-negative (V. parahaemolyticus) bacteria [100]. However, this compound did not display antibacterial activity when tested alone [100]. Different phospholipid classes, such as phosphatidylethanolamine and phosphatidylserine were identified in the red seaweed Pyropia orbicularis, as putative antimicrobial compounds, together with MGDG [113].

Photosynthetic Pigments
Pigments are not recognized as lipids, but they are present in the total lipid extracts of algae since they are extracted from the biomass along with lipids. Besides, in most studies, they were not removed from the lipid extracts. Pigments are mainly recognized by their antioxidant activity [134,135], but some studies have attributed a weak antibacterial activity to photosynthetic pigments such as chlorophylls and carotenoids [93]. Pheophytin α and chlorophyllide α from the microalga Isochrysis galbana were found to have a high antibacterial activity against marine strains of Brevibacterium and Micrococcus [136]. Carotenoid derivatives (β-cyclocitral and αand β-ionone) from the microalga D. salina extracts were suggested to be responsible for its antimicrobial activity [123]. Fucoxanthin-rich fractions from the brown macroalga L. cichorioides demonstrated weak activity against yeast (Safale and C. albicans) and bacteria (S. aureus and E. coli), while chlorophyll fractions showed antibacterial activity against S. aureus and E. coli [91]. Photosynthetic pigments isolated from the brown macroalga S. pallidum's extracts presented antifungal and antibacterial activities, chlorophylls (chlorophyll α as the main component) being the mediator to inhibit the growth of the fungi S. glycines and A. niger, while fucoxanthin was effective against E. coli [118].

Marine Invertebrates
The chemotaxonomic diversity of marine invertebrates is responsible for the large number of novel compounds identified in their phyla. Tropical biodiversity-rich benthic communities have been the most explored, thus the most fruitful in the identification of new potential antimicrobial compounds [137][138][139]. However, less conventional environments such as the Arctic ocean or mesopelagic communities have been started to be surveyed [140,141].
Marine invertebrates comprise a growing source of natural compounds, showing novel structures for biomedical and health-promoting applications [142]. Bioprospection of new compounds from marine invertebrates has revealed to be a prolific work to discover diverse bioactive compounds with action toward a broad spectrum of microorganisms [140,143]. Some of these reports have identified total lipid extracts as a potential source of bioactive compounds, lacking a sequential workflow of isolation, characterization, and purification of the metabolites responsible for the activity [99,143]. Although most of these studies used classical bioprospection methods to identify the bioactive compounds from marine species, others followed eco-friendly approaches by using fishing waste [141] or seafood by-products [144].
The main antibacterial natural products identified in marine invertebrates were peptides, polyketides, alkaloids, terpenes, and lipopeptides [14,34,150,152]. However, several antimicrobial lipids classes have been identified. Marine invertebrates produce an array of unique lipids originating from unusual biosynthetic pathways that are not common in other environments, as a result of thriving in diverse and extreme environments [142,153].
Porifera represents the most studied phylum of marine invertebrates for antimicrobial compounds' bioprospection, including lipids [154,155]. The high contribution of these ancestral metazoans for bioactive compounds' research seems to be related to their high filtering activity, pumping water during feeding, which expose them to viruses, bacteria, and eukaryotic organisms (pathogenic and non-pathogenic) [156,157]. Table 5 assembles the information regarding lipids from marine invertebrate species having antimicrobial activity. Table 6 summarizes the information about the antimicrobial properties, the tested organisms, and the antimicrobial assays for each marine invertebrates' species listed in Table 5. Figure 2 illustrates the chemical structure of the main lipid classes with antimicrobial properties from these natural sources.

Fatty Acids
FA identified in marine invertebrates display a high diversity of chemical structures that are scarce or inexistent in other environments, such as very long chain PUFA or cyclic forms [158][159][160]. A C14 acetylenic FA isolated from the sponge Oceanapia sp. revealed antimicrobial activity against nine microbial strains, including yeasts and the bacteria E. coli, P. aeruginosa, B. subtilis, and S. aureus [161]. Complex FA mixtures containing very long chain FA were isolated from the sponge Agelas oroides and tested as anti-infectious agents [162]. These FA inhibited the enoyl reductases, enzymes that catalyze the last step of the elongation cycle in FA synthesis, in P. falciparum, M. tuberculosis, and E. coli [162]. An acetylenic FA having antibacterial activity against S. aureus and E. coli was discovered in the marine sponge Paragrantia cf. waguensis [163]. Ravichandran et al. (2010) suggested 18:1 and 18:2 FA as antimicrobial compounds from the hemolymph and hemocytes of brachyuran crabs with greatest activity against V. cholerae, S. flexneri, S. pyogenes, and E. coli [148]. Brominated FA from a sponge of the genus Xestospongia showed antimicrobial activity against MRSA, S. mutans, and S. sobrinus [138]. Anti-infective brominated long-chain acids, termed motualevic acids, and an enantiomer of antazirine, were isolated from the sponge Siliquariaspongia sp. with activity against S. aureus and MRSA [164]. A recent study screening mesopelagic species as a source of potential antimicrobial compounds recognized the extracts of the lanternfish Myctophum punctatum and the Mediterranean krill Meganyctiphanes norvegica to inhibit the growth of MRSA, MSSA, and M. tuberculosis [141]. Extracts of both marine species were fractionated, and their composition was elucidated by LC-UV analysis. EPA, 4,7,10,13,16,19-docosahexaenoate (DHA, 22:6 n-3) and 8,11,14,17-eicosatetraenoic (ETA, 20:4 n-3) acids were the most abundant components in the lipid fractions of both marine species [141]. The 14-methyl-5,9-pentadecadienoic FA from the phospholipid pool was isolated and identified in the gorgonian coral Eunicea succinea and it was active against S. aureus and E. faecalis [137]. Quantitative data on antimicrobial efficiency are shown in Table 6.               Eunicea succinea Antibacterial

Sterols
Sterols with singular structures were isolated from marine invertebrates, exhibiting novel carbon skeletons. Marine sponges were found to be sources of steroid sulfates with antiviral activity against HIV [165,166] and against feline leukemia virus (FeLV) [166] and antifungal activity against the yeasts C. albicans [167,168] and S. cerevisiae [168,169]. Three sterols were isolated and identified in the marine sponge Haliclona simulans with anti-mycobacterial and anti-trypanosomal activity [170]. The EtOAc extract of the flesh of the rock oyster Saccostrea glomerate was fractionated, and the best active fraction showed several sterols (e.g., cholesterol, stigmasterol, sitosterol) and FA as putative active compounds with antibacterial and antifungal activities against a broad spectrum of microorganisms [171]. Steroidal glycosides (e.g., eryloside, wondosterols, sarasinoside, sokodosides) isolated from marine sponges also showed antimicrobial properties [172][173][174][175].

Polar Lipids
Polar lipids are the main constituents of biological membranes and are also present in biofluids. The methanolic extracts of both the hemolymph (plasma) and hemocytes (plasma cells) of six brachyuran crabs revealed antimicrobial activity against sixteen bacterial and fungal pathogenic strains [148]. The antimicrobial activity of the hemolymph extracts was assigned to the presence of polar lipids [148].
A new family of antimicrobial glycolipids, caminosides, isolated from the marine sponge Caminus sphaeroconia showed a potent in vitro inhibition against a panel of human and plant pathogens (e.g., MRSA, vancomycin-resistant Enterococcus) and enteropathogenic E. coli (EPEC) [147,176]. Several ceramide and glycolipid molecular species have been isolated from soft corals of the genus Sinularia, and exhibited antibacterial and antifungal activity against seven microbial strains [139]. A ceramide from the cnidaria Lobophytum crassum showed moderate antibacterial activity against P. aeruginosa, S. epidermis, B. subtilis, and S. aureus [177].
Terpenes with functional activities were identified in the cnidaria phylum, as from the soft coral Antillogorgia elisabethae [192]. Seven diterpenes showed potent antibacterial activity against the Gram-positive bacteria S. pyogenes, S. aureus, and E. faecalis [192]. A new series of lipids, termed mololipids, were found in the lipidome of a sponge from the order Verongida (recognized as "the Moloka'i sponge") and were reported to be active against HIV-1 [191]. These lipids present a core moloka'iamine nucleus with two FA-derived side chains, saturated linear and iso-methyl branched ranging from 14 to 20 carbons [191].

Pigments
Xanthophyll, β-cryptoxanthin, and β-carotene obtained from the flesh and coelomic fluid of Holothuria scabra (sea cucumbers) showed antibacterial activity against S. aureus [193]. Skin pigments of the jumbo squid Dosidicus gigas exhibited growth inhibition against several bacteria, fungi, and yeast species [144]. The highest inhibition zone diameter was verified against S. enterica, and ommatins, specifically of the xanthommatin type, were suggested as the main active compounds [144].

Cytotoxicity of Natural Antimicrobial Lipids against Mammalian Cells
Some lipid and lipid-rich extracts from natural sources showed high MIC and MBC values, in the range of mg/mL, as can be seen in Table 2, Table 4, and Table 6. This is more than one thousand times larger than is of pharmaceutical significance and raises an important question regarding cytotoxicity to mammalian cells and safety to human beings. Some studies emphasized the low toxicity to eukaryotic cells of antimicrobial lipids from natural sources, namely of FA [158]. But most studies that searched for natural lipids with antimicrobial activity did not perform in vitro or in vivo cytotoxicity experiments. Data regarding mammalian cytotoxicity are not yet available for most of these compounds, which is a limitation of the studies presented in this review about antimicrobial plant lipids. Most studies that tested antimicrobial and cytotoxicity activities from marine lipids are limited to lipid-rich extracts [145,194] or lipid-rich fractions [97,195]. For instance, petroleum ether fractions of Grateloupia livida extracts demonstrated a higher antibacterial activity and this fraction did not display acute oral toxicity in mice (100% survival rate at doses up to 2.0 g/Kg) [195]. Even so, careful conclusions should be taken from these evidences, since petroleum ether is a carcinogenic solvent. The MeOH and acetone fractions from O. obtusiloba demonstrated activity against HSV-1 and low toxicity to Vero cell (African Green monkey kidney cells) cultures, showing 50% cytotoxic concentration (CC 50 ) of 172 and >200 µg/mL in MeOH and acetone fractions, respectively [97]. A sulfolipid-rich fraction from the same red macroalga showed a higher cytotoxicity with CC 50 of 72 µg/mL in Vero cell lines [111]. SQDG fractions from S. vulgare with antiviral activity (HSV-1 and HSV-2), displayed CC 50 > 200 µg/mL in Vero cells [119]. The ethanolic extract as well as the glycolipid, FA and pigments' fractions from L. cichorioides demonstrated hemolytic activity at 200 µg/mL in erythrocytes of mongrel white mice [91]. SQDG(16:0/16:0) isolated from C. racemosa with antiviral activity (HSV-2) had very low toxicity to Vero cells ATCC CCL-81 with a CC 50 of 1.0 g/mL [95]. An acetylenic FA isolated from the calcareous sponge P. cf. waguensis demonstrated weak cytotoxic effect against NBT-T2 rat bladder epithelial cells (IC 50 > 20 mg/mL) [163]. FA mixtures isolated from the sponge A. oroides exhibited in vitro antiplasmodial, trypanocidal, and leishmanicidal activities with weak cytotoxicity toward mammalian (rat skeletal myoblast L6) cells (IC 50 from 43 to >90 µg/mL) [162]. Sterols isolated from the sponge H. simulans demonstrated anti-mycobacterial and anti-trypanosomal activities, but low cytotoxicity on normal fibroblasts derived from human foreskin (Hs27) cells with IC 50 ranges from 58 to >100 µM [170]. Meroterpene sulfate molecules isolated from the sponge Fasciospongia sp. was inactive (IC 50 > 30 µM) toward human foreskin fibroblast (HFF-1) cell lines [186]. Finally, lipid extracts from different body compartments (hemolymph, hemocyte, eggs, muscle, and exoskeleton) of four marine crustaceans demonstrated hemolytic activity against human red blood cells, hemolymph and exoskeleton extracts being those that presented the highest activity [149].
Molecules, in this case lipids, with such high MIC values have little chance of undergoing clinical development. Nevertheless, the results of the reviewed reports should allow to focus on the most interesting molecules, having a low MIC value, and reserve the molecules with a high MIC value for other applications, such as veterinary clinic or agriculture.

Lipidomics for the Analysis of Bioactive Lipids in Plants and in Marine Organisms
The identification and characterization of lipids as natural antimicrobial agents is of utmost importance, either for their exploitation from natural sources or to understand their mechanisms of action. Lipid identification from natural sources is achieved by using lipidomic strategies. Lipidomics consists in the analysis, identification, and structural characterization of the lipid molecular species from a biological matrix, based on chromatography and MS techniques. It also involves optimizing extraction and fractionation processes, considering the polarity of the different lipids that make up the biological systems. As such, lipidomics is a valuable tool to discover bioactive lipids with antimicrobial activity.
The main phases of the lipidomics workflow include lipid extraction, fractionation, and/or enrichment steps, and analysis of total extracts or lipid fractions by MS combined with LC and/or GC ( Figure 3).
Molecules, in this case lipids, with such high MIC values have little chance of undergoing clinical development. Nevertheless, the results of the reviewed reports should allow to focus on the most interesting molecules, having a low MIC value, and reserve the molecules with a high MIC value for other applications, such as veterinary clinic or agriculture.

Lipidomics for the Analysis of Bioactive Lipids in Plants and in Marine Organisms
The identification and characterization of lipids as natural antimicrobial agents is of utmost importance, either for their exploitation from natural sources or to understand their mechanisms of action. Lipid identification from natural sources is achieved by using lipidomic strategies. Lipidomics consists in the analysis, identification, and structural characterization of the lipid molecular species from a biological matrix, based on chromatography and MS techniques. It also involves optimizing extraction and fractionation processes, considering the polarity of the different lipids that make up the biological systems. As such, lipidomics is a valuable tool to discover bioactive lipids with antimicrobial activity.
The main phases of the lipidomics workflow include lipid extraction, fractionation, and/or enrichment steps, and analysis of total extracts or lipid fractions by MS combined with LC and/or GC (Figure 3). Lipid extraction is usually performed with organic solvents. Mixtures of CHCl3, MeOH, and H2O are the most used, such as the Bligh and Dyer [196] or Folch [197] methods and their modifications. These methods are considered the most efficient to extract the total lipid content. However, there are other solvents used for lipid extraction, such as EtOH, MeOH, n-hexane, BuOH, EtOAc, giving rise to different lipid yields [198,199]. CHCl3 is being replaced by CH2Cl2 in these methods because the latter is less toxic and allows a similar extraction efficiency. The solvent or the mixture of solvents used in the extraction step(s) exert a great influence in the lipid composition of the extracts obtained from natural sources. In most cases, extractions with different solvents lead to crude extracts with the same components but in different amounts. It is also possible to selectively extract some lipid classes, such as the extraction performed with apolar solvents, as n-hexane, which recover mostly apolar lipids (e.g., triacylglycerols). Nevertheless, an effective extraction is very important for the identification of bioactive lipids. Non-conventional methods of extraction as ultrasound-and microwave-assisted extraction or SFE [63] are used to help the extraction, particularly to improve the lipids yield, or to obtain extracts with specific functions and applications. Assisted extraction methods are frequently combined with green solvents, such as EtOH. Soxhlet-assisted extraction with less polar solvents, such as n-hexane, is normally used to isolate lipophilic and more apolar lipids, as terpenoids and sterols. Testing potential antimicrobial lipid extracts obtained with different solvents,  Lipid extraction is usually performed with organic solvents. Mixtures of CHCl 3 , MeOH, and H 2 O are the most used, such as the Bligh and Dyer [196] or Folch [197] methods and their modifications. These methods are considered the most efficient to extract the total lipid content. However, there are other solvents used for lipid extraction, such as EtOH, MeOH, n-hexane, BuOH, EtOAc, giving rise to different lipid yields [198,199]. CHCl 3 is being replaced by CH 2 Cl 2 in these methods because the latter is less toxic and allows a similar extraction efficiency. The solvent or the mixture of solvents used in the extraction step(s) exert a great influence in the lipid composition of the extracts obtained from natural sources. In most cases, extractions with different solvents lead to crude extracts with the same components but in different amounts. It is also possible to selectively extract some lipid classes, such as the extraction performed with apolar solvents, as n-hexane, which recover mostly apolar lipids (e.g., triacylglycerols). Nevertheless, an effective extraction is very important for the identification of bioactive lipids. Non-conventional methods of extraction as ultrasound-and microwave-assisted extraction or SFE [63] are used to help the extraction, particularly to improve the lipids yield, or to obtain extracts with specific functions and applications. Assisted extraction methods are frequently combined with green solvents, such as EtOH. Soxhlet-assisted extraction with less polar solvents, such as n-hexane, is normally used to isolate lipophilic and more apolar lipids, as terpenoids and sterols. Testing potential antimicrobial lipid extracts obtained with different solvents, even from the same source, would provide dissimilar bioactive properties due to different lipid compositions.
Total lipid extracts include a great diversity of lipid classes and molecular species. Fractionation of the total lipid extracts to obtain fractions enriched with specific lipid classes can be achieved by solid-phase extraction (SPE), using columns of simple silica (SPE-SiO 2 ) or functionalized silica, by preparative HPLC, or by TLC, although the latter is less common nowadays. TLC can also be used as an identification tool in lipid extracts or fractions, by applying spots of well-known lipid standards in the TLC plate and compare their retention factor with the samples' bands after elution. However, it has the disadvantages of lacking information at lipid molecular level and being time-consuming [200]. A careful choice of the eluents and gradients allows obtaining a selective separation of lipid fractions from the initial extract. The fractions obtained can be used for diverse purposes: to quantify the different lipid classes (e.g., by gravimetry), to separate neutral lipids (e.g., triacylglycerols and sterols) from polar lipids (e.g., phospholipids, glycolipids), or to identify different phospholipid classes, that can be further separated by TLC and estimated by phosphorus quantification of each TLC spot. The lipid fractions can be then used to evaluate the bioactive properties of a specific lipid class and analyzed by GC-MS and/or LC-MS to provide an insight on the structure-activity relationship.
The characterization of total lipids or lipid fractions is done, nowadays, by MS-based approaches using several kinds of mass spectrometers coupled with chromatography. GC-MS is used to identify and quantify FA after derivatization (usually methylation) of apolar lipids as terpenoids and sterols (after derivatization by silylation). GC-MS allows the identification of these molecules by specific retention times, mass spectra analysis, and data base comparison. Nevertheless, GC-MS neither covers large molecules nor non-volatile molecules. Thus, when this approach is used to characterize bioactive lipids, a lot of information is missing. LC-MS allows a broader coverage than GC-MS of lipid species and larger molecules, usually the most abundant ones in the lipid extracts, such as triacylglycerols, polar lipids, or esterified sterols. Besides, small molecules like free FA and free sterols are also detected.
LC-MS identifies the lipid molecular species by their retention time, accurate molecular weight observed in the mass spectra (i.e., LC-MS level), and detailed structural features by interpretation of their LC-MS/MS spectra. The chromatographic peaks are integrated to plot the quantification of each molecular species, which is normalized by an appropriate internal standard to reach a relative quantification. Routine lipidomics analyses use reversed-phase (e.g., C18 or C30), normal-phase, or hydrophilic interaction liquid chromatography (HILIC) columns [200]. In the former, the elution of the lipid molecules is based on their FA composition. So, an overlap of molecular species from different lipid classes but presenting the same FA composition can occur. In the latter two types of columns, the lipid molecules are separated by their hydrophilic properties, allowing to separate the different lipid classes present in the sample, which will depend on the polar head features.
Recent developments in MS technologies permit to acquire a huge amount of data in a short time frame, covering more than three hundred lipid species in one single LC-MS run. This can be done by means of the high-resolution mass spectrometers Orbitrap or quadrupole-time-of-flight (Q-TOF), available for untargeted or targeted analysis, or the triple quadrupoles (QQQ) and Q-traps, that are more suitable for targeted analysis, i.e., quantification of previously selected molecular species. However, data analysis is still time-consuming because of the lack of universal and exhaustive lipidomics databases and software. Some databases and software currently used are the LIPID MAPS database, the MZmine, LipidBlast, LipoStar, LipidSearch, and LPPtiger software.
The identification of bioactive molecules from natural sources is a laborious work that requires performing extraction, bioassays, fractionation, and in some cases, identification, in a sequential way. This is to achieve a specific class of bioactive metabolites or, preferentially, a unique and well-defined bioactive molecule. This process is called bioassay-guided fractionation and is emerging in the field of drug discovery from natural sources [201]. This approach usually starts with different extracts obtained by using solvents with different polarities, in order to have a set of extracts enriched in different types of metabolites. These extracts are evaluated for their putative bioactivities and the one(s) with the highest activity is(are) further fractionated and the new fractions are assayed. The bioassayfractionation-bioassay sequence can go around until a promising candidate molecule or family of molecules are considered and then it is characterized. The first step of fractionation is typically a liquid-liquid extraction (solvent-solvent partition), and normally uses chromatography techniques, as TLC and column chromatography [201][202][203]. Isolated fractions are assayed afterwards. A final deep characterization using MS and NMR spectroscopy will reveal the exact structure of the bioactive molecule(s). This workflow will be a valuable tool to guide new drug discoveries and for understanding the structure-activity relationships. In this regard, lipidomics represents a valuable platform to advance in the characterization of lipid structures, survey lipid pathways, and can help in the elucidation of the interaction of lipids with microorganisms' membranes, because of the high sensitivity and selectivity of MS.

Prospection and Applications of Antimicrobial Lipids
Several antimicrobial lipids are commercially available or can be used in different formulations in the cosmetic, pharmaceutical, and food industries, as well as in agriculture and aquaculture.

Cosmetic and Drug Formulations
In cosmetics, the free FA 10:0 and 12:0 and their corresponding MAG, monocaprin MAG(10:0) and monolaurin MAG(12:0), respectively, are commonly used in topical applications [21]. As a general rule, the esterification of free FA to glycerol increases the antimicrobial activity [127]. MAG(10:0) has been suggested as an ingredient for drug formulations to avoid oral infections by C. albicans [204].

Food Additives
The free FA 8:0 (commercial lipid standard) was shown to be effective against E. coli, Salmonella and other foodborne pathogens [21]. MAG(10:0) is known as a safe food additive, widely used as an emulsifier in the food industry [205].
Lipid extracts from marine organisms demonstrated to be a source of antibacterial compounds. The FA 16:1 inhibited the growth of the foodborne pathogens Bacillus cereus and B. weihenstephanensis [125]. Extracts from the microalga D. salina inhibited the growth of important food industry pathogens (i.e., E. coli, S. aureus, C. albicans, and A. niger) [123]. EPA, a high abundant PUFA in marine species, presented a potent action against the foodborne pathogen B. cereus, likely by disrupting the pathogen's cell membrane, ultimately leading to cell lysis [206].

Herbicides and Pesticides
Some studies evaluated the antimicrobial activity of plants' lipid extracts against both phytopathogenic fungi and bacteria. The sphingolipid [(2S,3S,4R,10E)-2-[(2'R)-2-hydroxytetracosanoylamino]-1,3,4octadecanetriol-10-ene], a low polarity aglycone isolated from cucumber (C. sativus) stems, had strong antibacterial and antifungal activity against plant pathogens [74]. A mixture of lipids (FA, FAME, squalene, and β-sitosterol) extracted from andiroba (Carapa sp.) seeds demonstrated a MIC range of 156-250 µg/mL against several phytopathogenic fungi: A. flavus, A. niger and F. oxysporum [58]. Antimicrobial compounds from plants are envisioned as eco-friendly alternatives to chemical pesticides that harm both the environment and public health [58], because of their structural diversity, unique bioactivity, and environmental compatibility [74]. It is, thus, necessary to invest in a new approach to understand the mode of action of antimicrobial lipids isolated from plants in microbial cells.
Marine macroalgae have been traditionally used by several populations as soil fertilizers in agriculture, enriching them with minerals and plant growth promoters. The presence of antimicrobial molecules in macroalgae may contribute to develop biologically active compounds that protect agricultural crops toward pathogenic bacteria and fungi. This is the case of the sulfolipids isolated from the brown macroalga S. wightii that showed activity against the Gram-negative bacterium Xanthomonas oryzae pv. oryzae that causes the bacterial blight of rice [120]. Specific glycolipids, termed caminosides, isolated from the marine sponge C. sphaeroconia were active against the plant pathogen Pythium ultimum, the cause of damping off and root rot diseases in food crops and ornamental species [176]. Three sesterterpenes identified in the sponge Thorectandra sp. showed high inhibitory effect against the plant pathogenic fungus Cladosporium herbarum [190].
A recent study has mapped soil-borne fungal plant pathogens and projected their distribution under different climate change and soil uses scenarios [207]. The most dominant pathogens that harm global crop production belong to the genera Alternaria and Fusarium [207]. Enriched fractions of glycolipids from the macroalgae L. cichorioides and S. pallidum, as well as free FA and PUFA fractions from L. cichorioides, and FA esters, triacylgycerols/free sterols, free sterols and diacylglycerol fractions from S. pallidum demonstrated activity against the fungus F. oxysporum [91,118]. Lipid fractions with several steroid compounds from the oyster S. glomerata were very active against Fusarium sp. [171]. A bioprospection study in macroalgae of the order Caulerpales isolated ten sesquiterpenoids and diterpenoids and tested their activity in several bacterium and fungi strains. Eight of these compounds inhibited the growth of Alternaria sp. [98].

Aquaculture
Nowadays, aquaculture production accounts for almost half of the animal and aquatic plants catches, with 82 million tons [208]. Aquaculture represents an important food production sector, so finding environmental-friendly compounds with antibiotic activity instead of synthetic drugs is of utmost importance. This is even more urgent if we consider that, although vaccination is the best alternative to prevent infectious diseases in general, in aquaculture it is not likely to protect juvenile fish (without a mature immune system), shrimp, and bivalves (without an adaptive immune system, only with innate system), which are the most affected aquaculture animals by infectious diseases.
Several antimicrobial compounds identified in marine organisms are salt-tolerant [14] and have demonstrated to be efficient against aquaculture-relevant pathogens such as Listonella anguillarum, Lactococcus garvieae, and Vibrio species, showing advantage in pathogen control of fish and shellfish farming [99,108,112,125,126,209]. Extracts of Falkenbergia, a heteromorphic sporophyte of Asparagopsis taxiformis, inhibited the growth of several fish and shrimp Vibrio pathogens [106]. The FA 16:3 n-4 (HTA) isolated from the diatom P. tricornutum inhibited the growth of the Gram-negative marine pathogenic bacterium L. anguillarum [125]. A lipid fraction of the oyster S. glomerata extract inhibited the shrimp white spot syndrome virus (WSSV) in Fenneropenaeus indicus [171].

Conclusions
Lipids isolated from plants and marine organisms have demonstrated a broad spectrum of antimicrobial activities. These biomolecules can be envisaged as a promising alternative to help control pathogenic microbial infections. They have shown positive results in in vitro studies and microorganisms do not develop resistance to them, as far as it is known. However, the study of natural sources-derived antimicrobial lipids presents yet some shortcomings. There is a low number of systematic studies performing the isolation of pure lipid molecular species and their structural characterization, since they are mainly studied as complex extracts. There is also a lack of in vivo tests, indispensable to understand their mechanism of action, their cytotoxicity to eukaryotic cells, and consequently allowing their use at a large scale. Lipidomics is a fundamental tool to advance in the characterization of lipid structures and elucidate the lipids-microorganisms' interaction. This can be achieved along with complementary biophysical, spectroscopic, and spectrometric studies that will be useful to understand their mechanism of action toward microbes and to define the structure-activity relationship. In addition, it is necessary to study the synergistic effects of these compounds against a wide range of microorganisms' species to evidence broad-spectrum effects. The bioprospection of new natural lipids will benefit from the association among complementary research areas and the industry. Furthermore, the potential applications of antimicrobial lipids in different economic sectors, such as pharmaceutical, cosmetics, agriculture, or aquaculture, are still underexplored. So, the combination of these natural resources with sophisticated analytical tools applied to lipidomics research will open new perspectives on antimicrobial lipids for clinical and non-clinical applications. It is hoped that the future could bring about a greater interaction between lipidomics and natural chemotherapy to reveal the true microbicidal action of lipids toward pathogens. More studies must be done to understand which lipids can effectively be responsible by the inhibitory or microbicidal effect and their structure-activity relationship. Nuclear