Marine Peptides and Their Anti-Infective Activities

Marine bioresources are a valuable source of bioactive compounds with industrial and nutraceutical potential. Numerous clinical trials evaluating novel chemotherapeutic agents derived from marine sources have revealed novel mechanisms of action. Recently, marine-derived bioactive peptides have attracted attention owing to their numerous beneficial effects. Moreover, several studies have reported that marine peptides exhibit various anti-infective activities, such as antimicrobial, antifungal, antimalarial, antiprotozoal, anti-tuberculosis, and antiviral activities. In the last several decades, studies of marine plants, animals, and microbes have revealed tremendous number of structurally diverse and bioactive secondary metabolites. However, the treatments available for many infectious diseases caused by bacteria, fungi, and viruses are limited. Thus, the identification of novel antimicrobial peptides should be continued, and all possible strategies should be explored. In this review, we will present the structures and anti-infective activity of peptides isolated from marine sources (sponges, algae, bacteria, fungi and fish) from 2006 to the present.


Introduction
Marine organisms are important sources of bioactive molecules that have been used to treat various diseases. Unusual marine environments are associated with chemical diversity, leading to a resource of novel active substances for the development of bioactive products [1][2][3][4].
Oceans, which cover more than 70% of the earth's surface, represent an enormous resource for the discovery of potential therapeutic agents. Over the last several decades, numerous compounds have been found in marine organisms with interesting pharmaceutical activities [1,5,6]. Therefore, marine organisms are thought to be a potential source of essential and novel biologically active substances for the development of therapeutics. In particular, marine peptides have attracted a great deal of attention due to their potential effects in promoting health and reducing disease.
Despite tremendous progress in medicine, infectious diseases caused by bacteria, fungi, and viruses are still a major threat to public health. Their impact is particularly large in developing countries due to the lack of access to medicines and the emergence of widespread drug resistance. Increasing numbers of microbial pathogens that have acquiring antibiotic-resistance have resulted in the increase demand for novel and effective antimicrobial compounds. In particular, studies evaluating the anti-infective actions of marine peptides are attracting increased researchers' interest, and marine peptides have been increasingly considered as anti-infective drugs. The diversity of the marine environment has provided a unique source of bioactive chemical compounds that could lead to potential new drugs candidates.
Marine peptides are specific protein fragments that in addition to acting as sources of nitrogen and amino acids have numerous potential physiological functions [7]. These peptides have been obtained from algae, fish, mollusk, crustacean, crab and marine bacteria and fungus. Bioactive marine peptides based on their structural properties, amino acid composition and sequences have been shown to display a variety of bioactivities such as anti-tumor, antiviral, anticoagulant, antioxidant, immunoinflammatory effects, and other medicinal properties [1,2,8,9].
In this review, we will present the structures and anti-infective activity of peptides isolated from the main marine organisms and microorganisms of interest (sponges, algae, bacteria, fungi and fish) from 2006 to the present.

Sponges
Sponges are sessile marine filter feeders that have developed efficient defense mechanisms against foreign attackers such as viruses, bacteria, or eukaryotic organisms. Marine sponges are one of the richest sources of pharmacologically-active chemicals in marine environments. It has been suggested that many of the bioactive secondary metabolites isolated from sponges are produced by functional enzyme clusters, which originated from the sponges and their associated microorganisms [10,11]. More than 5300 different products have been identified in sponges and their associated microorganisms, and more than 200 new metabolites from sponges are reported each year [10]. As infectious microorganisms evolve and develop resistance to existing pharmaceuticals, the marine sponge provides novel leads against bacterial, viral, fungal and parasitic diseases. Many marine natural products have successfully advanced to the late stages of clinical trials. The chemical diversity of sponge products is remarkable. In addition to the unusual nucleosides, bioactive terpenes, sterols, peptides, alkaloids, fatty acids, peroxides, and amino acid derivatives (all of them frequently halogenated) have been described in sponges. The early appearance of sponges in evolution has given them a lot of time for the development of an advanced chemical defense system. The synthesis of secondary metabolites is regulated, depending on conditions that the sponge experiences. The huge variety of secondary metabolites in marine sponges and the complexity of the compounds and their biosynthetic pathways can be regarded as an indication of their importance for survival. As infectious microorganisms evolve and develop resistance to existing pharmaceuticals, marine sponges provide novel leads against bacterial, fungal, and viral diseases [10,11].

Algae
Algae are very simple chlorophyll-containing organisms composed of one cell or grouped together in colonies or as organisms with many cells, sometimes collaborating together as simple tissues [12]. Algae are heterogeneous group of plants with a long fossil history. Two major types of algae have been identified. Macroalgae (seaweeds) occupy the littoral zone, whereas microalgae are found in both benthic and littoral habitats, as well as throughout the ocean waters as phytoplankton [13]. Phytoplankton include numerous organisms, including diatoms (Bacillariophyta), dinoflagellates (Dinophyta), green and yellow-brown flagellates, and blue-green algae (cyanobacteria). As photosynthetic organisms, this group plays a key role in the productivity of oceans and constitutes the basis of the marine food chain [12].
Marine algae produce a cocktail of metabolites with interesting biological activities, including anti-infective, anti-inflammatory and anti-proliferative, and with potential commercial value [14][15][16][17]. Structures exhibited by these compounds go from acyclic entities with a linear chain to complex polycyclic molecules. Their medical and pharmaceutical applications have been investigated for several decades [18].
Marine macroalgae have been used as foods, especially in China and Japan, and as crude drugs for the treatment of many diseases, such as iodine deficiency (goiter, Basedow's disease, and hyperthyroidism). Some seaweeds have also been used as a source of vitamins, treatment for various intestinal disorders, such as vermifuges, and as hypocholesterolemic and hypoglycemic agents [19].
Marine eukaryotic microalgae are known to produce numerous useful products, but have attracted little attention in the search for novel anti-infective compounds. Current reports mainly concern diatoms and cyanobacteria [20].
Diatoms are ubiquitous and constitute an important group within the phytoplankton community, as well as making an important contribution to the total marine primary production. These microalgae exhibit a characteristic golden-brown color, due to a high amount of xanthophyll fucoxanthin, which plays a major role in the light-harvesting complex of photosystems. In the water column, diatoms are exposed to rapidly varying light intensities. Diatoms produce an array of biologically active metabolites, many of which have been ascribed to a form of chemical defense, and which may have potential as candidate marine drugs [21].
Blue-green algae (cyanobacteria) show many structural features that are in common with bacteria. However, they are classified with algae because they contain chlorophyll a and related compounds. These algae are ancient photosynthetic prokaryotic organisms that produce biologically active secondary metabolites with diverse chemical structures such as nitrogenous compounds and cyclic polyethers [17]. Cyanobacteria may have evolved this extensive capacity to produce bioactive molecules because they are prokaryotes that have developed beyond a microscopic lifestyle, and hence require an arsenal of defensive substances to ward off predation by diverse macrograzers. Recently, several marine cyanobacterial natural products have been the focus of much attention, due to their intriguing structures and exciting biological activities [22].

Microorganism
Microorganisms have been the source of many valuable compounds in medicine, the pharmaceutical industry, and agriculture; however, most of these organisms are derived from terrestrial habitats. After intensive studies on terrestrial microorganisms, attention has now been focused on other ecosystems, such as the sea. Marine microorganisms, including bacteria and fungi, are rich sources of chemical products. Marine microorganisms are a promising new source of a large number of biologically active products [23][24][25][26]. Some of these marine species survive in a stressful habitat, under cold, lightless, and high-pressure conditions. These factors have resulted in the development of unique metabolisms, which result in the production of novel metabolites that differ from terrestrial organisms. Thus, marine microorganisms offer a wonderful resource for the discovery of new compounds with interesting biological activities, including antimicrobial, antifungal, anti-protozoan, anti-tuberculosis, and antiviral properties [27,28]. Thus far, only a small number of microorganisms have been investigated for their bioactive metabolites; however, a large number of active substances have been isolated, some of which feature unique structural skeletons.

Fish
Fishery products, including underutilized fish, jellyfish, and mollusks, also contain diverse and novel compounds. Some of these function as hypotensive agents, cardio-protective substances, muscle relaxants, and antimicrobial, antiviral, and anti-tumor agents [2,29,30].
Fish peptides exhibit broad-spectrum antimicrobial activity, killing both fish and human pathogens. These peptides have immunomodulatory effects, and the genes encoding these are highly responsive to microbes and innate immunostimulatory molecules. Recent research has demonstrated that some of the unique properties of fish peptides, including their ability to act even at very high salt concentrations, make them good potential targets for development as therapeutic antimicrobials [2,29,30].
In this review, we have surveyed peptides derived from marine sponges, algae, bacteria, fungi, and fish that have shown efficacy or activity against infectious and parasitic diseases, including bacterial, viral, fungal and protozoan infections. Table 1 presents new anti-infective peptides reported from 2006 to the present, along with their composition, source of origin, and reported bioactivity.

Antibacterial Activity
Several different organisms use antimicrobial peptides, typically 20-40 amino acids in length, for defense against infection. Most are capable of rapidly killing a wide range of microbes. Large antimicrobial proteins (>100 amino acids), are often lytic, nutrient-binding proteins or target specific microbial peptides by disrupting the structure or function of microbial cell membranes. A multitude of antimicrobial proteins have been found in the epithelial layers, phagocytes, and body fluids of multicellular animals, including humans. In addition to their role as endogenous antibiotics, antimicrobial peptides (AMP) contribute to inflammation, wound repair, and regulation of the adaptive immune system [91]. As part of ongoing global efforts to discover novel antimicrobials to treat infections caused by resistant pathogenic organisms, 35 studies have since 2006 reported novel antibacterial peptides isolated from marine sources such as bacteria, sponges, mud crabs, ascidin, spider crabs, jellyfish, and fish.

Aurelin
Ovchinnikova et al. reported in the purification of a 40-residue AMP from the mesoglea of a scyphoid jellyfish, Aurelia aurita, Linnaeus, 1758 [31] (Figure 1). The peptide was named aurelin (1), and exhibited activity against gram-positive and gram-negative bacteria. Minimum inhibitory concentrations (MIC) were defined under low salt conditions (10 mM sodium phosphate buffer, pH 7.4). Aurelin inhibited the growth of microorganisms (MICs: 22.64 μg/mL against Listeria monocytogenes and 7.66 μg/mL against Escherichia coli). Interestingly, aurelin showed no sequence homology to any previously identified AMP, but displayed partial similarity to both defensins and K-channel-blocking toxins of sea anemones [31].  (1). Aurelin was isolated from the mesoglea of a scyphoid jellyfish, Aurelia aurita [31]. Aurelin has six cysteine residues, forming three disulfide bonds.

Arenicin-1
Lee et al. reported that the 21-residue peptide arenicin-1 (2), isolated from the marine polychaete Arenicola marina, exhibited significant antibacterial activity against Pseudomonas aeruginosa and Staphylococcus aureus (MIC = 2 μg/mL) [32]. Arenicin-1 contained one disulfide bond (Cys3-Cys20), and formed a typical β-hairpin structure [33] (Figure 2). Arenicin-1 can disrupt the cell membrane through the formation of highly oligomerized states, which cause pore formation [92]. These results emphasize the inherent difficulty in predicting the antimicrobial potential and the mode of action of a rather simple scaffold. Furthermore, arenicin-1 induced the release of calcein from PE/PG liposomes, thus suggesting that the bacterial cell membrane is the main molecular target of the peptide [33].

Tauramamide
Tauramamide (3) is produced by cultures of the marine bacterial isolate Brevibacillus laterosporus PNG276, obtained from Papua New Guinea [34]. Tauramamide is a new lipopeptide antibiotic that contains two D-amino acids and is acylated at the N-terminus, two hallmarks of a non-ribosomal peptide synthase biosynthetic origin. Tauramamide showed potent (MIC = 0.1 μg/mL) and relatively selective inhibition of the gram-positive human pathogen Enterococcus sp. Moreover, tauramamide showed weak activity against Methicillin resistant S. aureus (MRSA; MIC = 200 μg/mL), although it was not appreciably active against Candida albicans (MIC = 50 μg/mL).

Hepcidins
Hepcidins (4) are cysteine-rich peptides isolated from tilapia (Oreochromis mossambicus), with MICs (50-100 μg/mL) against L. monocytogenes, S. aureus, and Enterococcus faecium [35]. Hepcidins are antimicrobial peptides that play important roles in resisting pathogenic infection. Through hybridization of a phage library, the cDNA sequences of three hepcidin-like antimicrobial peptides (named TH1-5, TH2-2, and TH2-3) were identified in tilapia ( Figure 3). The predicted molecular weights of TH1-5, TH2-2, and TH2-3 are 9.5, 9.4, and 9.8 kDa, respectively. Fish hepcidins are active against a wide variety of bacteria, both gram-positive and -negative in the low μM range, including potent activity against a large number of fish pathogens. In contrast, their quantified activity against fungi appears to be rather low [35].  (4) were isolated from tilapia (Oreochromis mossambicus) [35]. Identical or similar amino acid residues are in same colors. Gaps are inserted to obtain maximum homology.

Tunichromes
Tunichromes (6) are small peptides containing one or more dehydrodopa-derived units that have been identified in several species of tunicates. Incubation of tunichromes isolated from Ascidia nigra hemocytes under oxidative conditions possess inherent crosslinking properties in vitro [38]. Three tunichromes have also been isolated from the blood cells of Ascidia nigra (An-1, An-2, An-3) [39]. Tunichromes show potent (MICs = 0.1 μg/mL) and relatively selective activity against the gram-positive human pathogen Enterococcus sp. [38].

Bacillistatins 1 and 2
Two new cyclodepsipeptides, designated bacillistatins 1 (7) and 2 (8), have been isolated from cultures of a sample of Bacillus silvestris that was obtained from a Pacific Ocean (southern Chile) crab. Each 12-unit cyclodepsipeptide strongly inhibited growth in a human cancer cell line panel, with a 50% growth inhibition values (GI50s) of 10 −4 -10 −5 μg/mL. Further, each compound was active against antibiotic-resistant Streptococcus pneumoniae [40].

Thiopeptide TP-1161
The thiopeptide TP-1161 (9) was identified in a Norwegian marine sediment-derived gram-positive bacterium, Nocardiopsis sp. [41]. The in vitro antibacterial activity of TP-1161 was assessed against a panel of gram-negative and gram-positive bacteria, mostly representing clinical isolates. All of the gram-negative strains tested (six E. coli, two Klebsiella pneumoniae, one Salmonella enterica serovar Choleraesuis, and four P. aeruginosa) were not susceptible to TP-1161. However, the MICs against gram-positive strains ranged from 0.25 to 4 μg/mL, which was comparable to or lower than those of the reference antibiotic vancomycin. TP-1161 also inhibited the growth of vancomycin-resistant bacterial strains, including Enterococcus faecalis 560 and Enterococcus faecium 569, with an MIC of 1 μg/mL.
Papillosin possesses potent activity against gram-positive and gram-negative bacteria. Halocyntin is active against all the gram-positive bacteria tested, but its activity against gram-negative bacteria is effective but much less potent. In addition, for all strains tested, papillosin present a higher antibacterial activity than halocyntin.

Hyastatin
Hyastatin (14) is a glycine-rich multi-domain peptide from hemocytes of the Norwegian spider crab Hyas araneus [44]. It consists of three distinctly different domains: an N-terminal region enriched in Gly residues, a short Pro/Arg-rich region, and a C-terminal region containing six Cys residues with a Cys pattern resembling the one found in penaeidins ( Figure 7). The hyastatin transcript is constitutively expressed, and is primarily found in hemocytes. Hyastatin was purified in its native form Hyas araneus hemocytes and its antimicrobial potential was estimated against selected bacteria and fungi. The MIC value was defined as 50% growth inhibition compared to the growth control, and the highest concentration tested was 50 μM. Hyastatin had moderate activity against E. coli (MIC = 12.5 μM), but showed no activity against P. aeruginosa. The activity against gram-positive bacteria was either very strong or absent, with MIC values of 0.4 μM and >50 μM against Corynebacterium glutamicum and S. aureus, respectively [44].  (14), isolated from the hemocytes of Hyas araneus [44]. The different regions are distinguished with boxes and given a designation below. The sequence of hyastatin has been submitted to the NCBI GenBank database with the accession number FJ764995.

Indigoidine
A blue pigment is associated with Phaeobacter sp. Y4I-mediated inhibition of Vibrio fischeri [45]. This pigment, referred to as indigoidine (15), is produced by the condensation of two glutamine residues via a nonribosomal peptide synthase-based biosynthetic pathway. The indigoidine biosynthetic genes are igiB, which codes for a 6-phosphogluconate dehydrogenase, igiC, for a glutamate racemase, and igiD for indigoidine synthase [94].

Unnarmicins
Unnarmicins are depsipeptides isolated from the fermentation broth of a marine bacterium Photobacterium sp. MBIC06485. Unnarmicins A (16) and C (17) contain two Leu, and two Phe. The difference between the two peptides was due to replacement of the 3-hydroxyoctanoyl moiety in A (16) with the 3-hydroxyhexanoyl group in C (17) [46]. These compounds exert antibacterial activity against only one species in the Pseudovibrio genus, a common α-Proteobacteria genera in the marine environment [46]. The antibacterial activity of unnarmicins against two gram-positive bacteria (Firmicutes) and seven halophilic gram-negative bacteria belonging to Bacteroidetes, Alphaproteobacteria, and Gammaproteobacteria was evaluated by a plate diffusion assay [46].

Ngercheumicins
Depsipeptides from Photobacterium strains that are active against the non-pathogenic Pseudovibrio denitrificans were patented in 2007, and called ngercheumicins [47]. To date, five depsipeptides, ngercheumicins A-E, have been purified and characterized. Ngercheumicins A (18) and B (19) have a depsipeptide macrocycle containing one Phe and two Leu residues with different fatty acid tails. Ngercheumicins C (20) and D (21) have a macrocycle composed of three Leu, two Thr, and one Ser, with no fatty acid tails [47]. Although these compounds only target bacteria for which no pathologies have been described to date, ngercheumicins can be added to culture media to enhance the proliferation of slow-growing marine bacteria.

Solonamides
Marine Photobacterium produce cyclodepsipeptides, called solonamides. Various depsipeptides have been identified in this genus, suggesting that this type of compound may be a common feature in Photobacterium [48]. No antibacterial activity has been observed for solonamides A (22) and B (23), although the activity has only been assessed against Vibrio anguillarum and S. aureus. Nevertheless, solonamide B (23) has been reported to reduce the expression of MRSA hla and rnaIII, two genes involved in strain virulence, controlled by an agr-dependent quorum sensing system. The structural similarity of the solonamides and the auto-inducing peptides involved in S. aureus quorum sensing suggests that solonamides may be competitive inhibitors of the agr system [48]. The reduced activity of solonamide A compared with solonamide B indicates that the overall hydrophobicity of the depsipeptides, due to the fatty acid chain, may have an important impact on inhibiting the expression of virulence genes in S. aureus.
Although the fermentation broth extracted with methanol inhibits several target bacteria, no inhibitory activity is observed when cyclo-peptides are used alone. However, employing a solid phase synthesis method, the cyclo-(Phe-Pro-Leu-Pro) has been shown to exert a significant inhibitory activity against gram-negative bacteria, including P. aeruginosa and Klebsiella oxytoca [95]. The biosynthetic pathways involved in the synthesis of cyclo-peptides have not been investigated. The structural analogy with other tetrapeptides, however, suggests that NRPS modules are at the origin of this peptide, as in Streptomyces [96].  [51]. Its precursor has a segmented organization comprising a signal peptide, an acidic proregion, and the C-terminal AMP. The 40-residue AMP is cationic, C-terminally amidated, and characterized by the presence of six cysteine molecules joined by three intramolecular disulfide bridges. Its cysteine array is common to another AMP and toxins from cnidarians [51]

Clavanins
Clavanins (28) are antimicrobial peptides isolated from the marine tunicate Styela clava, showing 23 amino acid residues in length, cationic properties, and also high bactericidal activity [52]. In spite of clear benefits from the use of peptides, currently 95% of peptide properties have limited pharmaceutical applicability, such as low solubility and short half-life in the circulatory system. Saude et al. was reported that nanostructured clavanin A form shows improved antimicrobial activity and has the potential to be used to treat polymicrobial infections [52]. In vitro bioassays showed that the nanostructured clavanin was partially able to control development of S. aureus, Klebsiella pneumoniae and P. aeruginosa. In vivo sepsis bioassays were performed using C57BL6 mice strain inoculated with a polymicrobial suspension. Assays led to 100% survival rate under sub-lethal sepsis assays and 40% under lethal sepsis assays in the presence of nanoformulated clavanin A until the seventh day of the experiment [52].

Cadiolides
Cadiolides C-F (29-32), butenolide metabolites (Figure 9), were isolated from the tunicate Pseudodistoma antinboja by activity-guided fractionations [53]. These compounds were evaluated for their antibacterial activity, and most of them exhibited moderate to significant activity that selectively targeted Gram-positive strains (S. aureus, S. epidermidis, Kocuria rhizophila and B. subtilis; MIC = 0.2-12.5 μg/mL). However, they did not show inhibitory activity toward the Gram-negative bacteria. Cadiolides C-F (29-32) was also showed antibacterial activities with methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) ranging from 0.13 to 4 μg/mL of MICs [53]. In addition, it should be noted that none of these compounds showed significant cytotoxicity in the MTT assay at 100 μM. Therefore, cadiolides could serve as new lead compounds for the development of antibiotics for the treatment of bacterial infections caused by Gram-positive bacteria, such as S. aureus [53]. Furthermore, at MIC, a 2-fold reduction in biofilm formation was observed, and at twice the MIC, 168-fold reduction occurred. At higher concentrations, it appears strongly active toward biofilms, which is uncommon for antibiotics; however, cytosporone B is cytotoxic toward A549 cells (TI90 = IC90 A549/MIC90 = 6) [54]. Cytosporone E (34) was equipotent against USA100 and methicillin-sensitive S. aureus (MSSA) strains (72 μM), indicating the intrinsic drug resistant properties of MRSA strains are not helpful in resisting the action of this cytosporone. In addition, at MIC, it resulted in >5000-fold reduction in bacterial viability, indicating it is strongly bactericidal, and not just bacteriostatic. The cytosporone E MBC90 is significantly below its MIC, further demonstrating developmental potential. Cytosporone E is also cytotoxic, but reasonably selective for bacteria relative to mammalian cells (TI90 = 10). Cytosporone E (34), however, displayed an IC90 of 13 μM, which represents significant selectivity (TI90 = 33) for a moderately potent antimalarial drug [54].

Antifungal Activity
Pathogenic fungi cause diseases in humans and other organisms. Candida species are important human pathogens that are best known for causing opportunistic infections in immunocompromised hosts (e.g., transplant, AIDS, and cancer patients). Aspergillus can cause disease through the production of mycotoxins through the induction of allergenic responses and through localized or systemic infections. Aspergillus flavus produces aflatoxin, which is a carcinogen. Cryptococcus neoformans can cause severe forms of meningitis and meningo-encephalitis in patients with HIV infection and AIDS [97]. Fungal infections in humans range in severity from easily treated superficial infections of the skin and hair to life threatening disseminated infections that involve multiple organ systems. Among fungal infections (mycoses), crytococcosis and candidiasis, whether systemic (affecting deeper tissues and organs) or superficial (affecting skin, nails, scalp or mucous membrane), are serious problems in public health. Although fungi have existed for millions of years, their role in causing infections in humans was not recognized until the 1830s to 1840s. Early treatment modalities [98] of human infections were based on the plant mycologists' experience, and included the use of toxic reagents, such as copper, mercury, iodide, Whitefield's ointment (a combination of benzoic and salicylic acids), selenium sulfide, castellani paint, and gentian violet [97]. Griseofulvin, amphotericin B, allyl amines, and azole antifungals were developed during the 1950s to 1980s. Azole antifungals are remarkable as a drug class for their broad-spectrum targeting, oral bioavailability, and lower toxicity, as compared to other antifungals, such as polyenes (e.g., nystatin) and pyrimidines (e.g., 5-flucytosine). Nevertheless, new compounds being developed as resistance to azoles is slowly emerging, particularly in C. albicans. Diverse protein classes have emerged as novel, potent natural therapeutics. Prominent among metabolites with wide bioactivities in recent years are cyclic peptides, especially members of echinocandin family [99]. This suggests that, in principle, new antifungal agents based on peptidic structures are developing. From 2006 to the present, 18 studies reported antifungal marine peptides isolated from a diverse group of marine bacteria, ascidians, sponges and mollusk.

Halocidin
Jang et al. found that a synthetic analog of halocidin (36), a previously reported antimicrobial peptide isolated from the hemocytes of a marine ascidian, had potent antifungal activity (MIC = 1-4 μg/mL) [56]. The synthetic Di-K19Hc peptide was shown to bind C. albicans rapidly (30 s) via an interaction with β-1,3-glucan, a component of the fungal cell wall, and concomitantly inducing ion channel formation, K + efflux, and death of the fungal cell. Halocidin is composed of two subunits, containing 18 and 15 amino acid residues joined by a single disulfide bond ( Figure 12). An antimicrobial assay performed with synthetic congeners of halocidin containing the 18-residue monomer, showed that this subunit is more active than the heterodimer or the 15-residue monomer against MRSA and multi-drug resistant P. aeruginosa [100].  (36). Halocidin were identified from the hemocytes of Hyas araneus [56]. Vertical bars in the sequence indicate a disulfide bond between two cysteine residues. The asterisk (*) denotes C-terminal amidation.

Callipeltins J and K
Callipeltins are a group of marine peptides with unusual structural features and remarkable biological properties, isolated from the sponges Callipelta sp. [57]. From a structural point of view, the most distinctive feature of callipeltins is the presence of several non-proteinogenic units. Callipeltins K (37) and J (38) inhibit the growth of C. albicans ATCC 24433 in a standard disk assay with MIC value of 10 −4 M. The antibiotic mixture selectively inhibited the growth of fungus, among which C. albicans (MIC = 1 μM) was the most affected [57].

Pedein A
The cyclopeptide pedein A (39) is an antifungal peptides from the myxobacterium Chondromyces pediculatus. These cyclic peptides are notably composed of (6-chloro)-Trp, Gly, and several unusual amino acids. Pedein A inhibits many yeasts and fungi, especially C. albicans and Rhizopus arrhizus. (MIC = 0.6-1.6 μg/mL) [58]. Hopefully, future studies will evaluate the mechanism of action of these marine compounds.

Theonellamide
Nishimura and colleagues evaluated the pharmacology of the bicyclic antifungal dodecapeptide theonellamide F (40), previously isolated from a sponge Theonella sp. [59]. Chemical-genomic profiling analysis, together with detailed subcellular localization studies, determined that the antifungal theonellamides represent a new class of sterol-binding molecules that induce membrane damage and activate Rho1-mediated 1,3-β-D-glucan synthesis.
Theonellamide G (41) showed potent antifungal activity towards wild and amphotericin B-resistant strains of C. albicans, with IC50s of 4.49 and 2.0 μM, respectively. Additionally, it displayed cytotoxic activity against the human colon adenocarcinoma cell line (HCT-16) with an IC50 of 6.0 μM. These findings provide further insight into the chemical diversity and biological activities of this class of compounds [60].

Anti-CA Cyclic Lipopeptide
Anti-CA (45) from the bacterium B. amyloliquefaciens, isolated from a mangrove system, manifested fungicidal activity against a clinical strain of C. albicans. The main bioactive substance was a cyclic lipopeptide containing a heptapeptide, Asp-Leu-Leu-Val-Val-Glu-Leu, and a 3-OH fatty acid with 15 carbon atoms. The lipopeptide also killed other yeast strains including C. tropicalis, Metschnikowia bicuspidata, Saccharomyces cerevisiae, and Yarrowia lipolytica [63].  (7), and an anteiso-type of saturated β-fatty acid side chain. These lipopeptides displayed dose-dependent antifungal activity against a broad spectra of phytopathogens, as well as being weakly antagonistic to S. aureus. Moreover, they all displayed cytotoxic activities against a human leukemia cell line (HL-60) with IC50 values of 100, 100, and 1.6 μM, respectively [65].

Kahalalides
Another well-characterized, unusual group of peptides are the kahalalides, a family of depsipeptides with variable size and peptide series, ranging from C31 (tripeptide) to C77 (tridecapeptide) and carrying different fatty acid chains [66]. Hill et al. suggested that the Hawaiian sea slug Elysia rufescens acquires kahalalide-producing microbes from the surface of Bryopsis, and then retains these microbes as symbionts. Kahalalides, especially kahalalide F (49), are active against several fungi, such as C. albicans (IC50 = 3.02 μM), C. neoformans (IC50 = 1.53 μM), Mycobacterium intracellulare, and A. fumigates (IC50 = 3.21 μM) [102]. Their chemical synthesis has made it possible to characterize structure-activity relationships, resulting in the synthesis of derivatives with increased biological activity.

Callyaerin
Callyaerins are cytotoxic cyclic peptides from the Indonesian marine sponge Callyspongia aerizusa [69]. Callyaerin A (52) exhibited strong inhibitory properties towards C. albicans and moderate activity against gram-negative E. coli, while it was only mildly active or inactive against gram-positive S. aureus and B. subtilis, respectively. Callyaerin E (53) displayed strong antimicrobial activity towards C. albicans and B. subtilis, but was only mildly active against the two remaining bacterial strains [69].

Anti-Malarial Activity
Malaria is the most threatening parasitic infection in humans. Each year, between 300 and 500 million new clinical cases are reported by the World Health Organization [103], resulting in annual deaths of approximately one million people [104], of which 75% are children below five years of age [105]. The most exposed continent is Africa; however, Southeast Asia, Oceania, and Central and South America are also under severe threat. Among the most affected are developing nations, which suffer from health care systems that lack the necessary infrastructure to cope with such diseases.
There are four species of Plasmodium infecting humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae, but most malaria cases are caused by the protozoan parasite P. falciparum [106]. The parasite is transmitted by Anopheles mosquitos. Therefore, malaria can be controlled, to some extent, by conventional prevention strategies, such as mosquito repellents, mosquito traps, insecticides, or biological control. The introduction of efficient vaccines is still problematic, despite recent progress in this area [107]. As a direct consequence, the treatment of choice remains parasite chemotherapy with small molecule drugs, both of natural origin or synthetic [108,109]. In parallel, resistance to commonly used anti-malarial therapeutics is growing. Thus, there is an urgent need to develop new drugs against this disease. Moreover, combination treatment should be explored to reduce the risk of resistance. For example, combinations of artemisinin-amodiaquine and the artemether-lumefantine (Coartem) have been used. Contributing to the global search for novel antimalarial peptides, and as presented in Table 1, eleven novel marine peptides were shown to possess antimalarial activity between 2006 and the present.

Aerucyclamides
Four new haxacyclopeptides, aerucyclamides A-D (57-60), were isolated from the cyanobacterium Microcystis aeruginosa PCC 7806, and their structural characterization, synthesis, and biological activity have been reported [72,73]. All four aerucyclamides were evaluated for anti-plasmodial activity. The most active compound was aerucyclamide B (58), which displayed a submicromolar IC50 value of 0.7 μM against the chloroquine-resistant Plasmodium falciparum strain K1. In addition, this compound displays a large selectivity for the parasite in L6 rat myoblasts, with an IC50 value of 120 μM. Interestingly, a reduction of the thiazole to a thiazoline (structural modification from A to B) decreases the antiplasmodial activity by 1 order of magnitude. Similar aerucyclamides C (59) and D (60) (IC50 = 2.3 and 6.3 μM, respectively) had low micromolar activities.

Antiprotozoal Activity
Seventeen marine peptides were reported to possess antimicrobial activity against other protozoa, thus contributing to the ongoing global search for novel agents to treated so-called neglected diseases, including Leishmaniasis (caused by several species of the genus Leishmania), amoebiasis, trichomoniasis, African sleeping sickness (caused by Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense) and American sleeping sickness or Chagas disease (caused by Trypanosoma cruzi).

Viridamides
An investigation of the marine cyanobacterium Oscillatoria nigro-viridis from Panama led to the isolation of two novel bioactive lipopeptides, viridamides A (65) and B (66) [77]. Viridamide A was tested against a series of relevant tropical pathogens and cancer cell lines, and displayed significant activity against the parasitic protozoan Plasmodium falciparum (IC50 = 5.8 μM). Further, viridamide A also exhibited activity against Leishmania mexicana (IC50 = 1.1 μM) and Trypanosoma cruzi (IC50 = 1.5 μM).

Antituberculosis Activity
Antituberculosis agents are drugs used to treat tuberculosis, an infectious disease caused by Mycobacterium tuberculosis. This infection mainly affects the lungs, but can also affect many other organ systems. Many classes of drugs with different mechanism of action have activity against M. tuberculosis. As shown in Table 1, one paper (three compounds) reported work regarding anti-tuberculosis pharmacology with marine peptides from 2006 to the present.

Trichoderins
Three new aminolipopeptides, trichoderins A (82), A1 (83), and B (84), were isolated from a culture of the marine sponge-derived fungus Trichoderma sp. as anti-mycobacterial substances with activity against active and dormant bacilli [106]. Trichoderins showed potent antimycobacterial activity against Mycobacterium smegmatis, Mycobacterium bovis, and Mycobacterium tuberculosis under standard aerobic growth conditions, as well as dormancy-inducing hypoxic conditions, with MIC values in the range of 0.02-2 μg/mL [81].

Anti-Viral Activity
Influenza continues to be a danger to human and animal health. The ability of the virus to undergo rapid mutation means that antigenic variants constantly arise, breeching the host-adaptive immune response. Pathogenic variants may arise at any time, and through reassortment, can generate new subtypes with the potential to cause pandemics. Approaches to broaden the availability of novel antiviral compounds include the development of synthetic peptides that disrupt the entry of the virus into cells. Antiviral peptides have been developed against hepatitis C [110], West Nile virus [111], respiratory syncytial virus [112], human immunodeficiency virus (HIV) [113], and the influenza virus [114]. The clinical application of the peptide Enfuvirtide (FUZEON), which blocks the entry of HIV into cells, points to the success of peptide-based therapies in a clinical setting. Other key therapeutic peptides include bivalirudin (Angiomax), a platelet aggregation inhibitor peptide, and abarelix (Plenaxis), a gonadotropin-releasing hormone (GnRH) antagonist. These are typical of the new generation of peptides with excellent therapeutic efficacy.
As shown in Table 1, thirty reports were published from 2006 to the present on the antiviral pharmacology of novel marine peptides against human cytomegalovirus and herpes simplex virus. Three articles reported preclinical pharmacology of marine compounds active against the human immunodeficiency virus type-1 (HIV-1), the causative agent of the acquired immunodeficiency disease syndrome (AIDS).
Four other homogenous depsipeptides, mirabamides E-H (88-91), from the sponge Stelletta clavosa also showed strong inhibition of HIV-1 in a neutralization assay using the viral strain YU2-V3 with IC50 values of 121, 62, 68, and 41 nM, respectively [115]. The primary feature distinguishing mirabamides E-H from mirabamides A, C, and D was the presence of 2-amino-2-butenoic acid in place of threonine.

Mollamide B
Mollamide B (92) is a cyclic hexapeptide isolated from the Indonesian tunicate Didemnum molle. Mollamide B exhibited moderate antimalarial activity against P. falciparum (D6 clone and W2 clone), with IC50 values of 2.0 and 2.1 μg/mL, respectively. Further, mollamide B also exhibited marginal activity against Leishmania donovani, with IC50 and IC90 values of 18 and 35 μg/mL, respectively, and against HIV-1 in human PBM cells with an EC50 value of 48.7 μM [83].
Mollamide B was examined for antimicrobial activity against MRSA, Mycobacterium intracellulaire, C. albicans, C. glabrata, C. krusei, and C. neoformans, and exhibited no activity. Mollamide B has shown no anti-inflammatory activity in rat neonatal microglia, and did not inhibit cyclooxygenase-2 (COX-2) activity in a cell-based assay [116].

Papuamides
Papuamides are a class of marine sponge-derived cyclic depsipeptides, which are thought to have cytoprotective activity against HIV in vitro through the inhibition of viral entry [84]. Papuamide A (93) is a novel marine compound isolated from the sponge Theonella sp. Papuamide A was tested in a fusion assay and against the HCT-116 cell line, with IC50 values of 73 nM and 3.5 μM. The anti-HIV activity of papuamide was shown to occur through a membrane-targeting mechanism, in which the hydrophobic tail of the molecule inserts into the viral membrane and the tyrosine residue interacts with cholesterol [84].

Celebesides A and C
Plaza et al. isolated several new cyclic depsipeptides from the Indonesian marine sponge Siliquariaspongia mirabilis, including celebesides A (94) and C (95), which inhibited HIV-1 in an infectivity assay (IC50 = 1.9 ± 0.4 μg/mL) [61]. Celebeside A was active against both HIV-1 (IC50 = 0.002 μM) and HCT 116 cancer cells (IC50 = 0.009 μM) [61]. The lipid component of celebeside A is of mixed origin. Celebesides A and C were tested against HCT-116 cells, giving IC50 values of 9.9 and >31 μM, respectively. The ability to inhibit HIV-1 entry was also evaluated for both celebeside A and C, with IC50 values of 2.1, and >62 μM, respectively. Interestingly, for both celebesides A and C in biological assays, loss of activity was correlated with the loss of the phosphate group.
Plaza et al. isolated cyclic peptides, theopapuamides B (42), C (43), and D (97), from an extract of Siliquariaspongia mirabilis collected off Sulawesi Island, Indonesia [61]. Theopapuamides B (41) and C (42) were tested against HCT-116 cells giving IC50 values of 2.5 and 1.3 μM, respectively. The ability to inhibit HIV-1 entry was also evaluated for theopapuamides B (42), with IC50 values of 0.5 μM. Theopapuamides A (96), B (42), and C (43) were evaluated for their ability to inhibit the growth of both wild type and amphotericin B-resistant strains of C. albicans. Theopapuamide A inhibited the growth of both strains with zones of inhibition of 8 mm at 1 μg/disk, while theopapuamides B and C displayed zones of 10 mm against both strains at 5 μg/disk [61].
3.6.7. Homophymine A Homophymines A-E (99-103) and A1-E1 (104-108) are a series of cyclodepsipeptides isolated from Homophymia sp. collected from shallow waters off the east coast of New Caledonia [87,88]. The anti-viral properties of Homophymines A were tested in an assay with peripheral blood mononuclear cells (PBMC) infected with the III B strain of HIV-1. Homophymine A had cytoprotective properties through the inhibition of infection production, with an IC50 value of 75 nM. Homophymine A was cytotoxic against uninfected PBMC cells with an IC50 of 1.19 μM, but it was almost sixteen times more effective against infected cells. Homophymines A-E and A1-E1 were evaluated against a panel of cell lines including human cancer and the Vero green monkey kidney cell lines. Homophymines A-E and A1-E1 exhibited potent cytotoxicity with IC50 values ranging from 2 to 100 nM. They were the most potent in the PC3 human prostate adenocarcinoma and the SK-OV3 human ovarian adenocarcinoma cell lines [87].

Conclusions
Although marine peptides are largely unexplored, they have inherent activity and an ability to inhibit infection. Marine peptides are structurally diverse, have a wide spectrum of therapeutic action, a low bio-deposition rate in body tissues, and are highly specific to targets. Marine derived peptides also possess reduced risk of unwanted adverse side-effects. As marine peptides are composed of metabolically tolerable amino acids, they are generally safe and non-toxic. In addition to their use as active ingredients, marine peptides also have the ability to be used as excipients in drug formulations for the modification of biological activity, targeted delivery, or transport across cellular membranes. There are also a number of issues that have to be considered from the earliest stages of development when a marine chemical entity is selected for clinical development, including, the effects, sourcing, technological and scientific difficulties, and legal uncertainties. Finding new anti-infective peptides in marine living resources, particularly those living in deep-sea and special marine environments, is an important approach to identify novel active agents. Moreover, screening anti-infective activities in the existing marine peptide library, as well as semi-synthetic modifications of anti-infective agents, can be used to identify novel compounds. Extensive studies of anti-infective peptides will contribute to the generation of novel pharmaceutical products. Thus, marine peptides are a valuable source of bioactive compounds, which could be introduced for development in pharmaceutical industries.