As the search for new antibiotics continues, the resistance to known antimicrobial compounds continues to increase. Many researchers around the world, in response to antibiotics resistance, have continued to search for new antimicrobial compounds in different ecological niches such as the marine environment. Marine habitats are one of the known and promising sources for bioactive compounds with antimicrobial potentials against currently drug-resistant strains of pathogenic microorganisms. For more than a decade, numerous antimicrobial compounds have been discovered from marine environments, with many more antimicrobials still being discovered every year. So far, only very few compounds are in preclinical and clinical trials. Research in marine natural products has resulted in the isolation and identification of numerous diverse and novel chemical compounds with potency against even drug-resistant pathogens. Some of these compounds, which mainly came from marine bacteria and fungi, have been classified into alkaloids, lactones, phenols, quinones, tannins, terpenes, glycosides, halogenated, polyketides, xanthones, macrocycles, peptides, and fatty acids. All these are geared towards discovering and isolating unique compounds with therapeutic potential, especially against multidrug-resistant pathogenic microorganisms. In this review, we tried to summarize published articles from 2015 to 2019 on antimicrobial compounds isolated from marine sources, including some of their chemical structures and tests performed against drug-resistant pathogens.

Background: Malaria and neglected communicable protozoa parasitic diseases, such as leishmaniasis, and trypanosomiasis, are among the otherwise called diseases for neglected communities, which are habitual in underprivileged populations in developing tropical and subtropical regions of Africa, Asia, and the Americas. Some of the currently available therapeutic drugs have some limitations such as toxicity and questionable efficacy and long treatment period, which have encouraged resistance. These have prompted many researchers to focus on finding new drugs that are safe, effective, and affordable from marine environments. The aim of this review was to show the diversity, structural scaffolds, in-vitro or in-vivo efficacy, and recent progress made in the discovery/isolation of marine natural products (MNPs) with potent bioactivity against malaria, leishmaniasis, and trypanosomiasis. Main text: We searched PubMed and Google scholar using Boolean Operators (AND, OR, and NOT) and the combination of related terms for articles on marine natural products (MNPs) discovery published only in English language from January 2016 to June 2020. Twenty nine articles reported the isolation, identification and antiparasitic activity of the isolated compounds from marine environment. A total of 125 compounds were reported to have been isolated, out of which 45 were newly isolated compounds. These compounds were all isolated from bacteria, a fungus, sponges, algae, a bryozoan, cnidarians and soft corals. In recent years, great progress is being made on anti-malarial drug discovery from marine organisms with the isolation of these potent compounds. Comparably, some of these promising antikinetoplastid MNPs have potency better or similar to conventional drugs and could be developed as both antileishmanial and antitrypanosomal drugs. However, very few of these MNPs have a pharmaceutical destiny due to lack of the following: sustainable production of the bioactive compounds, standard efficient screening methods, knowledge of the mechanism of action, partnerships between researchers and pharmaceutical industries. Conclusions: It is crystal clear that marine organisms are a rich source of antiparasitic compounds, such as alkaloids, terpenoids, peptides, polyketides, terpene, coumarins, steroids, fatty acid derivatives, and lactones. The current and future technological innovation in natural products drug discovery will bolster the drug armamentarium for malaria and neglected tropical diseases.

Four new prenylxanthones (1–4) together with five known compounds (5–9) were isolated from the solid-substrate fermentation culture of Emericella sp. XL029 associated with the leaves of Panax notoginseng. Their structures and relative configurations were elucidated by extensive spectroscopic analysis and comparison of NMR data to those of known compounds. The anti-agricultural pathogenic fungal and antibacterial activities of all isolated compounds were evaluated against eight agricultural pathogenic fungi and nine human pathogenic bacteria. The anti-agricultural pathogenic fungal assay indicated that compounds 1, 2, 4, 8 and 9 displayed significant activity against Drechslera maydis with MIC value of 25 μg mL⁻¹. Moreover, compound 1 also exhibited significant activity against other three fungi including Rhizoctonia cerealis, Fusarium oxysporum and Physalospora piricola, and compounds 4 and 9 against Rhizoctonia cerealis with MIC value of 25 μg mL⁻¹. Compound 5 only demonstrated significant activity against Physalospora piricola with MIC value of 25 μg mL⁻¹. Further antibacterial activity tests revealed that compounds 1–9 showed significant antibacterial activity against all tested Gram-positive (except for drug resistant Staphylococcus aureus) and Gram-negative bacteria with MIC values ranging from 12.5 to 50 μg mL⁻¹. While only compounds 1, 4 and 9 displayed moderate activity against drug resistant Staphylococcus aureus with MIC value of 50 μg mL⁻¹.

Three new 3,4,6-trisubstituted α-pyrone derivatives, namely 6-(2′R-hydroxy-3′E,5′E-diene-1′-heptyl)-4-hydroxy-3-methyl-2H-pyran-2-one (1), 6-(2′S-hydroxy-5′E-ene-1′-heptyl)-4-hydroxy-3-methyl-2H-pyran-2-one (2), and 6-(2′S-hydroxy-1′-heptyl)-4 -hydroxy-3-methyl-2H-pyran-2-one (3), together with one known compound trichodermic acid (4), were isolated from the solid-substrate fermentation culture of Penicillium ochrochloronthe associated the roots of Taxus media. Compounds 1–4 displayed the antimicrobial activity selectively against tested fungal and bacterial strains with minimum inhibitory concentration (MIC) values ranging from 12.5 to 100 μg/ml. Furthermore, we found that only compound 4 exhibited moderate cytotoxicity against five human cancer cells (A549, LN229, MGC, LOVO, and MDA231) with IC₅₀ values of 51.45, 23.43, 39.16, 46.97, and 42.85 μg/ml, respectively.

In order to discover a broad-specificity and high stability chitinase, a marine fungus, Aspergillus fumigatus df347, was identified in the sediments of mangrove wetlands in Qinzhou Bay, China. The chitinase gene (AfChi28) from A. fumigatus df347 was cloned and heterologously expressed in Escherichia coli, and the recombinant enzyme AfChi28 was purified and characterized. AfChi28 is an acido-halotolerant- and temperature-resistant bifunctional enzyme with both endo- and exo-cleavage functions. Its enzymatic products are mainly GlcNAc, (GlcNAc)₂, (GlcNAc)₃ and (GlcNAc)₄. Na⁺, Mg²⁺, K⁺, Ca²⁺ and Tris at a concentration of 50 mM had a strong stimulatory effect on AfChi28. The crude enzyme and pure enzyme exhibited the highest specific activity of 0.737 mU/mg and 52.414 mU/mg towards colloidal chitin. The DxDxE motif at the end of strand β5 and with Glu154 as the catalytic residue was verified by the AlphaFold2 prediction and sequence alignment of homologous proteins. Moreover, the results of molecular docking showed that molecular modeling of chitohexaose was shown to bind to AfChi28 in subsites −4 to +2 in the deep groove substrate-binding pocket. This study demonstrates that AfChi28 is a promising chitinase for the preparation of desirable chitin oligosaccharides, and provides a foundation for elucidating the catalytic mechanism of chitinases from marine fungi.