Polyketide family of novel antibacterial 7-O-methyl-5'-hydroxy-3'-heptenoate-macrolactin from seaweed-associated Bacillus subtilis MTCC 10403

Natural Products and Pharmacological Properties of Symbiotic Bacillota (Firmicutes) of Marine Macroalgae

The shift from the terrestrial to the marine environment to discover natural products has given rise to novel bioactive compounds, some of which have been approved for human medicine. However, the ocean, which makes up nearly three-quarters of the Earth's surface, contains macro- and microorganisms whose natural products are yet to be explored. Among these underexplored marine organisms are macroalgae and their symbiotic microbes, such as Bacillota, a phylum of mostly Gram-positive bacteria previously known as Firmicutes. Macroalgae-associated Bacillota often produce chemical compounds that protect them and their hosts from competitive and harmful rivals. Here, we summarised the natural products made by macroalgae-associated Bacillota and their pharmacological properties. We discovered that these Bacillota are efficient producers of novel biologically active molecules. However, only a few macroalgae had been investigated for chemical constituents of their Bacillota: nine brown, five red and one green algae. Thus, Bacillota, especially from the marine habitat, should be investigated for potential pharmaceutical leads. Moreover, additional diverse biological assays for the isolated molecules of macroalgae Bacillota should be implemented to expand their bioactivity profiles, as only antibacterial properties were tested for most compounds.

Anti-Vibrio potential of natural products from marine microorganisms

The emergence of drug-resistant Vibrio poses a serious threat to aquaculture and human health, thus there is an urgent need for the discovery of new related antibiotics. Given that marine microorganisms (MMs) are evidenced as important sources of antibacterial natural products (NPs), great attention has been gained to the exploration of potential anti-Vibrio agents from MMs. This review summarizes the occurrence, structural diversity, and biological activities of 214 anti-Vibrio NPs isolated from MMs (from 1999 to July 2022), including 108 new compounds. They were predominantly originated from marine fungi (63%) and bacteria (30%) with great structural diversity, including polyketides, nitrogenous compounds, terpenoids, and steroids, among which polyketides account for nearly half (51%) of them. This review will shed light on the development of MMs derived NPs as potential anti-Vibrio lead compounds with promising applications in agriculture and human health.

Macrolactin a mediated biocontrol of Fusarium oxysporum and Rhizoctonia solani infestation on Amaranthus hypochondriacus by Bacillus subtilis BS-58

Plant diseases are one of the main hurdles for successful crop production and sustainable agriculture development world-wide. Though several chemical measures are available to manage crop diseases, many of them have serious side effects on humans, animals and the environment. Therefore, the use of such chemicals must be limited by using effective and eco-friendly alternatives. In view of the same, we found a Bacillus subtilis BS-58 as a good antagonist towards the two most devastating phytopathogens, i.e., Fusarium oxysporum and Rhizoctonia solani. Both the pathogens attack several agricultural crops (including amaranth) and induce a variety of infections in them. The findings of scanning electron microscopy (SEM) in this study suggested that B. subtilis BS-58 could inhibit the growth of both the pathogenic fungi by various means such as perforation, cell wall lysis, and cytoplasmic disintegration in the fungal hyphae. Thin-layer chromatography, LC–MS and FT-IR data revealed the antifungal metabolite to be macrolactin A with a molecular weight of 402 Da. Presence of the mln gene in the bacterial genome further endorsed that the antifungal metabolite produced by BS-58 was macrolactin A. Pot trial conducted in the present study showed that seed treatment by BS-58 effectively reduced seedling mortality (54.00 and 43.76%) in amaranth, when grown in pathogen infested soil (F. oxysporum and R. solani, respectively), when compared to their respective negative controls. Data also revealed that the disease suppression ability of BS-58 was almost equivalent to the recommended fungicide, carbendazim. SEM analysis of roots of the seedlings recovered from pathogenic attack substantiated the hyphal disintegration by BS-58 and prevention of amaranth crop. The findings of this study conclude that macrolactin A produced by B. subtilis BS-58 is responsible for the inhibition of both the phytopathogens and the suppression of the diseases caused by them. Being native and target specific, such strains under suitable conditions, may result in ample production of antibiotic and better suppression of the disease.

Classification and Multifaceted Potential of Secondary Metabolites Produced by Bacillus subtilis Group: A Comprehensive Review

Despite their remarkable biosynthetic potential, Bacillus subtilis have been widely overlooked. However, their capability to withstand harsh conditions (extreme temperature, Ultraviolet (UV) and γ-radiation, and dehydration) and the promiscuous metabolites they synthesize have created increased commercial interest in them as a therapeutic agent, a food preservative, and a plant-pathogen control agent. Nevertheless, the commercial-scale availability of these metabolites is constrained due to challenges in their accessibility via synthesis and low fermentation yields. In the context of this rising in interest, we comprehensively visualized the antimicrobial peptides produced by B. subtilis and highlighted their prospective applications in various industries. Moreover, we proposed and classified these metabolites produced by the B. subtilis group based on their biosynthetic pathways and chemical structures. The biosynthetic pathway, bioactivity, and chemical structure are discussed in detail for each class. We believe that this review will spark a renewed interest in the often disregarded B. subtilis and its remarkable biosynthetic capabilities.

Seaweed-associated heterotrophic Bacillus altitudinis MTCC13046: a promising marine bacterium for use against human hepatocellular adenocarcinoma

Since the report of the antibiotic with anticancer properties, scientists have been focusing to isolate and characterize novel anti-microbial natural products possessing anticancer activities. The current study describes the production of seaweed-associated heterotrophic Bacillus altitudinis MTCC13046 with potential anticancer properties. The bacterium was screened for its capacity to diminish the cell proliferation of the human hepatocellular adenocarcinoma (HepG2) cell line, without upsetting the normal cells. The bacterial extract showed anticancer properties in a dose-reactive form against HepG2 (IC₅₀, half maximal inhibitory concentration ~ 29.5 µg/ml) on tetrazolium bromide analysis with less significant cytotoxicity on common fibroblast (HDF) cells (IC₅₀ ~ 77 µg/ml). The potential antioxidant ability of the organic extract of B. altitudinis MTCC13046 (IC₉₀ 133 µg/ml) could corroborate its capacity to attenuate the pathophysiology leading to carcinogenesis. The results of the apoptosis assay showed that the crude extracts of B. altitudinis maintained 68% viability in normal cells compared to 11% in the cancer cells (IC₅₀ 76.9 µg/ml). According to the findings, B. altitudinis MTCC13046 could be used to develop prospective anticancer agents.

Pharmacological potential of seaweed-associated heterotrophic bacterium Bacillus atrophaeus

Extremities in marine environmental conditions led the marine macroalga-associated bacteria to adapt and biosynthesize potential bioactive agents. The myriad of marine macroalgae and the bacterial flora they are associated with constitute a potential source of bioactive components with significant biotechnological and pharmacological applications. Heterotrophic bacteria associated with the intertidal macroalgae were isolated and assessed for their pharmacological properties. Subsequently, Firmicutes dominated more than half of the 152 cultivable isolates from macroalgae-associated bacteria collected from the Gulf of Mannar (9°17'0'' N, 79°7'0'' E), on Peninsular India's southern coast. A total of 43 of those demonstrated steady antibacterial activities against a wide range of nosocomial pathogens. Among the bacteria isolated from marine macroalgae, Bacillus atrophaeus SHB2097 (MW821482) exhibited significant antimicrobial activities against clinically important pathogens. Organic extract of B. atrophaeus SHB2097 showed potential antimicrobial activities against test pathogens (minimum inhibitory concentration 6.25 µg/mL). Organic extract of B. atrophaeus SHB2097 revealed promising inhibition potential against cyclooxygenase-2 (IC₉₀ 53.26 µg/mL) and 5-lipoxygenase (IC₉₀ 9.74 µg/mL). The carbolytic enzyme α-glucosidase inhibition potential of the organic extract of the studied heterotrophic bacterium was significantly greater than (IC₉₀ 118 µg/mL) than that displayed by acarbose (IC₉₀ 645 µg/mL, p < 0.05). The significance of nuclear magnetic resonance-centered analyses of distinguishing signals in the organic extract and correlating those with bioactive potential was accentuated. The utilities of nuclear magnetic resonance-based fingerprinting emphasized the assessment of the distinctive signals in the solvent extracts and their correlation with the pharmacological properties. Thus, the heterotrophic B. atrophaeus SHB2097 could be used to develop potential therapeutic and biomedical agents.

Antibacterial and wound healing potential of topical formulation of marine symbiotic Bacillus

The inevitability to develop novel antimicrobial agents has considerably increased because of mounting alarms concerning multidrug-resistant microbial strains. The present study evaluated an antibacterial and wound healing topical formulation prepared with the ethyl acetate extract of marine symbiotic Bacillus amyloliquefaciens MTCC 12716 as the basic ingredient and the grafted macroalgal polysaccharide as the gel base with an appropriate proportion of natural stabilizing agents. The formulation exhibited potent antibacterial activities against clinical isolates of Staphylococcus aureus (18 mm inhibition zone) and Pseudomonas aeruginosa (19 mm) causing infection when compared with commercially available antimicrobial cream clindamycin. The in-vitro results indicated that the organic extract of B. amyloliquefaciens MTCC 12716 at its MIC and the formulation sealed the wound by 78 and 94%, respectively, at 48 h in the scratch-induced L929 cells, compared to 84% exhibited by clindamycin. The topical formulation of marine symbiotic Bacillus induced greater than 80% viability of the normal fibroblasts compared to 78% exhibited by clindamycin, when administered at a dose of 25 μg mL^-1. The studied antibacterial formulation could accelerate the wound healing by prompting the migration of fibroblasts towards the artificially created wound resulting in rapid wound closure, and at an even higher concentration of formulation, it displayed no cytotoxicity on L929 cells. The stability studies showed that the formulation maintained its physicochemical characteristics and minimal growth (<10 cfu g^-1) of bacteria on the plates throughout the time period of 18 months at 30 °C and 65% relative humidity. This study has established the antibacterial and wound healing potential of a topical formulation of marine symbiotic B. amyloliquefaciens.

Secondary Metabolites from Marine-Derived Bacillus: A Comprehensive Review of Origins, Structures, and Bioactivities

The marine is a highly complex ecosystem including various microorganisms. Bacillus species is a predominant microbialflora widely distributed in marine ecosystems. This review aims to provide a systematic summary of the newly reported metabolites produced by marine-derived Bacillus species over recent years covering the literature from 2014 to 2021. It describes the structural diversity and biological activities of the reported compounds. Herein, a total of 87 newly reported metabolites are included in this article, among which 49 compounds originated from marine sediments, indicating that marine sediments are majority sources of productive strains of Bacillus species Therefore, marine-derived Bacillus species are a potentially promising source for the discovery of new metabolites.

Marine Based Natural Products: Exploring the Recent Developments in the Identification of Antimicrobial Agents

The marine ecosystem is the less explored, biologically diverse, and vastest resource to discover novel antimicrobial agents. In recent decades' antimicrobial drugs are losing their effectiveness due to the growing resistance among pathogens, which causes diseases to have considerable death rates across the globe. Therefore, there is a need for the discovery of new antibacterials that can reach the market. There is a gradual growth of compounds from marine sources which are entering the clinical trials. Thus, the prominence of marine natural products in the field of drug design and discovery across the academia and pharmaceutical industry is gaining attention. Herein,  the present review covers nearly 200 marine based antimicrobial agents of 11 structural classes discovered from the year 2010 to 2022. All the discussed compounds have exhibited medium to high antimicrobial activity in inhibiting various microorganisms.

Pharmacological potential of seaweed-associated heterotrophic Firmicutes

Seaweed-associated bacterial symbionts are sources of potential pharmacological properties. The present study resulted in the culture-dependent isolation of bioactive heterotrophs belonging to the bacterial phylum Firmicutes, which were dominated more than 30% of the 127 cultivable isolates, among which 23 of them showed potential antimicrobial activities against a wide range of pathogens. The symbionts isolated from the seaweed Sargassum wightii showed significant bioactivity. Those were characterised as Bacillus safensis MTCC13040, B. valismortis MTCC13041, B. velezensis MTCC13044, B. methylotrophicus MTCC13042, Oceanobacillus profundus MTCC13045, B. tequilensis MTCC13043, and B. altitudinis MTCC13046. The organic extracts of the studied isolates showed potential antimicrobial properties against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci (minimum inhibitory concentration 6.25-12.5 μg ml^-1 ). The organic extract of B. altitudinis MTCC13046 displayed significantly greater radical quenching ability (IC₉₀ 133 μg ml^-1 , p < 0.05) other than attenuating hydroxymethyl glutaryl coenzyme A reductase (IC₉₀ 10.21 μg ml^-1 , p < 0.05) and angiotensin converting enzyme-1 (IC₉₀ 498 μg ml^-1 , p < 0.05) relative to other studied heterotrophs. The organic extract of B. tequilensis MTCC13043 displayed significantly greater attenuation potential against pro-inflammatory 5-lipooxygenase (IC₉₀ 5.94 μg ml^-1 , p < 0.05) and dipeptidyl peptidase-4 (IC₉₀ 271 μg ml^-1 , p < 0.05). The seaweed-associated B. altitudinis MTCC13046 and B. tequilensis MTCC13043 could be used to develop promising pharmacological leads.

Seaweed-associated heterotrophic bacteria: are they future novel sources of antimicrobial agents against drug-resistant pathogens?

Emergence of multidrug-resistant microorganisms and requirements for novel antimicrobial compounds necessitate exploring newer habitats to develop potential bioactive leads. Culture-contingent analysis of heterotrophic bacterial flora from the seaweeds led to the isolation of bioactive strains possessing potential antibacterial properties against wide-ranging clinical pathogens viz., methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecalis (VREfs). Seven of the most active strains belonging to the phylum Firmicutes isolated from a brown seaweed (Phaeophyceae) Sargassum wightii exhibited spot-over-lawn assay guided inhibition zone of larger than 30 mm. Integrated phenotypic and genotypic studies have led to the characterization of the seaweed-associated bacteria particularly belonging to the phylum Firmicutes. The organic extracts of the studied bacteria exhibited promising antibacterial properties against MRSA and VREfs with minimum inhibitory concentration ranging between 6.25 and 12.50 μg/mL. Time-kill kinetic profiles of those bacteria displayed rapid bactericidal activity against both E. coli and MRSA, showing a ≥ 3log₁₀ reduction in viable cell count than the initial. Among the studied bioactive Bacillus spp, B. tequilensis MTCC13043 and B. altiitudinis MTCC13046 were found to possess functional polyketide synthase (pks) gene (MW027664 and MW027660) that could be amplified. The outcome of amplified genes encrypting for polyketide synthase in conjunction with antibacterial activities unveiled the broad-spectrum antimicrobial activities of the marine heterotrophic Firmicutes, which could be further used against the emergent problem of antibiotic-resistant bacterial pathogens.

Antibiotic-active heterotrophic Firmicutes sheltered in seaweeds: can they add new dimensions to future antimicrobial agents?

Appearance of drug-resistant microorganisms prompted researchers to unravel new environments for development of novel antimicrobial agents. Culture-supported analysis of heterotrophic bacteria associated with seaweeds yielded 152 strains, in that larger share of the isolates was embodied by Bacillus atrophaeus SHB2097 (54%), B. velezensis SHB2098 (24%), B. subtilis SHB2099 (12%), and B. amyloliquefaciens SHB20910 (10%). One of the most active strains characterized as B. atrophaeus SHB2097 (MW821482) with an inhibition zone more than 30 mm on spot-over-lawn experiment, was isolated from a seaweed Sargassum wightii, was selected for bioprospecting studies. Significant antibacterial potential was displayed by bacterial organic extract against vancomycin-resistant Enterococcus faecalis, Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus, and Klebsiella pneumonia with minimum inhibitory concentration 6.25 µg/mL and comparable to the antibiotics ampicillin and chloramphenicol. The genes of type 1 pks (MZ222383, 700 bp) and hybrid nrps/pks (MZ222389, 1000-1400 bp) of B. atrophaeus MW821482 could be amplified. The bacterium displayed susceptibility to the commercially available antibiotic agents, and was negative for the pore-forming non-hemolytic hemolysin BL (hbl) and enterotoxin (nhe) genes, and therefore, was not pathogenic. The bacterium was found to possess genes (1000-1400 bp) involved in the biosynthesis of siderophore-class of compounds (MZ222387 and MZ222388) that showed 99% of similarity in BLAST search, and showed production of siderophore. Noteworthy antibacterial activities against clinically important pathogenic bacteria in conjunction with occurrence of genes coding for antimicrobial metabolites inferred that the marine heterotrophic bacterium B. atrophaeus SHB2097 could be used for the development of antibacterial agents against the emerging antibiotic resistance.

Macrolactin Metabolite Production by Bacillus sp. ZJ318 Isolated from Marine Sediment

A total of 172 microbial strains were screened and isolated from Arctic Ocean marine sediments at a depth of 42 ~ 3,763 m. A microorganism with strong antibacterial activity against Staphylococcus aureus was identified as Bacillus sp. ZJ318 according to the results of 16S rDNA sequencing and phylogenetic tree analyses. Bioactivity-guided isolation of the new/novel metabolite in the ethyl acetate (EA) extract obtained from the fermentation broth of this strain was followed by chromatographic fractionation and subsequent HPLC purification, leading to the isolation of one known macrolactin. The chemical structure of the macrolactin, which indicated macrolactin J isolation from marine microorganisms for the first time, was assigned based on a high-resolution electrospray ionization mass spectrometer system (HR-EMI-MS), nuclear magnetic resonance (NMR) spectral analyses, and a literature review. To improve macrolactin J production, the corresponding effects of nitrogen sources were investigated, and (NH₄)₂SO₄ was determined to produce the best effect. In addition, the optimal culture conditions were determined by an orthogonal experiment. Under these conditions, the yield of macrolactin J was increased to 2.41 mg/L, which was 2.2 times the original yield. This work lays a foundation for follow-up mechanistic and application research on macrolactin J.

Polyketide-derived macrobrevins from marine macroalga-associated Bacillus amyloliquefaciens as promising antibacterial agents against pathogens causing nosocomial infections

Marine heterotrophs are treasured bio-resources of antimicrobial metabolites, and herein we report the biosynthetic potential of Bacillus amyloliquefaciens (ex. Fukumoto) Priest et al. (Bacillaceae) strain MTCC 12713 isolated from an intertidal macroalga Kappaphycus alvarezii (Doty) L.M.Liao (Rhodophyta: Solieriaceae). The bacterium showed promising biological activities against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis. Genome mining of B. amyloliquefaciens MTCC 12713 predicted the gene clusters coding for biosynthesis of antibacterial metabolites. Bioactivity-guided purification was directed to isolate four homologous members of trans-acyltransferase polyketide synthase-derived antibiotics, which were classified as macrobrevin analogues. The compounds exhibited antibacterial activities against nosocomial pathogens, for example, methicillin-resistant S. aureus, vancomycin-resistant E. faecalis, Klebsiella pneumoniae and Pseudomonas aeruginosa with a range of MIC values from 1.56 to 6.25 μg/mL, although standard antibiotic chloramphenicol was active at 6.25-12.5 μg/mL. Conspicuously, the macrobrevin compound encompassing hexahydro-41-hydroxy-macrobrevin-31-acetate functionality, displayed considerably greater antagonistic activities against methicillin-resistant S. aureus, vancomycin-resistant E. faecalis, Vibrio parahaemolyticus, P. aeruginosa, K. pneumoniae, and Streptococcus pyogenes (MIC 1.56 μg/mL) compared to the positive controls and other macrobrevin analogues. Trans-AT polyketide synthase-stimulated biosynthetic pathway of macrobrevin compounds, through repetitive decarboxylative Claisen condensation between acetyl-S-KS domain and malonate-S-ACP units could corroborate the structural elucidation. In the genome mining study, among the 34 biosynthetic gene clusters, a hybrid trans-acyltransferase (trans-AT) pks/nrps gene cluster, which extends up to ∼81 Kb, was recognized in the genome of B. amyloliquefaciens MTCC 12713. The pks/nrps cluster revealed 46% similarity to trans-AT PKS-derived macrobrevin isolated from a mesophilic bacterium Brevibacillus sp. Leaf182 associated with the phyllosphere of the wild-type genotype of Arabidopsis thaliana. The binding positions for macrobrevins with S. aureus peptide deformylase showed docking score of larger than 14 kcal/mol, which was considerably greater than macrolactin N and actinonin (<10 kcal/mol). These present findings documented that the marine heterotrophic B. amyloliquefaciens MTCC 12713 could be used to develop prospective antibacterial agents belonging to macrobrevin analogues for biotechnological and pharmaceutical applications.

Novel amylomacins from seaweed-associated Bacillus amyloliquefaciens as prospective antimicrobial leads attenuating resistant bacteria

The rise in antibiotic-resistant bacterial strains prompting nosocomial infections drives the search for new bioactive substances of promising antibacterial properties. The surfaces of seaweeds are rich in heterotrophic bacteria with prospective antimicrobial substances. This study aimed to isolate antibacterial leads from a seaweed-associated bacterium. Heterotrophic Bacillus amyloliquefaciens MTCC 12716 associated with the seaweed Hypnea valentiae, was isolated and screened for antimicrobial properties against drug-resistant pathogens. The bacterial crude extract was purified and three novel amicoumacin-class of isocoumarin analogues, 11'-butyl acetate amicoumacin C (amylomacin A), 4'-hydroxy-11'-methoxyethyl carboxylate amicoumacin C (amylomacin B) and 11'-butyl amicoumacin C (amylomacin C) were isolated to homogeneity. The studied amylomacins possessed potential activities against Pseudomonas aeruginosa, vancomycin-resistant Enterococcus faecalis, Klebsiella pneumoniae, methicillin-resistant Staphylococcus aureus, and Shigella flexneri with a range of minimum inhibitory concentration values from 0.78 to 3.12 µg/mL, although standard antibiotics ampicillin and chloramphenicol were active at 6.25-25 µg/mL. Noticeably, the amylomacin compound encompassing 4'-hydroxy-11'-methoxyethyl carboxylate amicoumacin C functionality (amylomacin B), displayed considerably greater antagonistic activities against methicillin-resistant S. aureus, vancomycin-resistant E. faecalis, Vibrio parahaemolyticus, Escherichia coli, and K. pneumoniae (minimum inhibitory concentration 0.78 μg/mL) compared to the positive controls and other amylomacin analogues. Antimicrobial properties of the amylomacins, coupled with the presence of polyketide synthase-I/non-ribosomal peptide synthetase hybrid gene attributed the bacterium as a promising source of antimicrobial compounds with pharmaceutical and biotechnological applications.

A Leap Forward Towards Unraveling Newer Anti-infective Agents from an Unconventional Source: a Draft Genome Sequence Illuminating the Future Promise of Marine Heterotrophic Bacillus sp. Against Drug-Resistant Pathogens

During the previous decade, genome-built researches on marine heterotrophic microorganisms displayed the chemical heterogeneity of natural product resources coupled with the efficacies of harnessing the genetic divergence in various strains. Herein, we describe the whole genome data of heterotrophic Bacillus amyloliquefaciens MB6 (MTCC 12,716), isolated from a marine macroalga Hypnea valentiae, a 4,107,511-bp circular chromosome comprising 186 contigs, with 4154 protein-coding DNA sequences and a coding ratio of 86%. Simultaneously, bioactivity-guided purification of the bacterial extract resulted in six polyketide classes of compounds with promising antibacterial activity. Draft genome sequence of B. amyloliquefaciens MB6 unveiled biosynthetic gene clusters (BGCs) engaged in the biosynthesis of polyketide-originated macrolactones with prospective antagonistic activity (MIC ≤ 5 µg/mL) against nosocomial pathogens. Genome analysis manifested 34 putative BGCs necessitated to synthesize biologically active polyketide-originated frameworks or their derivatives. These results provide insights into the genetic basis of heterotrophic B. amyloliquefaciens MTCC 12,716 as a prospective lead for biotechnological and pharmaceutical applications.

Marine Bacterial Secondary Metabolites: A Treasure House for Structurally Unique and Effective Antimicrobial Compounds

The prevalence of antimicrobial resistance reduces the effectiveness of antimicrobial drugs in preventing and treating infectious diseases caused by pathogenic organisms, such as bacteria, fungi, and viruses. Because of the burgeoning growth of microbes with antimicrobial-resistant traits, there is a dire need to identify and develop novel and effective antimicrobial agents to treat infections from antimicrobial-resistant strains. The marine environment is rich in ecological biodiversity and can be regarded as an untapped resource for prospecting novel bioactive compounds. Therefore, exploring the marine environment for antimicrobial agents plays a significant role in drug development and biomedical research. Several earlier scientific investigations have proven that bacterial diversity in the marine environment represents an emerging source of structurally unique and novel antimicrobial agents. There are several reports on marine bacterial secondary metabolites, and many are pharmacologically significant and have enormous promise for developing effective antimicrobial drugs to combat microbial infections in drug-resistant pathogens. In this review, we attempt to summarize published articles from the last twenty-five years (1996–2020) on antimicrobial secondary metabolites from marine bacteria evolved in marine environments, such as marine sediment, water, fauna, and flora.

Difficidin class of polyketide antibiotics from marine macroalga-associated Bacillus as promising antibacterial agents

A heterotrophoic Bacillus amyloliquefaciens MTCC12713 isolated from an intertidal macroalga Kappaphycus alverezii displayed promising antibacterial activities against multidrug-resistant bacteria. Genome mining of the bacterium predicted biosynthetic gene clusters coding for antibacterial secondary metabolites. Twenty-one membered macrocyclic lactones, identified as difficidin analogues bearing 6-hydroxy-8-propyl carboxylate, 9-methyl-19-propyl dicarboxylate, 6-methyl-9-propyl dicarboxylate-19-propanone, and (20-acetyl)-6-methyl-9-isopentyl dicarboxylate (compounds 1 through 4) functionalities were purified through bioassay-guided fractionation. The difficidin analogues exhibited bactericidal activities against vancomycin-resistant Enterococcus faecalis, methicillin-resistant Staphylococcus aureus, and other drug-resistant strains, such of Klebsiella pneumonia and Pseudomonas aeruginosa with the minimum inhibitory concentration of about 2-9 × 10^-3 μM. A plausible enzyme-catalyzed biosynthetic pathway that is generated through addition of acrylyl initiator unit by repetitive decarboxylative Claisen condensation modules with malonate units was recognized, and their structures were corroborated with gene organization of the dif operon, which could comprehend dif A-O (~ 70 kb). Drug-likeness score for 5-ethoxy-28-methyl-(9-methyl-19-propyl dicarboxylate) difficidin (compound 2, 0.35) was greater than those of other difficidin analogues, which corroborated the potential in vitro antibacterial properties of the former. The present study demonstrated the potential of difficidin analogues for pharmaceutical and biotechnological uses against the bottleneck of emergent drug-resistant pathogens. KEY POINTS: • Difficidins were isolated from marine alga associated Bacillus amyloliquefaciens. • Whole-genome mining of bacterial genome predicted biosynthetic gene clusters. • Greater drug-likeness for difficidin 2 confirmed its potent antibacterial activity.

Isolation and Purification of a New Bacillus Subtilis Strain from Deer Dung with Anti-microbial and Anti-cancer Activities

OBJECTIVE

Bacillus strains are well known for their natural bioactive products that have antimicrobial and/or anti-cancer activities. Many of Bacillus' structurally unique metabolites can combat human diseases, including cancers. However, because Bacillus' metabolites are so abundant, few have been studied extensively enough to fully characterize their chemical constitutions and biological functions.

METHODS

In this study, we focused on the isolation and purification of a new Bacillus strain, and determined the effects of its metabolites on bacteria and cancer cells. Our study focused on a new strain of Bacillus isolated from deer dung. Based on BLAST results, this isolate belongs to Bacillus subtilis, and therefore we named the strain Bacillus subtilis NC16. Congo red assay was used to test the cellulase activity. The inhibition zone was measured to test the antimicrobial activity. CCK-8, wound healing and flow cytometry were used to test the anti-cancer activity.

RESULTS

Metabolites from Bacillus subtilis NC16 have both antimicrobial and anti-cancer activities. They can both suppress the growth of Trichoderma vride and Staphylococcus aureus, and inhibit the proliferation and promote the apoptosis of non-small cell lung cancer cell lines.

CONCLUSION

Our results suggest that Bacillus subtilis NC16 can not only degrade cellulose, but its metabolites may be sources of antibiotics and anti-cancer drugs.

Antagonistic Activity of Antimicrobial Metabolites Produced from Seaweed-Associated Bacillus amyloliquefaciens MTCC 10456 Against Malassezia spp

Members of the genus Malassezia are known to be opportunistic pathogens responsible for causing skin disorders such as seborrheic dermatitis or dandruff, pityriasis versicolor, folliculitis, atopic dermatitis, and psoriasis. Due to the side effects caused by prolonged use of current topical antifungal agents, development of an alternative treatment is necessary. Fermentative production of antimicrobial metabolites from Bacillus amyloliquefaciens MTCC 10456 was carried out, and their antagonistic activity against Malassezia furfur and Malassezia globosa was evaluated. The antifungal metabolites were isolated by acid precipitation, and bioassay-guided simultaneous separation of the antimicrobial compounds was done by reversed-phase high-performance liquid chromatography (RP-HPLC). The fraction which demonstrated antifungal activity consisted of bacilysin, homologues of bacillomycin D, and members of the macrolactin family. The presence of bacilysin was detected using specific inhibitor assays and homologues of bacillomycin D, and macrolactins were identified using liquid chromatography/high-resolution electrospray ionization-mass spectrometry (LC/HRESI-MS/MS) analysis. Synergism among the identified compounds was observed which enhanced the antagonistic activity against Malassezia spp. To our knowledge, this is the first study to report the co-production and separation of members of macrolactin antibiotics, lipopeptides such as bacillomycin D and dipeptide antibiotic bacilysin, by any Bacillus strain from marine environment. Activity of individual compounds against Malassezia has been reported which may facilitate their application in the field of dermatology and in cosmetic products.

Macrolactins: biological activity and biosynthesis

Marine microorganisms have proven to be a rich source of natural products with unique structures and novel activities, due to their special living conditions. Macrolactins (MLNs), mostly produced by marine-derived microorganisms, are a group of 24-membered lactone natural products, which exhibit potent antibacterial, antifungal, antiviral, anticancer, anti-inflammatory, anti-angiogenic and other activities. Their extensive biological activities make them potential compounds for drug development. MLNs are biosynthesized via a type I polyketide synthase (PKS) pathway with different tailoring steps, such as epoxidation, glycosylation and acylation. These modification steps provide opportunities to diversify their structures by combinatorial biosynthesis strategies. This review mainly focuses on the newly discovered MLNs in the past five years, including their biological activities and relevant biosynthetic studies.

Marine macroalga-associated heterotroph Bacillus velezensis as prospective therapeutic agent

Marine macroalgae and their accompanying microbial flora were proved to be the reservoir of potential bioactive compounds with promising pharmacological applications. Heterotrophic bacteria concomitant with the marine algae were isolated and screened for their antibacterial potential against clinically recognized pathogens. The bacterial isolate with greater bioactive properties was identified as Bacillus velezensis MBTDLP1 (phylum Firmicutes), which was isolated from the marine macroalga Laurencia papillosa, by integrated morphological, biochemical and molecular characterization. B. velezensis showed promising antibacterial property against methicillin-resistant Staphylococcus aureus and Vibrio parahemolyticus with inhibition zone of 32-36 mm. Organic ethyl acetate extract of the isolate also displayed prospective antibacterial activity against the test pathogens (minimum inhibitory concentration 7.5-15 µg/mL), coupled with promising antioxidant (IC₅₀ 0.1-0.9 mg/mL against oxidants), anti-inflammatory (IC₅₀ 0.01 mg/mL against 5-lipoxygenase), and carbolytic enzyme attenuation properties (IC₅₀ 0.1-0.4 mg/mL in response to α-amylase and α-glucosidase). Significant anticancer potential against breast carcinoma (MCF-7) cells (IC₅₀ 0.03 mg/mL) coupled with lesser cytotoxicity to the normal fibroblast (3T3L) cells (IC₅₀ 0.14 mg/mL) were also recognized. The apoptosis assay could give reasonable outcome as the organic extract of B. velezensis induced apoptosis to 81% of the cancer cells while maintaining almost 60% viability in normal cells. The results put forward that B. velezensis MBTDLP1 could be used to isolate bioactive compounds with therapeutic potential and biomedical applications.

Macrolactin Antibiotics: Amazing Natural Products

The resistance among various microbial species (infectious agents) to different antimicrobial drugs has emerged as a cause of serious public health problem all over the world. In this sense, natural products have been a rich source of compounds for drug discovery with antibiotic activity. Macrolactins are amazing structures which have antibiotic activity against some clinically relevant pathogens. In addition, they have anti-inflammatory, antifungal, antimicrobial, and antitumor activities. They are macrolides containing 24-membered lactone ring with some differences in their chemical structures. The synthesis of these compounds is a difficult task which has attracted attention of researchers; however few syntheses have been reported. In this review, the isolation of all reported macrolactins, their syntheses and biological activities are revisited.

Antibacterial activity in secondary metabolite extracts of heterotrophic bacteria against Vibrio alginolyticus, Aeromonas hydrophila, and Pseudomonas aeruginosa

Background: Disease causing bacteria such as Vibrio alginolyticus, Aeromonas hydrophila, and Pseudomonas aeruginosa present a problem for fish farming. Treatment to remove them are generally carried out using antibiotics which have side effects on fish, the environment and humans. However, the use of antibacterial compounds derived from heterotrophic bacteria serve as a good alternative for antibiotics. Therefore, this study aimed to explore antibacterial activity in the secondary metabolite extracts of heterotrophic bacteria against Vibrio alginolyticus, Aeromonas hydrophila, and Pseudomonas aeruginosa. Methods: Heterotrophic bacteria namely Bacillus sp. JS04 MT102913.1, Bacillus toyonensis JS08 MT102920.1, Bacillus cereus JS10 MT102922.1, Bacillus sp. JS11 MT102923.1, Pseudoalteromonas sp. JS19 MT102924.1, Bacillus cereus JS22 MT102926.1, and Bacillus sp. strain JS25 MT102927.1 were used in this study. The sequences of these bacteria have been deposited and are available from NCBI GenBank. Each heterotrophic bacterium was cultured on 6L nutrient broth for 8 days, and extracts produced using ethyl acetate to obtain their secondary metabolites. These extracts were tested for their phytochemical contents using FT-IR and also tested for their inhibitory property in pathogenic bacteria by agar diffusion method. Results: Phytochemical test results showed that the seven heterotrophic bacterial isolates produced terpenoid compounds. Based on the inhibitory test, the secondary metabolite extracts from Bacillus sp strain JS04 had the highest inhibitory effect on the growth of pathogenic bacteria namely, V. alginolyticus (17.5 mm), A. hydrophila (16.8 mm), and P. aeruginosa (17.3 mm). Conclusion: It was concluded that the secondary metabolite extracts of heterotrophic bacteria inhibit the growth of V. alginolyticus, A. hydrophila, and P. aeruginosa.

Chemical mining of heterotrophic Shewanella algae reveals anti-infective potential of macrocyclic polyketides against multidrug-resistant pathogens

Heterotrophic Gamma-proteobacterium Shewanella algae MTCC 12715, associated with an intertidal red algae Hypnea valentiae, presented broad-spectra of antibacterial activities against pathogenic bacteria bringing about nosocomial infection. Bioassay-guided fractionation of the bacterial crude extract resulted in two undescribed macrocyclic polyketide analogs, with anti-infective activities against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis (MIC 3.1-5.0 µg/mL). In order to identify the polyketide biosynthetic machinery termed type-I polyketide synthase (pks-I) encoding biologically active secondary metabolites in this strain, the ketosynthase-coding regions of DNA with ≈700 bp size, were amplified, and the partial sequence was submitted in the GenBank (accession number MH157093). The titled compounds were classified under macrocyclic polyketides bearing dodecahydropyrano-trioxacyclooctadecine-dione and trioxo-octadecahydro-1H-benzo[o]tetraoxacyclopentacosine-carboxylate functionalities. Structure-activity correlation analysis displayed that hydrophobic descriptor of the studied compounds could play a prominent role in its anti-infective property against the opportunistic pathogens. Further, in silico molecular docking studies were performed in the allosteric sites of penicillin-binding protein (PBP2a) coded by mecA genes of MRSA, and the best binding pose for each compound (docking score -8.47 kcal/mol and -9.58 kcal/mol, respectively) could be correlated with their in vitro antibacterial activities. The pks-I assisted biosynthetic pathway of macrocyclic polyketides through step-wise decarboxylative condensation initiated by malonate-acyl carrier protein corroborated their structural attributes. Chemical mining of the studied macroalgae-associated heterotrophic bacterium thus revealed the promising antagonistic properties of macrocyclic polyketides isolated from Shewanella algae MTCC 12715 against multidrug-resistant pathogens.

Macrocyclic polyketides with siderophore mode of action from marine heterotrophic Shewanella algae: Prospective anti-infective leads attenuate drug-resistant pathogens

AIMS

Biotechnological and chemical characterization of previously undescribed homologous siderophore-type macrocyclic polyketides from heterotrophic Shewanella algae Microbial Type Culture Collection (MTCC) 12715 affiliated with Rhodophycean macroalga Hypnea valentiae of marine origin, with significant anti-infective potential against drug-resistant pathogens.

METHODS AND RESULTS

The heterotrophic bacterial strain in symbiotic association with intertidal macroalga H. valentiae was isolated to homogeneity in a culture-dependent method and screened for bioactivities by spot-over-lawn assay. The bacterial organic extract was purified and characterized by extensive chromatographic and spectroscopic methods, respectively, and was assessed for antibacterial activities with disc diffusion and microtube dilution methods. The macrocyclic polyketide compounds exhibited wide-spectrum of anti-infective potential against clinically significant vancomycin-resistant Enterococcus faecalis (VREfs), methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa and Klebsiella pneumonia with minimum inhibitory concentration of about 1-3 µg ml^-1 , insomuch as the antibiotics chloramphenicol and ampicillin were active at ≥6·25 µg ml^-1 . The studied compounds unveiled Fe³⁺ chelating activity, which designated that their prospective anti-infective activities against the pathogens could be due to their siderophore mechanism of action. In support of that, the bacterium exhibited siderophore production on bioassay involving the cast upon culture agar plate, and the presence of siderophore biosynthetic gene (≈1000 bp) (MF 981936) further corroborated the inference. In silico molecular modelling with penicillin-binding protein (PBP2a) coded by mecA genes of MRSA (docking score -11·68 to -12·69 kcal mol^-1 ) verified their in vitro antibacterial activities. Putative biosynthetic pathway of macrocyclic polyketides through stepwise decarboxylative condensation initiated by malonate-acyl carrier protein further validated their structural and molecular attributes.

CONCLUSIONS

The studied siderophore-type macrocyclic polyketides from S. algae MTCC 12715 with significant anti-infective potential could be considered as promising candidates for pharmaceutical and biotechnological applications, especially against emerging multidrug-resistant pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study exhibited the heterotrophic bacteria in association with intertidal macroalga as propitious biological resources to biosynthesize novel antibacterial agents.

A Strong Antifungal Activity of 7-O-Succinyl Macrolactin A vs Macrolactin A from Bacillus amyloliquefaciens ELI149

Chemical analysis of the crude extract of bacterial strain Bacillus amyloliquefaciens ELI149, which had been previously isolated from soil, resulted in the isolation and characterization of two known macrolactin derivatives, macrolactin A (1) and 7-O-succinyl macrolactin A (2). The structures of two compounds were assigned by 1D/2D NMR techniques. The two compounds were demonstrated antifungal activity against some important phytopathogens. However, the presence of the succinyl moiety at C-7 gives to the molecule more activity being the second compound more active than the first, showing for the first time, a structure/activity relationship. The cellular damage was also studied in two important phytopathogen fungi.

Pharmacological properties of some mangrove sediment-associated bacillus isolates

Mangrove sediment-associated bacteria are of significantly important in the field of medicine and pharmaceuticals as new promising sources of biologically active pharmacophores due to the extreme conditions, such as high salt concentration and soil anoxia. The sediment bacteria associated with Acanthus ilicifolius and Avicennia officinalis collected from the Mangalavanam mangrove ecosystem of the Kerala State of India were evaluated using various in vitro models for the assessment of their pharmacological properties. The bacteria exhibiting significant antioxidant and antimicrobial activities were isolated, identified, and characterized by the integrated microbiological, biochemical, and 16S rRNA sequencing. Among the varied bacteria isolated from mangrove sediments, Bacillus amyloliquefaciens MBMS5 (GenBank accession number MK765025) exhibited significant antimicrobial activities against various pathogenic bacteria, such as Aeromonas caviae, Vibrio parahemolyticus, and methicillin-resistant Staphylococcus aureus. The extracellular extracts of B. amyloliquefaciens MBMS5 exhibited potential antioxidant activity against free radical species coupled with anti-inflammatory property as displayed by the attenuation activity against pro-inflammatory 5-lipoxygenase.

Moving away from traditional antibiotic treatment: can macrocyclic lactones from marine macroalga-associated heterotroph be the alternatives?

Intertidal red algae Hypnea valentiae associated Bacillus amyloliquefaciens MTCC 12716 revealed potential inhibitory effects on the growth of drug-resistant pathogens. In the genome of B. amyloliquefaciens MTCC 12716, biosynthetic gene clusters encoding antibacterial metabolites were predicted, which might be expressed and contributed to the broad-spectrum anti-infective activity. Three homologue members of the 24-membered macrocyclic lactone family, named as bacvalactones 1-3 bearing 13-O-ethyl (1); 15-O-furanyl-13-O-isobutyl-7-O-propyl-propanoate (2); and 15-O-furanyl-13-O-isobutyl-7-O-propyl-propanoate-7,24-dimethyl (3) functionalities, were acquired through bioactivity-guided purification. The macrocyclic lactones displayed bactericidal activity against opportunistic pathogens causing nosocomial infections, for instance, methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecalis (VREfs), and multidrug-resistant strains of Pseudomonas aeruginosa and Klebsiella pneumonia with MIC ≤ 3.0 μg/mL, whereas standard antibiotics ampicillin and chloramphenicol were active only at concentrations of ≥ 6.25 mg/mL. The biosynthetic pathway of macrocyclic lactones that are generated by trans-AT polyketide synthases through stepwise extension of an acetyl starter unit by eleven sequential Claisen condensations with malonyl-CoA was established, and the structures were correlated with the gene organization of the mln operon, which encompasses nine genes mln A-I (approximately 47 kb in size). The best binding poses for each compounds (1-3) with Staphylococcus aureus peptide deformylase (SaPDF) unveiled docking scores (≥ 9.70 kcal/mol) greater than that of natural peptide deformylase inhibitors, macrolactin N and actinonin (9.14 and 6.96 kcal/mol, respectively), which supported their potential in vitro bioactivities. Thus, the present work demonstrated the potential of macrocyclic lactone for biotechnological and pharmaceutical applications against emerging multidrug-resistant pathogens. Key Points •Three antibacterial bacvalactones were identified from the symbiotic bacterium. •The symbiotic bacterial genome was explored to identify the biosynthetic gene clusters. •Trans-AT pks-assisted mln biosynthetic pathway of the macrocyclic lactone was proposed. •In silicomolecular interactions of the bacvalactones with S. aureus PDF were analyzed.

Salicornolides A-C from Gracilaria salicornia attenuate pro-inflammatory 5-lipoxygense: Prospective natural anti-inflammatory leads

Three macrolides bearing the carbon framework of oxabicyclo[21.3.1]heptacosa-ene-diones (A and B) and oxabicyclo[19.3.1]pentacosa-ene-dione (C) were isolated and characterised from the organic extract of the intertidal red seaweed Gracilaria salicornia (family Gracilariaceae), which were named as salicornolides A-C. These natural macrolides were conformationally pre-organised ring structure providing diverse functionalities, and their potential bioactive properties led to the development of pharmacophores with anti-inflammatory properties. The 21-membered pyran-enclosed salicornolide B displayed greater cyclooxygenase-2 (IC₅₀ COX-2 1.13 mM) inhibitory activity than those exhibited by the 21-membered aryl salicornolide A and 19-membered salicornolide C (IC₅₀ COX-2-1.2 mM). The attenuating potential of the studied compounds against pro-inflammatory enzyme, 5-lipoxygenase (IC₅₀ LOX < 1.5 mM) was significantly greater than that displayed by the non-steroidal anti-inflammatory ibuprofen (IC₅₀ 4.5 mM), whereas the selectivity indices exhibited by salicornolides against cyclooxygenase-2 was significantly higher (1.18-1.41, P < 0.05) when compared to that of ibuprofen (SI 0.43) attributing the greater selectivity profile of the former towards inducible pro-inflammatory mediators than the latter. The minimal binding energy of salicornolide B (-9.64 kcal mol^-1), a greater number of hydrogen-bonds and lesser inhibitory constant (Ki 85.15 nM) might be responsible for effective binding towards 5-lipoxygenase, and that could attribute its greater anti-inflammation potential than those displayed by other compounds. The putative biosynthetic cascade initiated by malonate-acyl carrier protein unambiguously confirmed the structural attributions of the titled macrocyclic lactones. The undescribed salicornolides A-C from seaweed Gracilaria salicornia attenuating pro-inflammatory 5-lipoxygense might be considered as prospective natural anti-inflammatory leads for pharmaceutical applications.

Marine bacteria as source of antimicrobial compounds

The marine environment encompasses a huge biological diversity and can be considered as an underexplored location for prospecting bioactive molecules. In this review, the current state of art about antimicrobial molecules from marine bacteria has been summarized considering the main phylum and sources evolved in a marine environment. Considering the last two decades, we have found as most studied group of bacteria producers of substances with antimicrobial activity is the Firmicutes phylum, in particular strains of the Bacillus genus. The reason for that can be attributed to the difficult cultivation of typical Actinobacteria from a marine sediment, whose members are the major producers of antimicrobial substances in land environments. However, a reversed trend has been observed in recent years with an increasing number of reports settling on Actinobacteria. Great diversity of chemical structures have been identified, such as fijimicyns and lynamicyns from Actinomycetes and macrolactins produced by Bacillus.

Oxygenated elansolid-type of polyketide spanned macrolides from a marine heterotrophic Bacillus as prospective antimicrobial agents against multidrug-resistant pathogens

Three homologous oxygenated elansolid-type of polyketide spanned macrolides were isolated from a heterotrophic marine bacterium, Bacillus amyloliquefaciens MTCC 12716, associated with an intertidal red alga Hypnea valentiae. The complete genome of the bacterium was sequenced and all detectable natural product gene clusters were analysed. The B. amyloliquefaciens MTCC 12716 genome features polyketide synthase (pks) systems of every known formally classified family, nonribosomal peptide synthetases and hybrid clusters. Comprehensive spectroscopic studies revealed the compounds to possess isobenzofuranyl benzoate and 1H-furopyrano[2,3-c]oxacyclononadecine-6-carboxylate moieties. The identified compounds displayed broad-spectrum bactericidal activity against methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecalis, and drug-resistant strains of Pseudomonas aeruginosa and Klebsiella pneumoniae with minimum inhibitory concentrations (MICs) of ≤1.0 µg/mL, whereas the standard antibiotics ampicillin and chloramphenicol were active only at concentrations of ≥6.25 µg/mL. The plausible mechanism of elansolid-type macrolide biosynthesis by trans-AT polyketide synthases through the pks starter unit para-hydroxybenzoic acid was hypothesised, and the structures were correlated with the gene organisation, with the predicted gene cluster comprising 16 genes (~81 kb in size). The best binding poses for each compound with the peptide deformylase (PDF) protein of S. aureus revealed docking scores (>11.30 kcal/mol) greater than actinonin (6.96 kcal/mol), a natural PDF inhibitor. The higher electronic values along with optimum lipophilic parameters support the potential anti-infective properties of the studied macrolides. These antibacterial elansolid-type of polyketide spanned macrolides in marine symbiotic B. amyloliquefaciens could be potential leads for biotechnological and pharmaceutical applications against emerging multidrug-resistant pathogens.

Marine Pharmacology in 2014-2015: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis, Antiviral, and Anthelmintic Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The systematic review of the marine pharmacology literature from 2014 to 2015 was completed in a manner consistent with the 1998-2013 reviews of this series. Research in marine pharmacology during 2014-2015, which was reported by investigators in 43 countries, described novel findings on the preclinical pharmacology of 301 marine compounds. These observations included antibacterial, antifungal, antiprotozoal, antituberculosis, antiviral, and anthelmintic pharmacological activities for 133 marine natural products, 85 marine compounds with antidiabetic, and anti-inflammatory activities, as well as those that affected the immune and nervous system, and 83 marine compounds that displayed miscellaneous mechanisms of action, and may probably contribute to novel pharmacological classes upon further research. Thus, in 2014-2015, the preclinical marine natural product pharmacology pipeline provided novel pharmacology as well as new lead compounds for the clinical marine pharmaceutical pipeline, and thus continued to contribute to ongoing global research for alternative therapeutic approaches to many disease categories.

Bioactive Secondary Metabolites from Bacillus subtilis: A Comprehensive Review

Bacillus subtilis is widely underappreciated for its inherent biosynthetic potential. This report comprehensively summarizes the known bioactive secondary metabolites from B. subtilis and highlights potential applications as plant pathogen control agents, drugs, and biosurfactants. B. subtilis is well known for the production of cyclic lipopeptides exhibiting strong surfactant and antimicrobial activities, such as surfactins, iturins, and fengycins. Several polyketide-derived macrolides as well as nonribosomal peptides, dihydroisocoumarins, and linear lipopeptides with antimicrobial properties have been reported, demonstrating the biosynthetic arsenal of this bacterium. Promising efforts toward the application of B. subtilis strains and their natural products in areas of agriculture and medicine are underway. However, industrial-scale availability of these compounds is currently limited by low fermentation yields and challenging accessibility via synthesis, necessitating the development of genetically engineered strains and optimized cultivation processes. We hope that this review will attract renewed interest in this often-overlooked bacterium and its impressive biosynthetic skill set.

Previously Undescribed Antibacterial Polyketides from Heterotrophic Bacillus amyloliquefaciens Associated with Seaweed Padina gymnospora

A heterotrophic marine bacterium Bacillus amyloliquefaciens isolated from seaweed Padina gymnospora exhibited broad spectra of antibacterial activities against pathogenic bacteria Aeromonas hydrophila, Vibrio harveyi, Vibrio vulnificus, and Vibrio parahaemolyticus. The seaweed-associated B. amyloliquefaciens was recognized to possess functional type I polyketide synthase-1 (pks-1) gene, and was used to isolate four homologous compounds with polyketide frameworks. The compounds were characterized as 11-(15-butyl-13-ethyl-tetrahydro-12-oxo-2H-pyran-13-yl) propyl-2-methylbenzoate (1), 9-(tetrahydro-12-isopropyl-11-oxofuran-10-yl)-ethyl-4-ethoxy-2-hydroxybenzoate (2), 12-(aminomethyl)-11-hydroxyhexanyl-10-phenylpropanoate (3), and 7-(14-hydroxypropan-13-yl)-8-isobutyl-7,8-dihydrobenzo[c]oxepin-1(3H)-one (4) by comprehensive nuclear magnetic resonance and mass spectroscopic experiments. The compounds 1-4 displayed significant antibacterial activities against clinically important pathogens V. parahaemolyticus and V. vulnificus (inhibitory zone diameter of ≥15 mm, 100 mcg on disk). The electronic and hydrophobic parameters appeared to hold a conspicuous part in directing the antibacterial properties of the compounds. This study revealed seaweed-associated B. amyloliquefaciens as potential source of antimicrobial polyketides for pharmaceutical applications.

Unprecedented antioxidative and anti-inflammatory aryl polyketides from the brown seaweed Sargassum wightii

Previously undescribed aryl polyketide lactones, 4-(8-ethyl-tetrahydro-7-oxo-2H-pyran-5-yl)-propyl-4'-methylbenzoate (compound 1) and methyl-2-(12-oxo-7-phenyl-8-vinyl-1-oxa-4,9-cyclododecadien-3-yl)-acetate (compound 2) were purified from ethyl acetate-methanol fraction of the brown seaweed Sargassum wightii. The structures were proposed based on their NMR and mass spectrometric data. The antioxidative activities of the lactones were significantly greater (P<0.05) (IC50 1,1-diphenyl-2-picrylhydrazyl radical scavenging 0.24-0.32mg/mL) than α-tocopherol (IC50 0.63mg/mL). The title compounds displayed considerably greater 5-lipoxygenase inhibitory activity (IC50 0.56 and 0.29mg/mL, respectively) in conjunction with higher selectivity indices (anti-cycloxygense-1IC50/anti-cycloxygense-2IC50 >1) compared to non-steroidal anti-inflammatory drugs (SIaspirin 0.03, SIibuprofen 0.43). Putative biosynthetic pathway of title polyketide products through polyketide synthase enzyme cascade catalyzed reactions substantiated the structural attributions of the hitherto unreported aryl polyketides. This is the first report of the occurrence and characterization of two rare skeletal types, oxo-2H-pyranyl and oxa-cyclododecadienyl macrolactone featuring the aryl substituent from marine organisms with potential antioxidative and anti-inflammatory activities.

O-heterocyclic derivatives with antibacterial properties from marine bacterium Bacillus subtilis associated with seaweed, Sargassum myriocystum

The brown seaweed, Sargassum myriocystum associated with heterotrophic bacterium, Bacillus subtilis MTCC 10407 (JF834075) exhibited broad-spectra of potent antibacterial activities against pathogenic bacteria Aeromonas hydrophila, Vibrio vulnificus, and Vibrio parahaemolyticus. B. subtilis MTCC 10407 was found to be positive for polyketide synthetase (pks) gene, and therefore, was considered to characterize secondary metabolites bearing polyketide backbone. Using bioassay-guided fractionation, two new antibacterial O-heterocyclic compounds belonging to pyranyl benzoate analogs of polyketide origin, with activity against pathogenic bacteria, have been isolated from the ethyl acetate extract of B. subtilis MTCC 10407. In the present study, the secondary metabolites of B. subtilis MTCC 10407 with potent antibacterial action against bacterial pathogens was recognized to represent the platform of pks-1 gene-encoded products. Two homologous compounds 3 (3-(methoxycarbonyl)-4-(5-(2-ethylbutyl)-5,6-dihydro-3-methyl-2H-pyran-2-yl)-butyl benzoate) and 4 [2-(8-butyl-3-ethyl-3,4,4a,5,6,8a-hexahydro-2H-chromen-6-yl)-ethyl benzoate] also have been isolated from the ethyl acetate extract of host seaweed S. myriocystum. The two compounds isolated from ethyl acetate extract of S. myriocystum with lesser antibacterial properties shared similar structures with the compounds purified from B. subtilis that suggested the ecological and metabolic relationship between these compounds in seaweed-bacterial relationship. Tetrahydropyran-2-one moiety of the tetrahydropyrano-[3,2b]-pyran-2(3H)-one system of 1 might be cleaved by the metabolic pool of seaweeds to afford methyl 3-(dihydro-3-methyl-2H-pyranyl)-propanoate moiety of 3, which was found to have no significant antibacterial activity. It is therefore imperative that the presence of dihydro-methyl-2H-pyran-2-yl propanoate system is essentially required to impart the greater activity. The direct involvement of polarisability (Pl) with the target bioactivity in 2 implied that inductive (field/polar) rather than the steric effect (parachor) appears to be the key factor influencing the induction of antibacterial activity. The present work may have a footprint on the use of novel O-heterocyclic polyketide products from seaweed-associated bacterium for biotechnological, food, and pharmaceutical applications mainly as novel antimicrobial secondary metabolites.

Genomics-Inspired Discovery of Three Antibacterial Active Metabolites, Aurantinins B, C, and D from Compost-Associated Bacillus subtilis fmb60

Fmb60 is a wild-type Bacillus subtilis isolated from compost with significant broad-spectrum antimicrobial activities. Two novel PKS clusters were recognized in the genome sequence of fmb60, and then three polyene antibiotics, aurantinins B, C, and D, 1-3, were obtained by bioactivity-guided isolation from the fermentation of fmb60. The structures of aurantinins B-D were elucidated by LC-HRMS and NMR data analysis. Aurantinins C and D were identified as new antimicrobial compounds. The three aurantinins showed significant activity against multidrug-resistant Staphylococcus aureus and Clostridium sporogenes. However, aurantinins B-D did not exhibit any cytotoxicity (IC50 > 100 μg/mL) against LO2 and Caco2 cell lines by MTT assay. Furthermore, using S. aureus as a model bacterium to explore the antibacterial mechanism of aurantinins B-D, it was revealed that the bactericidal activity of aurantinins B-D was related to their ability to disrupt the cell membrane.

Antimicrobial properties of cultivable bacteria associated with seaweeds in the Gulf of Mannar on the southeast coast of India

In this study, 234 bacterial strains were isolated from 7 seaweed species in the Gulf of Mannar on the southeast coast of India. The strains having consistent antimicrobial activity were chosen for further studies, and this constituted about 9.8% of the active strains isolated. Phylogenetic analysis using 16S rDNA sequencing with the help of classical biochemical identification indicated the existence of 2 major phyla, Firmicutes and Proteobacteria. Antimicrobial activity analysis combined with the results of amplifying genes encoding for polyketide synthetase and nonribosomal peptide synthetase showed that seaweed-associated bacteria had broad-spectrum antimicrobial activity. These epibionts might be beneficial to seaweeds by limiting or preventing the development of competing or fouling bacteria. Phylogenetic analysis of ketosynthase (KS) regions with respect to the diverse range of KS domains showed that the KS domains from the candidate isolates were of Type I. The bacterial cultures retained their antimicrobial activities after plasmid curing, which further suggested that the antimicrobial activity of these isolates was not encoded by plasmid, and the genes encoding the antimicrobial product might be present within the genome. Seaweed-associated bacteria with potential antimicrobial activity suggested that the seaweed species are an ideal ecological niche harboring specific bacterial diversity representing a largely underexplored source of antimicrobial secondary metabolites.

Characterization of substituted aryl meroterpenoids from red seaweed Hypnea musciformis as potential antioxidants

The ethyl acetate fraction of red seaweed Hypnea musciformis was purified to yield three substituted aryl meroterpenoids, namely, 2-(tetrahydro-5-(4-hydroxyphenyl)-4-pentylfuran-3-yl)-ethyl-4-hydroxybenzoate (1), 2-2-[(4-hydroxybenzoyl)-oxy]-ethyl-4-methoxy-4-2-[(4-methylpentyl)oxy]-3,4-dihydro-2H-6-pyranylbutanoic acid (2) and 3-((5-butyl-3-methyl-5,6-dihydro-2H-pyran-2-yl)-methyl)-4-methoxy-4-oxobutyl benzoate (3). The structures of these compounds, as well as their relative stereochemistries, were confirmed by exhaustive NMR spectroscopic data analyses. Compound 1 exhibited similar 2,2'-diphenylpicrylhydrazyl radical inhibiting and Fe(2+) ion chelating activities (IC50 25.05 and 350.7μM, respectively) as that of commercial antioxidant gallic acid (IC50 32.3 and 646.6μM, respectively), followed by 3 (IC50 231.2 and 667.9μM, respectively), and 2 (IC50 322.4 and 5115.3μM, respectively), in descending order of activities. Structure-activity relationship analysis revealed that the antioxidant activities of these compounds were directly proportional to the steric and hydrophobic parameters. The seaweed derived aryl meroterpenoids might serve as potential lead antioxidative molecules for use in pharmaceutical and food industries.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.