Isolation and identification of antimicrobial secondary metabolites from Bacillus cereus associated with a rhabditid entomopathogenic nematode

Antimicrobial profile of the culturable olive sporobiota and its potential as a source of biocontrol agents for major phytopathogens in olive agriculture

BACKGROUND

The phytopathogens Xylella fastidiosa and Verticillium dahliae present an unparalleled threat to olive agriculture. However, there is no efficient field treatment available today for either pest. Spore-forming bacteria (i.e., the sporobiota) are known for their extraordinary resistance properties and antimicrobial activity. The aim of this study was thus to identify potential novel sustainable spore-forming biocontrol agents derived from the culturable olive microbiome, termed the sporobiota, in general and in particular against X. fastidiosa and V. dahliae.

RESULTS

We demonstrate the wide-ranging antimicrobial profile of 415 isolates from the culturable olive sporobiota towards human and plant pathogens. We further identified five candidates with antagonistic activity against X. fastidiosa and V. dahliae. These belong to the Bacillus subtilis, Bacillus cereus and Peribacillus simplex clade. The activity was related to the species and their relative origin (soil versus leaf endophytic). It is of particular interest that two of these candidates are already naturally present at the site of disease-development that is, plant interior. We further confirmed the presence of lipopeptide genes potentially associated with the reported bioactivity.

CONCLUSIONS

The study provides insights into how members of the olive sporobiota may support the olive plant to ward off detrimental pathogens. It further yields five promising candidates for the development of eco-friendly, multi-active biocontrol agents in olive agriculture. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The Potential of Bacilli-Derived Biosurfactants as an Additive for Biocontrol against Alternaria alternata Plant Pathogenic Fungi

Fungal diseases caused by Alternaria alternata constitute a significant threat to the production and quality of a wide range of crops, including beans, fruits, vegetables, and grains. Traditional methods for controlling these diseases involve synthetic chemical pesticides, which can negatively impact the environment and human health. Biosurfactants are natural, biodegradable secondary metabolites of microorganisms that have also been shown to possibly have antifungal activity against plant pathogenic fungi, including A. alternata being sustainable alternatives to synthetic pesticides. In this study, we investigated the potential of biosurfactants of three bacilli (Bacillus licheniformis DSM13, Bacillus subtilis DSM10, and Geobacillus stearothermophilus DSM2313) as a biocontrol agent against A. alternata on beans as a model organism. For this fermentation, we describe using an in-line biomass sensor monitoring both permittivity and conductivity, which are expected to correlate with cell concentration and products, respectively. After the fermentation of biosurfactants, we first characterised the properties of the biosurfactant, including their product yield, surface tension decrement capability, and emulsification index. Then, we evaluated the antifungal properties of the crude biosurfactant extracts against A. alternata, both in vitro and in vivo, by analysing various plant growth and health parameters. Our results showed that bacterial biosurfactants effectively inhibited the growth and reproduction of A. alternata in vitro and in vivo. B. licheniformis manufactured the highest amount of biosurfactant (1.37 g/L) and demonstrated the fastest growth rate, while G. stearothermophilus produced the least amount (1.28 g/L). The correlation study showed a strong positive relationship between viable cell density VCD and OD600, as well as a similarly good positive relationship between conductivity and pH. The poisoned food approach in vitro demonstrated that all three strains suppressed mycelial development by 70–80% when applied with the highest tested dosage of 30%. Regarding in vivo investigations, B. subtilis post-infection treatment decreased the disease severity to 30%, whereas B. licheniformis and G. stearothermophilus post-infection treatment reduced disease severity by 25% and 5%, respectively. The study also revealed that the plant’s total height, root length, and stem length were unaffected by the treatment or the infection.

Natural products, including a new caboxamycin, from Streptomyces and other Actinobacteria isolated in Spain from storm clouds transported by Northern winds of Arctic origin

Actinobacteria, mostly Streptomyces species, are the main source of natural products essential in medicine. While the majority of producer microorganisms of secondary metabolite are reported from terrestrial or marine environments, there are limited reports of their isolation from atmospheric precipitations. Clouds are considered as atmospheric oases for microorganisms and there is a recent paradigm shift whereby atmospheric-derived Actinobacteria emerge as an alternative source for drug discovery. In this context, we studied a total of 18 bioactive Actinobacteria strains, isolated by sampling nine precipitation events with prevailing Northern winds in the Cantabrian Sea coast, Northern Spain. Backward trajectories meteorological analyses indicate that air masses were originated mostly in the Arctic Ocean, and their trajectory to downwind areas involved the Atlantic Ocean and also terrestrial sources from continental Europe, and in some events from Canada, Greenland, Mauritania and Canary Islands. Taxonomic identification of the isolates, by 16S rRNA gene sequencing and phylogenetic analyses, revealed that they are members of three Actinobacteria genera. Fifteen of the isolates are Streptomyces species, thus increasing the number of bioactive species of this genus in the atmosphere to a 6.8% of the total currently validated species. In addition, two of the strains belong to the genus Micromonospora and one to genus Nocardiopsis. These findings reinforce a previous atmospheric dispersal model, extended herein to the genus Micromonospora. Production of bioactive secondary metabolites was screened in ethyl acetate extracts of the strains by LC-UV-MS and a total of 94 secondary metabolites were detected after LC/MS dereplication. Comparative analyses with natural products databases allowed the identification of 69 structurally diverse natural products with contrasted biological activities, mostly as antibiotics and antitumor agents, but also anti-inflammatory, antiviral, antiparasitic, immunosuppressant and neuroprotective among others. The molecular formulae of the 25 remaining compounds were determined by HRMS. None of these molecules had been previously reported in natural product databases indicating potentially novel metabolites. As a proof of concept, a new metabolite caboxamycin B (1) was isolated from the culture broth of Streptomyces sp. A-177 and its structure was determined by various spectrometric methods. To the best of our knowledge, this is the first novel natural product obtained from an atmospheric Streptomyces, thus pointing out precipitations as an innovative source for discovering new pharmaceutical natural products.

The Age of Next-Generation Therapeutic-Microbe Discovery: Exploiting Microbe-Microbe and Host-Microbe Interactions for Disease Prevention

Humans are considered "superorganisms," harboring a diverse microbial collective that outnumbers human cells 10 to 1. Complex and gravely understudied host- and microbe-microbe interactions-the product of millions of years of host-microbe coevolution-govern the superorganism in almost every aspect of life functions and overall well-being. Abruptly disrupting these interactions via extrinsic factors has undesirable consequences for the host. On the other hand, supplementing commensal or beneficial microbes may mitigate perturbed interactions or enhance the interactive relationships that ultimately benefit all parties. Hence, immense efforts have focused on dissecting the innumerable host- and microbe-microbe relationships to characterize if a "positive" or "negative" interaction is at play and to exploit such behavior for broader implications. For example, microbiome research has worked to identify and isolate naturally antipathogenic microbes that may offer therapeutic potential either in a direct, one-on-one application or by leveraging its unique metabolic properties. However, the discovery and isolation of such desired therapeutic microbes from complex microbiota have proven challenging. Currently, there is no conventional technique to universally and functionally screen for these microbes. With this said, we first describe in this review the historical (probiotics) and current (fecal microbiota or defined consortia) perspectives on therapeutic microbes, present the discoveries of therapeutic microbes through exploiting microbe-microbe and host-microbe interactions, and detail our team's efforts in discovering therapeutic microbes via our novel microbiome screening platform. We conclude this minireview by briefly discussing challenges and possible solutions with therapeutic microbes' applications and paths ahead for discovery.

Antimicrobial Compounds from Microorganisms

Antimicrobial resistance is an exigent public health concern owing to the emergence of novel strains of human resistant pathogens and the concurrent rise in multi-drug resistance. An influx of new antimicrobials is urgently required to improve the treatment outcomes of infectious diseases and save lives. Plant metabolites and bioactive compounds from chemical synthesis have found their efficacy to be dwindling, despite some of them being developed as drugs and used to treat human infections for several decades. Microorganisms are considered untapped reservoirs for promising biomolecules with varying structural and functional antimicrobial activity. The advent of cost-effective and convenient model organisms, state-of-the-art molecular biology, omics technology, and machine learning has enhanced the bioprospecting of novel antimicrobial drugs and the identification of new drug targets. This review summarizes antimicrobial compounds isolated from microorganisms and reports on the modern tools and strategies for exploiting promising antimicrobial drug candidates. The investigation identified a plethora of novel compounds from microbial sources with excellent antimicrobial activity against disease-causing human pathogens. Researchers could maximize the use of novel model systems and advanced biomolecular and computational tools in exploiting lead antimicrobials, consequently ameliorating antimicrobial resistance.

Chiral secondary amino acids, their importance, and methods of analysis

Naturally occurring secondary amino acids, with proline as the main representative, contain an alpha-imino group in a cycle that is typically four-, five-, and six-membered. The unique ring structure exhibits exceptional properties-conformational rigidity, chemical stability, and specific roles in protein structure and folding. Many proline analogues have been used as valuable compounds for the study of metabolism of both prokaryotic and eukaryotic cells and for the synthesis of compounds with desired biological, pharmaceutical, or industrial properties. The D-forms of secondary amino acids play different roles in living organisms than the L-forms. They have different metabolic pathways, biological, physiological, and pharmacological effects, they can be indicators of changes and also serve as biomarkers of diseases. In the scientific literature, the number of articles examining D-amino acids in biological samples is increasing. The review summarises information on the occurrence and importance of D- and L-secondary amino acids-azetidic acid, proline, hydroxyprolines, pipecolic, nipecotic, hydroxypipecolic acids and related peptides containing these D-AAs, as well as the main analytical methods (mostly chromatographic) used for their enantiomeric determination in different matrices (biological samples, plants, food, water, and soil).

Therapeutic prospects of endophytic Bacillus species from Berberis lycium against oxidative stress and microbial pathogens

Endophytes are microorganisms residing within plant tissues. Bacterial endophytes are important sources for production of pharmaceutically important metabolites. Berberis lycium is an important medicinal plant and there exist no report regarding isolation and determination of bioactive potential of its bacterial endophytes. Therefore the present study was aimed to isolate and identify bacterial endophytes from Berberis lycium. The study resulted in isolation of 20 strains of bacterial endophytes. Based on their antibacterial activity three strains were identified as Bacillus cereus (LBL6), Bacillus thuringiensis (SBL3) and Bacillus anthracis (SBL4) on basis of 16SrRNA gene using universal primers. Crude ethyl acetate extracts of LBL6, SBL3 and SBL4 were further evaluated for antioxidant and antifungal activities. Moderate antioxidant activity (56 %) at a concentration of 1000 µg/mL was observed for LBL6 followed by 45 and 43 % activity by SBL4 and SBL3 respectively. Significant antifungal activity was observed against Aspergillus niger (60 %) and Aspergillus flavus (56 %) at concentration of 4 mg/mL of SBL3 and SBL4 respectively. GCMS analysis of extract (LBL6) exhibited presence of 12 bioactive secondary metabolites corresponding to antimicrobial, antifungal, antioxidant, antitumor and anticancer activities. In conclusion, present study highlighted the importance of Berberis lycium to host diverse bacterial endophytes of pharmaceutical importance.

Bioactive Natural Products in Actinobacteria Isolated in Rainwater From Storm Clouds Transported by Western Winds in Spain

Actinobacteria are the main producers of bioactive natural products essential for human health. Although their diversity in the atmosphere remains largely unexplored, using a multidisciplinary approach, we studied here 27 antibiotic producing Actinobacteria strains, isolated from 13 different precipitation events at three locations in Northern and Southern Spain. Rain samples were collected throughout 2013-2016, from events with prevailing Western winds. NOAA HYSPLIT meteorological analyses were used to estimate the sources and trajectories of the air-mass that caused the rainfall events. Five-day backward air masses trajectories of the diverse events reveals a main oceanic source from the North Atlantic Ocean, and in some events long range transport from the Pacific and the Arctic Oceans; terrestrial sources from continental North America and Western Europe were also estimated. Different strains were isolated depending on the precipitation event and the latitude of the sampling site. Taxonomic identification by 16S rRNA sequencing and phylogenetic analysis revealed these strains to belong to two Actinobacteria genera. Most of the isolates belong to the genus Streptomyces, thus increasing the number of species of this genus isolated from the atmosphere. Furthermore, five strains belonging to the rare Actinobacterial genus Nocardiopsis were isolated in some events. These results reinforce our previous Streptomyces atmospheric dispersion model, which we extend herein to the genus Nocardiopsis. Production of bioactive secondary metabolites was analyzed by LC-UV-MS. Comparative analyses of Streptomyces and Nocardiopsis metabolites with natural product databases led to the identification of multiple, chemically diverse, compounds. Among bioactive natural products identified 55% are antibiotics, both antibacterial and antifungal, and 23% have antitumor or cytotoxic properties; also compounds with antiparasitic, anti-inflammatory, immunosuppressive, antiviral, insecticidal, neuroprotective, anti-arthritic activities were found. Our findings suggest that over time, through samples collected from different precipitation events, and space, in different sampling places, we can have access to a great diversity of Actinobacteria producing an extraordinary reservoir of bioactive natural products, from remote and very distant origins, thus highlighting the atmosphere as a contrasted source for the discovery of novel compounds of relevance in medicine and biotechnology.

Applications of metallic nanomaterials for the treatment of water

Water scarcity is not a novel issue. It has already affected almost every continent in this blue planet. It is driven by two primary sources: increasing demand for fresh water due to the increase in population and overexhaustion of the available freshwater resources. During the past decade, stress has been given to extract fresh, clean and safe potable elixir of life from the bountiful stores of sea water by exploiting various technologies. As nanomaterials are providing promising solutions to almost all our problems, they are again being accessed in order to combat the problem of global freshwater scarcity. Desalination methods have marvellously improved under the impact of nanomaterials. Different metallic nanomaterials are being used to serve this purpose; for example, silver, iron, zinc, titanium dioxide in addition to natural and synthetically derived polymeric bionanomaterials. In the present paper, a brief account of all the metallic nanomaterials which are being used for treatment of water has been provided by thorough investigation on the research done till now. It strives to throw light on various materials and methods which are based on the exploitation of nanotechnology for the treatment of water.

In vitro evaluation of cytotoxic effects of di (2-ethylhexyl) phthalate (DEHP) produced by Bacillus velezensis strain RP137 isolated from Persian Gulf

Phthalates are widely used in polymer science and have potential toxicity related to their chemical structures. However, lots of evidence indicate that phthalate derivatives are undoubtedly produced as secondary metabolites by organisms, including plants, animals, and microorganisms. In the present study, Bacillus velezensis strain RP137 was cultured under optimized conditions. Its biomass was extracted with ethyl acetate with one fraction showing cytotoxic properties. A pure compound was isolated from the active fraction using combined silica gel and LH20 size exclusion column chromatography. Structural evaluation including FT-IR, ¹H NMR, ¹³C NMR, HR-MS and CHN analysis identified the purified compound as di(2-ethylhexyl)phthalate (DEHP) with the formula C₂₄H₃₈O₄ and the molecular weight of 389.29 Da. The microorganism-derived (stereospecific) DEHP was strongly reduced the proliferation and induced cytotoxic effects on various eukaryotic cell lines in compare to the synthetic racemic mixture of the compound when assessed by MTT assay. Furthermore, crystal violet assay and morphological changes confirmed the cytotoxic effect of DEHP. Interestingly, non-malignant SV40-immortalized fibroblast cells were less affected by the purified DEHP. Further evaluation on the antibacterial activity of DEHP documented no effect toward Gram-positive (S. aureus) and Gram-negative (E. coli and P. aeruginosa) pathogens even at a high concentration of 100 μM. In conclusion, existence of DEHP as byproduct of microorganism's metabolism can seriously be considered as a warning to human health.

Stilbenoids: A Natural Arsenal against Bacterial Pathogens

The escalating emergence of resistant bacterial strains is one of the most important threats to human health. With the increasing incidence of multi-drugs infections, there is an urgent need to restock our antibiotic arsenal. Natural products are an invaluable source of inspiration in drug design and development. One of the most widely distributed groups of natural products in the plant kingdom is represented by stilbenoids. Stilbenoids are synthesised by plants as means of protection against pathogens, whereby the potential antimicrobial activity of this class of natural compounds has attracted great interest in the last years. The purpose of this review is to provide an overview of recent achievements in the study of stilbenoids as antimicrobial agents, with particular emphasis on the sources, chemical structures, and the mechanism of action of the most promising natural compounds. Attention has been paid to the main structure modifications on the stilbenoid core that have expanded the antimicrobial activity with respect to the parent natural compounds, opening the possibility of their further development. The collected results highlight the therapeutic versatility of natural and synthetic resveratrol derivatives and provide a prospective insight into their potential development as antimicrobial agents.

Evaluation and entomopathogenicity of gut bacteria associated with dauer juveniles of Oscheius chongmingensis (Nematoda: Rhabditidae)

The nematodes of genus Oscheius are insect parasites with a potential role as biological control agents. The composition of gut microbiota and its potential assistant role in the complex pathogenic mechanism of nematodes have been poorly illustrated. In this study, the intestinal bacteria associated with dauer juveniles of the nematode Oscheius chongmingensis Tumian were classified by 16S rDNA high-throughput sequencing. The raw reads were assigned to 845 operational taxonomic units (OTUs) after quality filtering. The results showed that the genus Ochrobactrum, with a proportion of 59.82%, was the most abundant genus, followed by 7.13% Bacillus, 4.7% Albidiferax, 4.26% Acinetobacter, and 3.09% Rhodococcus. The two dominant bacteria, Ochrobactrum and Bacillus, were further isolated by culturing on NBTA and LB medium respectively, and then identified as Ochrobactrum tritici and Bacillus cereus by morphological and 16S rDNA sequence analysis. Furthermore, the entomopathogenicity of these two bacterial species was studied. The results showed that O. tritici caused 93.33% mortality within 144 hr in the 4th -instar larvae of Galleria mellonella treated with 2 × 109  CFU/ml, whereas B. cereus showed 100% mortality at a concentration of 3.3 × 107  CFU/ml within 48 hr. These findings, especially the presence of O. tritici, which had not been found in other nematode species in the genus Oscheius, indicate that the associated nematode O. chongmingensis may have particular utility as a biocontrol agent.

Gut bacteria of animals/pests living in polluted environments are a potential source of antibacterials

The morbidity and mortality associated with bacterial infections have remained significant despite chemotherapeutic advances. With the emergence of drug-resistant bacterial strains, the situation has become a serious threat to the public health. Thus, there is an urgent need to identify novel antibacterials. The majority of antibiotics available in the market are produced by bacteria isolated from soil. However, the low-hanging fruit has been picked; hence, there is a need to mine bacteria from unusual sources. With this in mind, it is important to note that animals and pests such as cockroaches, snake, crocodiles, and water monitor lizard come across pathogenic bacteria regularly, yet flourish in contaminated environments. These species must have developed methods to defend themselves to counter pathogens. Although the immune system is known to possess antiinfective properties, gut bacteria of animals/pests may also offer a potential source of novel antibacterial agents, and it is the subject of this study. This paper discusses our current knowledge of bacteria isolated from land and marine animals with antibacterial properties and to propose untapped sources for the isolation of bacteria to mine potentially novel antibiotic molecules.

Advances in Antarctic Research for Antimicrobial Discovery: A Comprehensive Narrative Review of Bacteria from Antarctic Environments as Potential Sources of Novel Antibiotic Compounds Against Human Pathogens and Microorganisms of Industrial Importance

The recent emergence of antibiotic-resistant bacteria has become a critical public health problem. It is also a concern for industries, since multidrug-resistant microorganisms affect the production of many agricultural and food products of economic importance. Therefore, discovering new antibiotics is crucial for controlling pathogens in both clinical and industrial spheres. Most antibiotics have resulted from bioprospecting in natural environments. Today, however, the chances of making novel discoveries of bioactive molecules from various well-known sources have dramatically diminished. Consequently, unexplored and unique environments have become more likely avenues for discovering novel antimicrobial metabolites from bacteria. Due to their extreme polar environment, Antarctic bacteria in particular have been reported as a potential source for new antimicrobial compounds. We conducted a narrative review of the literature about findings relating to the production of antimicrobial compounds by Antarctic bacteria, showing how bacterial adaptation to extreme Antarctic conditions confers the ability to produce these compounds. We highlighted the diversity of antibiotic-producing Antarctic microorganisms, including the phyla Proteobacteria, Actinobacteria, Cyanobacteria, Firmicutes, and Bacteroidetes, which has led to the identification of new antibiotic molecules and supports the belief that research on Antarctic bacterial strains has important potential for biotechnology applications, while providing a better understanding of polar ecosystems.

Atmospheric Precipitations, Hailstone and Rainwater, as a Novel Source of Streptomyces Producing Bioactive Natural Products

A cultivation-dependent approach revealed that highly diverse populations of Streptomyces were present in atmospheric precipitations from a hailstorm event sampled in February 2016 in the Cantabrian Sea coast, North of Spain. A total of 29 bioactive Streptomyces strains isolated from small samples of hailstone and rainwater, collected from this hailstorm event, were studied here. Taxonomic identification by 16S rRNA sequencing revealed more than 20 different Streptomyces species, with their closest homologs displaying mainly oceanic but also terrestrial origins. Backward trajectory analysis revealed that the air-mass sources of the hailstorm event, with North Western winds, were originated in the Arctic Ocean (West Greenland and North Iceland) and Canada (Labrador), depending on the altitude. After traveling across the North Atlantic Ocean during 4 days the air mass reached Europe and precipitated as hailstone and rain water at the sampling place in Spain. The finding of Streptomyces species able to survive and disperse through the atmosphere increases our knowledge of the biogeography of genus Streptomyces on Earth, and reinforces our previous dispersion model, suggesting a generalized feature for the genus which could have been essential in his evolution. This unique atmospheric-derived Streptomyces collection was screened for production of bioactive secondary metabolites. Analyses of isolates ethyl acetate extracts by LC-UV-MS and further database comparison revealed an extraordinary diversity of bioactive natural products. One hundred molecules were identified, mostly displaying contrasted antibiotic and antitumor/cytotoxic activities, but also antiparasitic, antiviral, anti-inflammatory, neuroprotector, and insecticide properties. More interestingly, 38 molecules not identified in natural products databases might represent new natural products. Our results revealed for the first time an extraordinary diversity of Streptomyces species in the atmosphere able to produce an extraordinary repertoire of bioactive molecules, thus providing a very promising source for the discovery of novel pharmaceutical natural products.

Antibacterial low-molecular-weight compounds produced by the marine bacterium Rheinheimera japonica KMM 9513T

Strain KMM 9513^T was isolated from a sediment sample collected from the Sea of Japan seashore and selected due to its ability to inhibit indicator bacterial growth. The strain KMM 9513^T has been recently described as a novel species Rheinheimera japonica. This study was undertaken to determine which substances produced by strain KMM 9513^T could be responsible for its antimicrobial activity. Eight compounds were obtained from an ethyl acetate extract of R. japonica KMM 9513^T. The structures of five diketopiperazines (4-8) and diisobutyl-, dibutyl- and bis(2-ethylhexyl) phthalates (1-3) were established on the basis of detailed interpretation of NMR data, by Marfey method and optical rotation data. The structures of diketopiperazines were determined as cyclo-(L-valyl-L-proline), cyclo-(L-valyl-D-proline), cyclo-(L-phenylalanyl-L-proline), cyclo-(L-leucyl-L-proline), and cyclo-(L-phenylalanyl-D-proline). Compounds 1-3, 5 and 8 revealed antimicrobial activities against Bacillus subtilis and/or Enterococcus faecium and Staphylococcus aureus. In this paper, we describe the isolation and structural elucidation of the isolated compounds 1-8. This is the first report of the characterisation of low molecular weight antibacterial metabolites produced by a member of the genus Rheinheimera.

Stilbene Derivatives from Photorhabdus temperata SN259 and Their Antifungal Activities against Phytopathogenic Fungi

Chemical investigation of an insect pathogenic enterobacterium, Photorhabdus temperata SN259, led to the isolation and identification of seven metabolites, which include three new compounds, 3-hydroxy-2-isopropyl-5-phenethylphenyl carbamate, 1, 2-(1-hydroxypropan-2-yl)-5-[2-phenylethyl]benzene-1,3-diol, 2, 2-(1-hydroxypropan-2-yl)-5-[(E)-2-phenylethenyl]benzene-1,3-diol, 3, and four known metabolites (4-7). Their structures were elucidated on the basis of MS and NMR data and by comparison with those reported previously. The activities of compounds 1-7 were evaluated against four phytopathogenic fungi (Pythium aphanidermatum, Rhizoctonia solani Kuhn, Exserohilum turcicum, and Fusarium oxysporum). In an agar medium assay, compounds 1 and 7 showed strong inhibition against P.  aphanidermatum with EC₅₀ values of 2.8 and 2.7 μg/mL, respectively. By comparing the structure of compounds 1-7, we deduced that the acylamino group in compound 1 and the isopropyl group in compound 7 contribute to the inhibitory activity.

Antimicrobial Activities of Bacteria Associated with the Brown Alga Padina pavonica

Macroalgae belonging to the genus Padina are known to produce antibacterial compounds that may inhibit growth of human- and animal pathogens. Hitherto, it was unclear whether this antibacterial activity is produced by the macroalga itself or by secondary metabolite producing epiphytic bacteria. Here we report antibacterial activities of epiphytic bacteria isolated from Padina pavonica (Peacocks tail) located on northern coast of Tunisia. Eighteen isolates were obtained in pure culture and tested for antimicrobial activities. Based on the 16S rRNA gene sequences the isolates were closely related to Proteobacteria (12 isolates; 2 Alpha- and 10 Gammaproteobacteria), Firmicutes (4 isolates) and Actinobacteria (2 isolates). The antimicrobial activity was assessed as inhibition of growth of 12 species of pathogenic bacteria (Aeromonas salmonicida, A. hydrophila, Enterobacter xiangfangensis, Enterococcus faecium, Escherichia coli, Micrococcus sp., Salmonella typhimurium, Staphylococcus aureus, Streptococcus sp., Vibrio alginoliticus, V. proteolyticus, V. vulnificus) and one pathogenic yeast (Candida albicans). Among the Firmicutes, isolate P8, which is closely related to Bacillus pumilus, displayed the largest spectrum of growth inhibition of the pathogenic bacteria tested. The results emphasize the potential use of P. pavonica associated antagonistic bacteria as producers of novel antibacterial compounds.

Methods for in vitro evaluating antimicrobial activity: A review

In recent years, there has been a growing interest in researching and developing new antimicrobial agents from various sources to combat microbial resistance. Therefore, a greater attention has been paid to antimicrobial activity screening and evaluating methods. Several bioassays such as disk-diffusion, well diffusion and broth or agar dilution are well known and commonly used, but others such as flow cytofluorometric and bioluminescent methods are not widely used because they require specified equipment and further evaluation for reproducibility and standardization, even if they can provide rapid results of the antimicrobial agent's effects and a better understanding of their impact on the viability and cell damage inflicted to the tested microorganism. In this review article, an exhaustive list of in vitro antimicrobial susceptibility testing methods and detailed information on their advantages and limitations are reported.

In Vitro Evaluation of the Antioxidant, 3,5-Dihydroxy-4-ethyl-trans-stilbene (DETS) Isolated from Bacillus cereus as a Potent Candidate against Malignant Melanoma

3,5-dihydroxy Q1 -4-ethyl-trans-stilbene (DETS) is a natural stilbene, which was first identified as bioactive bacterial secondary metabolite isolated from Bacillus cereus associated with a rhabditid entomopathogenic nematode. The present study was intended to investigate the antioxidant and anticancer activity of this compound in vitro. Antioxidant activity was investigated by assaying DPPH free radical scavenging, superoxide radical-(O2..) scavenging, hydroxyl radical scavenging and metal chelating activity, which proved that the compound is a powerful antioxidant. The metal chelating activity of DETS was higher than butylated hydroxyanisol (BHA) and gallic acid, two well-known antioxidants. As the molecule exhibited strong antioxidant potential, it was further evaluated for cytotoxic activity toward five cancer cells of various origins. Since the compound has a strong structural similarity with resveratrol (trans- 3,4,5-trihydroxystilbene), a well-studied chemopreventive polyphenolic antioxidant, its anticancer activity was compared with that of resveratrol. Among the five cancer cells studied, the compound showed maximum cytotoxicity toward the human melanoma cell line, [A375, IC50: 24.01 μM] followed by cervical [HeLa-46.17 μM], colon [SW480- 47.28 μM], liver [HepG2- 69.56 μM] and breast [MCF-7- 84.31 μM] cancer cells. A375 was much more sensitive to DETS compared to the non-melanoma cell line, A431, in which the IC50 of the compound was more than double (49.60 μM). In the present study, the anticancer activity of DETS against melanoma was confirmed by various apoptosis assays. We also observed that DETS, like resveratrol, down-regulates the expression status of major molecules contributing to melanoma progression, such as BRAF, β-catenin and Brn-2, all of which converge in MITF-M, the master regulator of melanoma signaling. The regulatory role of MITF-M in DETS-induced cytotoxicity in melanoma cells was confirmed by comparing the cytotoxicity of DETS in A375 cells (IC50-24.01 μM), with that in SK-MEL-2 (IC50-67.6 μM), another melanoma cells which highly over-express MITF-M. The compound arrests the cells at S-G2 transition state of the cell cycle, as resveratrol. Our results indicate that DETS is a powerful antioxidant, having anticancer efficacy comparable with that of resveratrol, and is a potential candidate to be explored by in vivo studies and in-depth mechanistic evaluation. To our knowledge, this is the first report on the antioxidant and anticancer properties of DETS.

Complete genome of Bacillus sp. Pc3 isolated from the Antarctic seawater with antimicrobial activity

Bacillus sp. Pc3 was isolated from the Antarctic seawater with strong antifungal activity against several plant pathogenic fungi. Here, we report the complete sequence of the 3.9-Mbp genome of this strain. The genome sequence may provide fundamental molecular information on elucidating the metabolic pathway of antimicrobial compounds in this strain.