Panning for chemical gold: marine bacteria as a source of new therapeutics

Production of metabolites as bacterial responses to the marine environment

Bacteria in marine environments are often under extreme conditions of e.g., pressure, temperature, salinity, and depletion of micronutrients, with survival and proliferation often depending on the ability to produce biologically active compounds. Some marine bacteria produce biosurfactants, which help to transport hydrophobic low water soluble substrates by increasing their bioavailability. However, other functions related to heavy metal binding, quorum sensing and biofilm formation have been described. In the case of metal ions, bacteria developed a strategy involving the release of binding agents to increase their bioavailability. In the particular case of the Fe³⁺ ion, which is almost insoluble in water, bacteria secrete siderophores that form soluble complexes with the ion, allowing the cells to uptake the iron required for cell functioning. Adaptive changes in the lipid composition of marine bacteria have been observed in response to environmental variations in pressure, temperature and salinity. Some fatty acids, including docosahexaenoic and eicosapentaenoic acids, have only been reported in prokaryotes in deep-sea bacteria. Cell membrane permeability can also be adapted to extreme environmental conditions by the production of hopanoids, which are pentacyclic triterpenoids that have a function similar to cholesterol in eukaryotes. Bacteria can also produce molecules that prevent the attachment, growth and/or survival of challenging organisms in competitive environments. The production of these compounds is particularly important in surface attached strains and in those in biofilms. The wide array of compounds produced by marine bacteria as an adaptive response to demanding conditions makes them suitable candidates for screening of compounds with commercially interesting biological functions. Biosurfactants produced by marine bacteria may be helpful to increase mass transfer in different industrial processes and in the bioremediation of hydrocarbon-contaminated sites. Siderophores are necessary e.g., in the treatment of diseases with metal ion imbalance, while antifouling compounds could be used to treat man-made surfaces that are used in marine environments. New classes of antibiotics could efficiently combat bacteria resistant to the existing antibiotics. The present work aims to provide a comprehensive review of the metabolites produced by marine bacteria in order to cope with intrusive environments, and to illustrate how such metabolites can be advantageously used in several relevant areas, from bioremediation to health and pharmaceutical sectors.

Development of novel drugs from marine surface associated microorganisms

While the oceans cover more than 70% of the Earth's surface, marine derived microbial natural products have been largely unexplored. The marine environment is a habitat for many unique microorganisms, which produce biologically active compounds ("bioactives") to adapt to particular environmental conditions. For example, marine surface associated microorganisms have proven to be a rich source for novel bioactives because of the necessity to evolve allelochemicals capable of protecting the producer from the fierce competition that exists between microorganisms on the surfaces of marine eukaryotes. Chemically driven interactions are also important for the establishment of cross-relationships between microbes and their eukaryotic hosts, in which organisms producing antimicrobial compounds ("antimicrobials"), may protect the host surface against over colonisation in return for a nutrient rich environment. As is the case for bioactive discovery in general, progress in the detection and characterization of marine microbial bioactives has been limited by a number of obstacles, such as unsuitable culture conditions, laborious purification processes, and a lack of de-replication. However many of these limitations are now being overcome due to improved microbial cultivation techniques, microbial (meta-) genomic analysis and novel sensitive analytical tools for structural elucidation. Here we discuss how these technical advances, together with a better understanding of microbial and chemical ecology, will inevitably translate into an increase in the discovery and development of novel drugs from marine microbial sources in the future.

An antimicrobial guanidine-bearing sesterterpene from the cultured cyanobacterium Scytonema sp

Scytoscalarol (1), a antimicrobial sesterterpene bearing a guanidino group, was isolated from the cultured cyanobacterium Scytonema sp. (UTEX 1163) by bioassay-guided fractionation. The chemical structure was determined by spectroscopic analysis including MS and 1D and 2D NMR. Scytoscalarol (1) showed antimicrobial activities against Bacillus anthracis, Staphylococcus aureus, Escherichia coli, Candida albicans, and Mycobacterium tuberculosis with MIC values in the range from 2 to 110 microM.

ZipN, an FtsA-like orchestrator of divisome assembly in the model cyanobacterium Synechocystis PCC6803

We pursued the characterization of the divisome of the spherical-celled cyanobacterium Synechocystis PCC6803, through deletion, site-directed mutagenesis, GFP tagging, two-hybrid and co-immunoprecipitation assays. We presently report that the DivIVA-like protein Cdv3 is essential to both cell growth and division, whereas the AmiC, AmpH, FtsE, FtsN, SpoIID, YlmD, YlmE and YlmG proteins are dispensable. With the exception of the self-interacting protein YlmD, none of these dispensable factors appeared to interact with ZipN, the crucial cytokinetic factor we previously characterized. By contrast, we found that ZipN interacts with itself and the self-interacting protein Cdv3, as well as with all other crucial cytokinetic factors we previously characterized, namely: FtsZ, FtsI, FtsQ, SepF and ZipS. We also identified ZipN amino acids selectively involved in ZipN interaction with one of its following partners, Cdv3, FtsQ or SepF. Finally, we found no direct interaction between Cdv3, SepF and ZipS. Collectively, these results indicate that ZipN is a central player of divisome assembly in cyanobacteria, similarly to the FtsA protein of E. coli that is absent in cyanobacteria and chloroplast.

Characterization of the FtsZ-interacting septal proteins SepF and Ftn6 in the spherical-celled cyanobacterium Synechocystis strain PCC 6803

Assembly of the tubulin-like cytoskeletal protein FtsZ into a ring structure at midcell establishes the location of the nascent division sites in prokaryotes. However, it is not yet known how the assembly and contraction of the Z ring are regulated, especially in cyanobacteria, the environmentally crucial organisms for which only one FtsZ partner protein, ZipN, has been described so far. Here, we characterized SepF and Ftn6, two novel septal proteins, in the spherical-celled strain Synechocystis PCC 6803. Both proteins were found to be indispensable to Synechocystis sp. strain PCC 6803. The depletion of both SepF and Ftn6 resulted in delayed cytokinesis and the generation of giant cells but did not prevent FtsZ polymerization, as shown by the visualization of green fluorescent protein (GFP)-tagged FtsZ polymers. These GFP-tagged Z-ring-like structures often appeared to be abnormal, because these reporter cells respond to the depletion of either SepF or Ftn6 with an increased abundance of total, natural, and GFP-tagged FtsZ proteins. In agreement with their septal localization, we found that both SepF and Ftn6 interact physically with FtsZ. Finally, we showed that SepF, but not Ftn6, stimulates the formation and/or stability of FtsZ polymers in vitro.

Antitumor compounds from marine actinomycetes

Chemotherapy is one of the main treatments used to combat cancer. A great number of antitumor compounds are natural products or their derivatives, mainly produced by microorganisms. In particular, actinomycetes are the producers of a large number of natural products with different biological activities, including antitumor properties. These antitumor compounds belong to several structural classes such as anthracyclines, enediynes, indolocarbazoles, isoprenoides, macrolides, non-ribosomal peptides and others, and they exert antitumor activity by inducing apoptosis through DNA cleavage mediated by topoisomerase I or II inhibition, mitochondria permeabilization, inhibition of key enzymes involved in signal transduction like proteases, or cellular metabolism and in some cases by inhibiting tumor-induced angiogenesis. Marine organisms have attracted special attention in the last years for their ability to produce interesting pharmacological lead compounds.

Actinomycetes for marine drug discovery isolated from mangrove soils and plants in China

The mangrove ecosystem is a largely unexplored source for actinomycetes with the potential to produce biologically active secondary metabolites. Consequently, we set out to isolate, characterize and screen actinomycetes from soil and plant material collected from eight mangrove sites in China. Over 2,000 actinomycetes were isolated and of these approximately 20%, 5%, and 10% inhibited the growth of Human Colon Tumor 116 cells, Candida albicans and Staphylococcus aureus, respectively, while 3% inhibited protein tyrosine phosphatase 1B (PTP1B), a protein related to diabetes. In addition, nine isolates inhibited aurora kinase A, an anti-cancer related protein, and three inhibited caspase 3, a protein related to neurodegenerative diseases. Representative bioactive isolates were characterized using genotypic and phenotypic procedures and classified to thirteen genera, notably to the genera Micromonospora and Streptomyces. Actinomycetes showing cytotoxic activity were assigned to seven genera whereas only Micromonospora and Streptomyces strains showed anti-PTP1B activity. We conclude that actinomycetes isolated from mangrove habitats are a potentially rich source for the discovery of anti-infection and anti-tumor compounds, and of agents for treating neurodegenerative diseases and diabetes.