David and Goliath: chemical perturbation of eukaryotes by bacteria

Biology and applications of co-produced, synergistic antimicrobials from environmental bacteria

Environmental bacteria, such as Streptomyces spp., produce specialized metabolites that are potent antibiotics and therapeutics. Selected specialized antimicrobials are co-produced and function together synergistically. Co-produced antimicrobials comprise multiple chemical classes and are produced by a wide variety of bacteria in different environmental niches, suggesting that their combined functions are ecologically important. Here, we highlight the exquisite mechanisms that underlie the simultaneous production and functional synergy of 16 sets of co-produced antimicrobials. To date, antibiotic and antifungal discovery has focused mainly on single molecules, but we propose that methods to target co-produced antimicrobials could widen the scope and applications of discovery programs.

The ARC2 response in Streptomcyes coelicolor requires the global regulatory genes afsR and afsS

ARC2 is a synthetic compound, related in structure and mechanism to the antibiotic triclosan, that activates the production of many specialized metabolites in the Streptomyces genus of bacteria. In this work, we demonstrate that the addition of ARC2 to Streptomyces coelicolor cultures results in considerable alterations in overall gene expression including most notably the specialized metabolic genes. Using actinorhodin production as a model system, we show that the effect of ARC2 depends on the pleiotropic regulators afsR and afsS but not afsK. We find that the constitutive expression of afsS can bypass the need for afsR but not the reverse, while the constitutive expression of afsK had no effect on actinorhodin production. These data are consistent with a model in which ARC2 activates a cell stress response that depends on AfsR activating the expression of the afsS gene such that AfsS then triggers the production of actinorhodin.

Chemical entrapment and killing of insects by bacteria

Actinobacteria produce antibacterial and antifungal specialized metabolites. Many insects harbour actinobacteria on their bodies or in their nests and use these metabolites for protection. However, some actinobacteria produce metabolites that are toxic to insects and the evolutionary relevance of this toxicity is unknown. Here we explore chemical interactions between streptomycetes and the fruit fly Drosophila melanogaster. We find that many streptomycetes produce specialized metabolites that have potent larvicidal effects against the fly; larvae that ingest spores of these species die. The mechanism of toxicity is specific to the bacterium’s chemical arsenal: cosmomycin D producing bacteria induce a cell death-like response in the larval digestive tract; avermectin producing bacteria induce paralysis. Furthermore, low concentrations of volatile terpenes like 2-methylisoborneol that are produced by streptomycetes attract fruit flies such that they preferentially deposit their eggs on contaminated food sources. The resulting larvae are killed during growth and development. The phenomenon of volatile-mediated attraction and specialized metabolite toxicity suggests that some streptomycetes pose an evolutionary risk to insects in nature.

Streptomyces bacteria produce metabolites with diverse biological activities. Here, Ho et al. show that some of these metabolites are volatile terpenes that attract fruit flies, while other metabolites can kill the insect’s larvae upon ingestion of bacterial spores.

Cytotoxic quinones from the aerial parts of Morinda umbellata L

Although Morinda umbellata L. has been used in numerous folk medicines, there is a lack of phytochemical studies on this plant. Sixteen undescribed quinones, namely, ten anthraquinones (umbellatas A-J), one naphthohydroquinone (umbellata K), one naphthohydroquinone dimer (umbellata L), and four dinaphthofuran quinones (umbellatas M-P), were isolated from the aerial parts of Morinda umbellata L. (Rubiaceae). The structures of all the isolated quinones were elucidated based on spectroscopic methods. Four of the unknown quinones (umbellatas A, H, K and M) showed potent cytotoxic effects against A431, A2780, NCI-H460, HCT116, HepG2, and MCF-7 human cancer cell lines with IC₅₀ values of 1.3-7.1 μM. These results reveal potential lead compounds for the development of new anticancer agents.

Screening and purification of natural products from actinomycetes that affect the cell shape of fission yeast

This study was designed to identify bioactive compounds that alter the cellular shape of the fission yeast Schizosaccharomyces pombe by affecting functions involved in the cell cycle or cell morphogenesis. We used a multidrug-sensitive fission yeast strain, SAK950 to screen a library of 657 actinomycete bacteria and identified 242 strains that induced eight different major shape phenotypes in S. pombe. These include the typical cell cycle-related phenotype of elongated cells, and the cell morphology-related phenotype of rounded cells. As a proof of principle, we purified four of these activities, one of which is a novel compound and three that are previously known compounds, leptomycin B, streptonigrin and cycloheximide. In this study, we have also shown novel effects for two of these compounds, leptomycin B and cycloheximide. The identification of these four compounds and the explanation of the S. pombe phenotypes in terms of their known, or predicted bioactivities, confirm the effectiveness of this approach.

Summary: A cell shape-based visual screen of S. pombe in the presence of actinomycete-derived bioactivities and an explanation for the phenotypes following identification of the compounds.