Inhibition of bacterial quorum sensing and biofilm formation by extracts of neotropical rainforest plants.

Bacterial biofilms are responsible for many persistent infections by many clinically relevant pathogens such as Staphylococcus aureus and Pseudomonas aeruginosa. Biofilms are much more resistant to conventional antibiotics than their planktonic counterparts. Quorum sensing, an intercellular communication system, controls pathogenesis and biofilm formation in most bacterial species. Quorum sensing provides an important pharmacological target since its inhibition does not provide a selective pressure for resistance. In this study, we investigated the quorum sensing and biofilm inhibitory activities of 126 plant extracts from 71 species collected from neotropical rainforests in Costa Rica. Quorum sensing and biofilm interference were assessed using a modified disc diffusion bioassay with Chromobacterium violaceum ATCC 12,472 and a spectrophotometric bioassay with Pseudomonas aeruginosa PA14, respectively. Species with significant anti-quorum sensing and/or anti-biofilm activities belonged to the Meliaceae, Melastomataceae, Lepidobotryaceae, Sapindaceae, and Simaroubaceae families. IC50 values ranged from 45 to 266 µg/mL. Extracts of these active species could lead to future development of botanical treatments for biofilm-associated infections.

Microbial regulation in gorgonian corals.

Gorgonian corals possess many novel natural products that could potentially mediate coral-bacterial interactions. Since many bacteria use quorum sensing (QS) signals to facilitate colonization of host organisms, regulation of prokaryotic cell-to-cell communication may represent an important bacterial control mechanism. In the present study, we examined extracts of twelve species of Caribbean gorgonian corals, for mechanisms that regulate microbial colonization, such as antibacterial activity and QS regulatory activity. Ethanol extracts of gorgonians collected from Puerto Rico and the Florida Keys showed a range of both antibacterial and QS activities using a specific Pseudomonas aeruginosa QS reporter, sensitive to long chain AHLs and a short chain N-acylhomoserine lactones (AHL) biosensor, Chromobacterium violaceium. Overall, the gorgonian corals had higher antimicrobial activity against non-marine strains when compared to marine strains. Pseudopterogorgia americana, Pseusopterogorgia acerosa, and Pseudoplexuara flexuosa had the highest QS inhibitory effect. Interestingly, Pseudoplexuara porosa extracts stimulated QS activity with a striking 17-fold increase in signal. The stimulation of QS by P. porosa or other elements of the holobiont may encourage colonization or recruitment of specific microbial species. Overall, these results suggest the presence of novel stimulatory QS, inhibitory QS and bactericidal compounds in gorgonian corals. A better understanding of these compounds may reveal insight into coral-microbial ecology and whether a therapeutic potential exists.

Anti-quorum sensing activity of medicinal plants in southern Florida.

Bacterial intercellular communication, or quorum sensing (QS), controls the pathogenesis of many medically important organisms. Anti-QS compounds are known to exist in marine algae and have the ability to attenuate bacterial pathogenicity. We hypothesized that terrestrial plants traditionally used as medicines may also produce anti-QS compounds. To test this hypothesis, 50 medicinal plants from southern Florida were screened for anti-QS activity using two biomonitor strains, Chromobacterium violaceum and Agrobacterium tumefaciens. Of these plants, six showed QS inhibition: Conocarpus erectus L. (Combretaceae), Chamaecyce hypericifolia (L.) Millsp. (Euphorbiaceae), Callistemon viminalis (Sol. ex Gaertn.) G. Don (Myrtaceae), Bucida burceras L. (Combretaceae), Tetrazygia bicolor (Mill.) Cogn. (Melastomataceae), and Quercus virginiana Mill. (Fagaceae). This study introduces not only a new mode of action and possible validation for traditional plant use, but also a potentially new therapeutic direction for the treatment of bacterial infections.

Arsenic complexes in the arsenic hyperaccumulator Pteris vittata (Chinese brake fern).

Pteris vittata (Chinese brake fern), the first reported arsenic (As) hyperaccumulating plant, can be potentially applied in the phytoremediation As-contaminated sites. Understanding the mechanisms of As tolerance and detoxification in this plant is critical to further enhance its capability of As hyperaccumulation. In this study, an unknown As species, other than arsenite (AsIII) or arsenate (AsV) was found in leaflets by using anion-exchange chromatography-hydride generation-atomic fluorescence spectroscopy and size-exclusion chromatography-atomic fluorescence spectrometry. The chromatographic behavior of this unknown As species and its stability suggest that it is likely an As complex. Although phytochelatin with two subunits (PC2) was the only major thiol in P. vittata under As exposure, this unknown As complex was unlikely to be an AsIII-PC2 complex by comparison of their chromatographic behaviors, stability at different pHs and charge states. The complex is sensitive to temperature and metal ions, but relatively insensitive to pH. In buffer solution of pH 5.9, it is present in a neutral form.

Thiol synthesis and arsenic hyperaccumulation in Pteris vittata (Chinese brake fern).

Pteris vittata (Chinese brake fern) has potential for phytoremediation of As-contaminated sites. In this study, the synthesis of total thiols and acid-soluble thiols in P. vittata was investigated under arsenic exposure. The strong and positive correlation between As concentration and acid-soluble thiols in plant leaflets suggests that acid-soluble thiols may play a role in As detoxification. A major As-induced thiol was purified and characterized. A molecular ion (M + 1) of 540 m/z suggests that the thiol was a phytochelatin (PC) with two base units (PC(2)). However, the ratios of acid-soluble thiols to As in leaflets exposed to As ranged from 0.012 to 0.026, suggesting that only a very small part of As is complexed by PC(2). PCs could play a minor detoxification role in this hyperaccumulator. A PC-independent mechanism appears to be mainly involved in As tolerance, while PC-dependent detoxification seems to be a supplement.

Light-activated plant defence.

Sunlight provides the energy required for all biochemical, physiological and developmental processes necessary for plant growth, reproduction and survival. The role of light in photosynthesis and photomorphogenesis has been appreciated for some time; however, the role of light in plant defence is a comparatively recent realization. Between 75-100 photosensitizers or phototoxins, molecules that become toxic in the presence of light, have been extracted from higher plant tissues. These biologically-active compounds have diverse biosynthetic origins and belong to at least 15 different phytochemical classes (i.e. acetophenones, acetylenes, benzophenanthrenes, ß-carbolines, coumarins, extended quinones, furanochromones, furanocoumarins, furanoquinolines, isoquinolines, lignans, pterocarpans, quinolines, sesquiterpenes and thiophenes). Of more than 100 angiosperm families assayed, phototoxins and/or phototoxic activity have been reported in c. 40 families representing 32 orders and 8 subclasses of the Magnoliophyta. Most of these allelochemicals are acute toxins with little organism-specificity. As such, they are effective biocides capable of killing a wide-range of potentially harmful organisms including: viruses, pathogenic bacteria and fungi, nematodes and herbivorous insects, as well as competing plant species. This review focuses on the phytochemistry, taxonomic occurrence and toxicological consequences of phototoxic metabolites in flowering plants. The available information suggests that phototoxic phytochemicals: (i) are broad-spectrum allelochemicals capable of protecting plants against a variety of detrimental organisms in nature, (ii) represent a successful defensive strategy in both primitive and advanced plant taxa, and (iii) may be an effective defence under a variety of photoenvironmental conditions. CONTENTS Summary 401 I. Introduction 401 II. Chemistry/biochemistry of plant photosensitizers 402 III. Botanical distribution of photosensitizers 403 IV. Toxicology 405 V. Biological activity/defensive role of photosensitizers 408 VI. Concluding remarks 413 References 415.