Protocetraric acid: an excellent broad spectrum compound from the lichen <i>Usnea albopunctata</i> against medically important microbes

<div><p>The aim of this study was to investigate the antimicrobial property of the compounds present in the lichen <i>Usnea albopunctata</i>. Ethyl acetate extract of the lichen was purified by column chromatography to yield a major compound which was characterised by spectroscopic methods as protocetraric acid. In this study, protocetraric acid recorded significant broad spectrum antimicrobial property against medically important human pathogenic microbes. The prominent antibacterial activity was recorded against <i>Salmonella typhi</i> (0.5 μg/mL). Significant antifungal activity was recorded against <i>Trichophyton rubrum</i> (1 μg/mL), which is significantly better that the standard antifungal agent. Protocetraric acid is reported here for the first time from <i>U. albopunctata.</i> Thus the results of this study suggest that protocetraric acid has significant antimicrobial activities and has a strong potential to be developed as an antimicrobial drug against pathogenic microbes.</p></div

Biocontrol of <i>Aspergillus</i> Species on Peanut Kernels by Antifungal Diketopiperazine Producing <i>Bacillus cereus</i> Associated with Entomopathogenic Nematode

<div><p>The rhabditid entomopathogenic nematode associated <i>Bacillus cereus</i> and the antifungal compounds produced by this bacterium were evaluated for their activity in reducing postharvest decay of peanut kernels caused by <i>Aspergillus</i> species in <i>in vitro</i> and <i>in vivo</i> tests. The results showed that <i>B. cereus</i> had a significant effect on biocontrol effectiveness in <i>in vitro</i> and <i>in vivo</i> conditions. The antifungal compounds produced by the <i>B. cereus</i> were purified using silica gel column chromatography and their structure was elucidated using extensive spectral analyses. The compounds were identified as diketopiperazines (DKPs) [cyclo-(L-Pro-Gly), cyclo(L-Tyr-L-Tyr), cyclo-(L-Phe-Gly) and cyclo(4-hydroxy-L-Pro-L-Trp)]. The antifungal activities of diketopiperazines were studied against five <i>Aspergillus</i> species and best MIC of 2 µg/ml was recorded against <i>A. flavus</i> by cyclo(4-hydroxy-L-Pro-L-Trp). To investigate the potential application of cyclo(4-hydroxy-L-Pro-L-Trp) to eliminate fungal spoilage in food and feed, peanut kernels was used as a food model system. White mycelia and dark/pale green spores of <i>Aspergillus</i> species were observed in the control peanut kernels after 2 days incubation. However the fungal growth was not observed in peanut kernels treated with cyclo(4-hydroxy-L-Pro-L-Trp). The cyclo(4-hydroxy-L-Pro-L-Trp) was nontoxic to two normal cell lines [fore skin (FS) normal fibroblast and African green monkey kidney (VERO)] up to 200 µg/ml in MTT assay. Thus the cyclo(4-hydroxy-L-Pro-L-Trp) identified in this study may be a promising alternative to chemical preservatives as a potential biopreservative agent which prevent fungal growth in food and feed. To the best of our knowledge, this is the first report demonstrating that the entomopathogenic nematode associated <i>B. cereus</i> and cyclo(4-hydroxy-L-Pro-L-Trp) could be used as a biocontrol agents against postharvest fungal disease caused by <i>Aspergillus</i> species.</p></div

Effect of different incubation times of <i>B. cereus</i> on control of <i>Aspergillus</i> species.

<p>The control is treated with sterile distilled water. Values followed by different letters were significantly different according to Duncan’s multiple range test <i>p</i> = 0.05.</p

Cytotoxicity of cyclo(4-hydroxy-L-Pro-L-Trp) against normal cell lines.

<p>Cytotoxicity of cyclo(4-hydroxy-L-Pro-L-Trp) against normal cell lines.</p

The structure of diketopiperazines.

<p>(<b>A</b>) Cyclo-(L-Pro-Gly), (<b>B</b>) Cyclo(D-Tyr-D-Tyr), (<b>C</b>) Cyclo-(L-Phe-Gly) and (<b>D</b>) Cyclo(4-hydroxy-L-Pro-L-Trp).</p

Microscopic images (Carl Zeiss stereomicroscope, Stemi 200C) of the growth of <i>A. flavus</i> in control and peanut kernels treated with crude ethyl acetate extract and cyclo(4-hydroxy-L-Pro-L-Trp) after 2 days.

<p>(<b>A</b>) Control plates: solvent control-treated with methanol, medium alone- treated with autoclaved modified medium and untreated- peanut kernels without methanol and modified medium (<b>B</b>) crude extract (<b>C</b>) cyclo(4-hydroxy-L-Pro-L-Trp). 1 ×, 2 ×, and 3 × are the 1-fold, 2-fold, and 3-fold MIC concentration of cyclo(4-hydroxy-L-Pro-L-Trp) or crude ethyl acetate extract.</p

Details of the pure compounds.

<p>Details of the pure compounds.</p

Microscopic images (Carl Zeiss stereomicroscope, Stemi 200C) of the growth of <i>A. niger</i> in control and peanut kernels treated with crude ethyl acetate extract and cyclo(4-hydroxy-L-Pro-L-Trp) after 2 days.

<p>(<b>A</b>) Control plates: solvent control-treated with methanol, medium alone- treated with autoclaved modified medium and untreated- peanut kernels without methanol and modified medium (<b>B</b>) crude extract (<b>C</b>) Cyclo(4-hydroxy-L-Pro-L-Trp). 1 ×, 2 ×, and 3 × are the 1-fold, 2-fold, and 3-fold MIC concentration of cyclo(4-hydroxy-L-Pro-L-Trp) or crude ethyl acetate extract.</p

Effect of <i>B. cereus</i> and cell free culture filtrate on <i>Aspergillus</i> species.

<p>Values followed by different letters in same row were significantly different according to Duncan’s multiple range test <i>p</i> = 0.05.</p><p>+ full growth up to the edge of the Petri plate.</p><p>Effect of <i>B. cereus</i> and cell free culture filtrate on <i>Aspergillus</i> species.</p

HPLC chromatogram of diketopiperazines on a reversed-phase C18 column (LC-20AD).

<p>Samples of 15 µl were injected to a column (250 mm×4.6 mm×5 mm), eluted with 100% methanol. Retention time is 2.778 min. The calculated purity is 96% based on the peak area. (<b>A</b>) Cyclo-(L-Pro-Gly), (<b>B</b>) Cyclo(D-Tyr-D-Tyr), (<b>C</b>) Cyclo-(L-Phe-Gly) and (<b>D</b>) Cyclo(4-hydroxy-L-Pro-L-Trp).</p