Abstract
One of the biggest challenges in public health is the rising number of antibiotic resistant pathogens and the lack of novel antibiotics. In recent years there is a rising focus on fungi as sources of antimicrobial compounds due to their ability to produce a large variety of bioactive compounds and the observation that virtually every fungus may still contain yet unknown so called “cryptic,” often silenced, compounds. These putative metabolites could include novel bioactive compounds. Considerable effort is spent on methods to induce production of these “cryptic” metabolites. One approach is the use of small molecule effectors, potentially influencing chromatin landscape in fungi. We observed that the supernatant of the fungus Doratomyces (D.) microsporus treated with valproic acid (VPA) displayed antimicrobial activity against Staphylococcus (S.) aureus and two methicillin resistant clinical S. aureus isolates. VPA treatment resulted in enhanced production of seven antimicrobial compounds: cyclo-(L-proline-L-methionine) (cPM), p-hydroxybenzaldehyde, cyclo-(phenylalanine-proline) (cFP), indole-3-carboxylic acid, phenylacetic acid (PAA) and indole-3-acetic acid. The production of the antimicrobial compound phenyllactic acid was exclusively detectable after VPA treatment. Furthermore three compounds, cPM, cFP, and PAA, were able to boost the antimicrobial activity of other antimicrobial compounds. cPM, for the first time isolated from fungi, and to a lesser extent PAA, are even able to decrease the minimal inhibitory concentration of ampicillin in MRSA strains. In conclusion we could show in this study that VPA treatment is a potent tool for induction of “cryptic” antimicrobial compound production in fungi, and that the induced compounds are not exclusively linked to the secondary metabolism. Furthermore this is the first discovery of the rare diketopiperazine cPM in fungi. Additionally we could demonstrate that cPM and PAA boost antibiotic activity against antibiotic resistant strains, suggesting a possible application in combinatorial antibiotic treatment against resistant pathogens.
Highlights
One of the biggest challenges in healthcare today is the rising number of antibiotic resistant pathogenic microorganisms isolated from human and veterinary sources and the lack of novel antimicrobial agents in the discovery pipeline (GarciaAlvarez et al, 2012)
We report the identification of seven induced antimicrobial compounds derived from primary and secondary metabolism of the fungus, including cyclo-(Lproline-L-methionine), which was for the first time discovered in fungi
We could show that cPM and to lesser extend phenylacetic acid (PAA) are able to enhance ampicillin activity against two resistant methicillin-resistant Staphylococcus (S.) aureus isolate (MRSA) strains decreasing the Minimal inhibitory concentrations (MICs) value to 10 mg/l
Summary
One of the biggest challenges in healthcare today is the rising number of antibiotic resistant pathogenic microorganisms isolated from human and veterinary sources and the lack of novel antimicrobial agents in the discovery pipeline (GarciaAlvarez et al, 2012). This has reignited the interest in the search of antimicrobial compounds derived from natural sources (Ling et al, 2015). It has been shown that this fungus has the ability to degrade the antifungal alkaloid sampangine (Orabi et al, 1999) Besides these studies little is known about the primary and secondary metabolite profile of this fungus
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