Natural products and antifungal drug discovery

Antimicrobial activity and mechanism of anti-MRSA of phloroglucinol derivatives

BACKGROUND

In previous studies, authors have completed the total synthesis of several phloroglucinol natural products and synthesized a series of their derivatives, which were tested with good biological activities.

OBJECTIVES

To discover anti-MRSA lead compound and study their mechanism of action.

METHODS

Phloroglucinol derivatives were tested to investigate their activities against several gram-positive strains including Methicillin-resistant Staphylococcus aureus (MRSA). The mechanism study was conducted by determining extracellular potassium ion concentration, intracellular NADPH oxidase content, SOD activity, ROS amount in MRSA and MRSA survival rate under A5 treatment. The in vitro cytotoxicity test of A5 was conducted.

RESULTS

The activity of monocyclic compounds was stronger than that of bicyclic compounds, and compound A5 showed the best MIC value of 0.98 μg/mL and MBC value of 1.95 μg/mL, which were 4-8 times lower than that of vancomycin. The mechanism study of A5 showed that it achieved anti-MRSA effect through membrane damage, which is proved by increased concentration of extracellular potassium ion after A5 treatment. Another possible mechanism is the over ROS production induced cell death, which is suggested by observed alternation of several reactive oxygen species (ROS) related indicators including NADPH concentration, superoxide dismutase (SOD) activity, ROS content and bacterial survival rate after A5 treatment. The cytotoxicity results in vitro showed that A5 was basically non-toxic to cells.

CONCLUSION

Acylphloroglucinol derivative A5 showed good anti-MRSA activity, possibly via membrane damage and ROS-mediated oxidative stress mechanism. It deserves further exploration to be a potential lead for the development of new anti-MRSA agent.

Synthesis and antifungal activity of C-21 steroids with an aromatic D ring

Six analogues of salpichrolides with a simplified side chain (6-11) were synthesized using a new methodology to obtain steroids with an aromatic D-ring. The key step was the elimination of HBr in a vicinal dibromo D-homosteroid by treatment with 1,4-diazabicyclo[2.2.2]octane (DABCO). All new compounds were completely characterized by 2D NMR techniques and tested on two fungal pathogenic species, Fusarium virguliforme and Fusarium solani.

Partial identification of antifungal compounds from Punica granatum peel extracts

Aqueous extracts of pomegranate peels were assayed in vitro for their antifungal activity against six rot fungi that cause fruit and vegetable decay during storage. The growth rates of Alternaria alternata , Stemphylium botryosum , and Fusarium spp. were significantly inhibited by the extracts. The growth rates were negatively correlated with the levels of total polyphenolic compounds in the extract and particularly with punicalagins, the major ellagitannins in pomegranate peels. Ellagitannins were also found to be the main compounds in the bioactive fractions using bioautograms, and punicalagins were identified as the main bioactive compounds using chromatographic separation. These results suggest that ellagitannins, and more specifically punicalagins, which are the dominant compounds in pomegranate peels, may be used as a control agent of storage diseases and to reduce the use of synthetic fungicides.

Antifungal activity of redox-active benzaldehydes that target cellular antioxidation

Background

Disruption of cellular antioxidation systems should be an effective method for control of fungal pathogens. Such disruption can be achieved with redox-active compounds. Natural phenolic compounds can serve as potent redox cyclers that inhibit microbial growth through destabilization of cellular redox homeostasis and/or antioxidation systems. The aim of this study was to identify benzaldehydes that disrupt the fungal antioxidation system. These compounds could then function as chemosensitizing agents in concert with conventional drugs or fungicides to improve antifungal efficacy.

Methods

Benzaldehydes were tested as natural antifungal agents against strains of Aspergillus fumigatus, A. flavus, A. terreus and Penicillium expansum, fungi that are causative agents of human invasive aspergillosis and/or are mycotoxigenic. The yeast Saccharomyces cerevisiae was also used as a model system for identifying gene targets of benzaldehydes. The efficacy of screened compounds as effective chemosensitizers or as antifungal agents in formulations was tested with methods outlined by the Clinical Laboratory Standards Institute (CLSI).

Results

Several benzaldehydes are identified having potent antifungal activity. Structure-activity analysis reveals that antifungal activity increases by the presence of an ortho-hydroxyl group in the aromatic ring. Use of deletion mutants in the oxidative stress-response pathway of S. cerevisiae (sod1Δ, sod2Δ, glr1Δ) and two mitogen-activated protein kinase (MAPK) mutants of A. fumigatus (sakAΔ, mpkCΔ), indicates antifungal activity of the benzaldehydes is through disruption of cellular antioxidation. Certain benzaldehydes, in combination with phenylpyrroles, overcome tolerance of A. fumigatus MAPK mutants to this agent and/or increase sensitivity of fungal pathogens to mitochondrial respiration inhibitory agents. Synergistic chemosensitization greatly lowers minimum inhibitory (MIC) or fungicidal (MFC) concentrations. Effective inhibition of fungal growth can also be achieved using combinations of these benzaldehydes.

Conclusions

Natural benzaldehydes targeting cellular antioxidation components of fungi, such as superoxide dismutases, glutathione reductase, etc., effectively inhibit fungal growth. They possess antifungal or chemosensitizing capacity to enhance efficacy of conventional antifungal agents. Chemosensitization can reduce costs, abate resistance, and alleviate negative side effects associated with current antifungal treatments.

Screening for antifungal peptides and their modes of action in Aspergillus nidulans

Many short cationic peptides have been identified as potent antimicrobial agents, but their modes of action are not well understood. Peptide synthesis on cellulose membranes has resulted in the generation of peptide libraries, while high-throughput assays have been developed to test their antibacterial activities. In this paper a microtiter plate-based screening method for fungi has been developed and used to test nine antibacterial peptides against the model fungus Aspergillus nidulans. Microscopical studies using sublethal peptide concentrations caused defects in polarized growth, including increased branch formation and depolarized hyphae. We characterized the mode of action for one of our target peptides, Sub5 (12 amino acids), which has already been shown to possess pharmacological potential as an antibacterial agent and is able to interact with ATP and ATP-dependent enzymes. The MIC for A. nidulans is 2 μg/ml, which is in the same range as the MICs reported for bacteria. Fluorescein isothiocyanate (FITC)-labeled Sub5 targeted the cytoplasmic membrane, particularly hyphal tips, and entered the cytoplasm after prolonged exposure, independent of endocytosis. Interestingly, Sub5 peptide treatment disturbed sterol-rich membrane domains, important for tip growth, at hyphal tips. A very similar peptide, FITC-P7, also accumulated on the cell membrane but did not have antibacterial or antifungal activity, suggesting that the cytoplasmic membrane is a first target for the Sub5 peptide; however, the antifungal activity seems to be correlated with the ability to enter the cytoplasm, where the peptides might act on other targets.