Screening for Novel Antifungal Agents Produced by Actinomycetes Found in Association with Plant Rhizospheres

Janice Strap, Antony Basil, Ana González-Franco, Min Jin Kang, Ranjeet Tokala, Don Crawford


Life-threatening fungal infections and a shift in the spectrum of fungal pathogens has recently become more prevalent due to prolonged use of broad-spectrum antibiotics, increased use of immunosuppressive drugs, and as a complication of acquired immunodeficiency syndrome. The majority of invasive fungal infections are due to Aspergillus and Candida species but their treatment is currently limited. The emergence of drug resistant fungal pathogens and the threat of biological warfare exacerbate the need for efficacious antifungals. Although pathogenic fungi are not recognized as likely bioweapons, the prophylactic use of antibiotics for biological threats such as anthrax could increase the number of infections caused by Candida. Actinomycetes are soil bacteria capable of producing an extensive array of chemically diverse secondary metabolites. Microbes associated with plant roots differ in number and type from that associated with the surrounding soil and are influenced by the plant species. Rhizosphere isolates are a potential source of novel antifungal agents of medical relevance. The basis of this hypothesis is that actinomycetes have antagonistic activity toward plant pathogenic fungi within the rhizosphere. This antifungal activity may also be effective against medically important fungi. Actinomycetes were isolated from several different rhizosphere-associated soils by the serial-dilution-spread plate method. A plate bioassay was used to test, on different media, the in vitro antagonism of the rhizosphere-isolated actinomycetes against the plant pathogens Pythium ultimum, P. infestans, P. irregulare, and Fusarium oxysporium; the human pathogens C. albicans and A. niger; and the yeasts Saccharomyces cerevisiae and S. pastorianus. Rhizosphere-associated actinomycetes were isolated that could inhibit a broad spectrum of fungi and yeasts including A. niger and C. albicans. Medium composition had an affect on the inhibition profiles observed. This study showed that plant rhizospheres provide an excellent source of actinomycetes that produce compounds with potential against human fungal and yeast pathogens.

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American Society for Microbiology General Meeting
    • ASM 102nd (2002)