Aims:  Assessment of antimicrobial activity of the mycoparasite Coniothyrium minitans and its macrolide antibiotic macrosphelide A. Methods and Results:  Thirteen isolates of C. minitans were tested for ability to inhibit a number of filamentous fungi, yeasts, oomycetes and bacteria in agar based tests. Activity was found against some ascomycetes, basidiomycetes, oomycetes and Gram‐positive bacteria, but not against zygomycetes, yeasts or Gram‐negative bacteria tested. Six C. minitans isolates (Conio, Contans, IVT1, CM/AP/3118, B279/1, A1/327/1) were found to produce macrosphelide A in liquid culture and no other antibiotics were detected. On agar, macrosphelide A inhibited growth of some ascomycetes, basidiomycetes, oomycetes and all four Gram‐positive bacteria tested, including the medically important Staphylococcus aureus with a minimum inhibitory concentration of ≤500 μg ml−1. There was no inhibition observed against the yeasts and Gram‐negative bacteria when macrosphelide A was tested at 700 μg ml−1. Conclusions:  The spectrum and level of activity of macrosphelide A produced by C. minitans against micro‐organisms are extended markedly compared to previous reports. Significance and Impact of the Study:  Macrosphelide A was effective against Staph. aureus. Further study on the control of this bacterium is merited in view of the development of antibiotic resistance.

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