Effects and incidence of volatile organic compound interactions between soil bacterial and fungal isolates

The effects of volatile organic compounds (VOCs) produced by a random selection of soil bacteria on the growth rates and activity of several fungi were assessed using experimental systems that had only atmospheric connections. All of the bacterial isolates either stimulated or inhibited the growth rate of at least one of the fungal species. Growth rates of some of the fungi were inhibited by up to 60% in some cases and stimulated by up to 35% in others (P<0.05). No single bacterial isolate was effective against all of the fungi. Of the bacterial isolates 54% both inhibited the growth rate of some fungi and stimulated others. Many bacteria, 42% of the total, could only inhibit growth, but none were solely stimulatory (P<0.05). Growth of some fungi was inhibited within 2 d of exposure to bacterial cultures (P<0.05) and only resumed when the fungus was placed on fresh PDA plates. Contrastingly, cores taken from the growing margins of cultures did not grow (P<0.05) when placed onto PDA plates that had previously been exposed to the bacterial cultures. Laccase activity in Phanaerochaete magnoliae ceased completely on exposure to all of the bacterial isolates and was significantly reduced in Trichoderma viride. Tyrosinase activity in T. viride was not affected by any of the bacterial isolates, but activity in P. magnoliae was increased, inhibited or not affected, depending on the bacterium to which it was exposed. These VOC-mediated effects appeared to be species-specific, with each fungus responding uniquely to the products of each of the bacterial cultures. The interactions varied according to the substrate on which the organisms were cultured, and on the size and age of the bacterial population. Interactions mediated by microbially produced VOCs could be widespread in soils, and the outcome of these interactions will be determined by the microorganisms involved.