Bacteria associated with Suillus grevillei sporocarps and ectomycorrhizae and their effects on in vitro growth of the mycobiont

Twenty seven bacterial species were isolated from both the sporocarps of Suillus grevillei and the ectomycorrhizae of Suillus grevillei-Larix decidua. The genera Pseudomonas, Bacillus and Streptomyces were predominant. Several species were common to both the sporocarps and the ectomycorrhizae. Dual culture trials between Gram-positive, Gram-negative, Streptomyces and five different isolates of S. grevillei showed several behavior patterns depending on the bacterial group, the fungal isolate and the time. Gram-positive bacteria seldom stimulated fungal growth. Among Gram-negative bacteria, Pseudomonas fluorescens strain 70 and Pseudomonas putida strain 42 showed the greatest enhancement of growth. Streptomyces always caused significant inhibition of the fungus. Bacterial supernatants never significantly stimulated fungal growth; volatile metabolites frequently enhanced fungal growth but seldom significantly. Most of the bacterial isolates produced siderophores. The results obtained suggest for some bacterial strains a very high fungus selectivity at the intraspecific level.