PSEUDOMONAS-FLUORESCENS IN ASSOCIATION WITH FRUIT BODIES OF THE ECTOMYCORRHIZAL MUSHROOM CANTHARELLUS-CIBARIUS

Aerobic bacteria were isolated from fruit bodies of Cantharellus cibarius to study bacterial influence on growth and development of mycorrhizal and vegetative hyphae. Comparisons were made with seven other mushroom species. The number of bacteria in the tissue of C. cibarius was 2.4 x 10(6) g-1 F.W. This population size was of the same magnitude as in the soil of its growing site and in Hydnum rufescens, but 100-1000 times greater than in the agarics studied. The dominating bacterium was Pseudomonas fluorescens biovar. 1. The population of P. fluorescens changed from 12 % of the total bacterial population in soil, to 78 % in fruit bodies of C. cibarius. Strains were tested with C. cibarius in vitro to study their effect on fungal growth. No growth stimulating strains were found. Non-inhibitory strains of P. fluorescens were inoculated in artificial ectomycorrhiza systems of C. cibarius with Picea abies. In TEM the naturally occurring bacteria in fruit bodies were found embedded in mucus and proliferating in the interhyphal spaces. In the mycorrhiza experiment the bacteria were only found on the surface of the mantle. Even though the bacteria are motile, they can not actively penetrate the fungal mantle. P. fluorescens is probably incidentally incorporated in the fruit body during its development. In King's B broth, the bacterium did not grow at pH 4, although the average pH (H2O) of the growing site of C. cibarius was 4.1. It is suggested that growth of P. fluorescens in the immediate environment of the vegetative mycelium might be favoured by its higher pH (5.8-6.2) and trehalose.