Quantification of the mycorrhization helper bacterium Streptomyces sp. AcH 505 in soil microcosms. The relative amounts of AcH 505 were measured by real-time quantitative PCR (qPCR) in the presence or absence of the mycorrhizal fungus Piloderma croceum, the soil microbial filtrate, and pedunculate oak microcuttings. In the presence of microcuttings quantification was performed with bulk soil as well as rhizosphere samples. The bars indicate the qPCR abundance of AcH 505 in the absence (a) and presence (rhizosphere (b) and bulk soil (c)) of the host plant. qPCR abundances are reported in terms of delta Ct values, which indicate the number of cycles at which the fluorescent signal exceeds the background level and surpasses the threshold established in the exponential section of the amplification plot. Error bars denote standard errors; bars with different letters are significantly different according to one-way ANOVA and the Tukey HSD test (P < 0.05). Note that co-inoculation with P. croceum stimulates the growth of AcH 505.