Selection of bacterial populations in the mycosphere of Laccaria proxima: is type III secretion involved?

The bacterial communities in the Laccaria proxima mycosphere (soil from beneath fruiting bodies) and the corresponding bulk soil were compared by cultivation-dependent and cultivation-independent methods. To assess the distribution of type III secretion systems (TTSS), a PCR-based system for the broad detection of a highly conserved gene involved in TTSS, that is hrcR, was developed and used to assay the cultured bacteria from the L. proxima mycosphere and surrounding bulk soil. PCR-DGGE based on the 16S ribosomal RNA gene showed the selection of presumably mycosphere-specific bacterial groups in the mycosphere of L. proxima compared to the bulk soil in 3 sampling years. Moreover, plate counts revealed that the numbers of culturable heterotrophic bacteria were increased in the mycosphere as compared to the bulk soil. Strikingly, the percentage of randomly picked isolates that carried the hrcR gene showed a significant increase, from 2.8 in the bulk to 13.4 in the mycosphere soil. The increase could be mainly attributed to the emergence of a hrcR positive Pseudomonas fluorescens, denoted BS053, which constituted the most dominant species in the culturable mycosphere communities. This organism was, together with a hrcR-positive Burkholderia terrae BS110, exclusively found in mycosphere soil. Direct detection of hrcR genes using a cultivation-independent approach showed the selection of several hrcR gene types uniquely in the mycosphere, indicating the selection of several TTSS-harboring bacterial species. Thus, different bacteria were found to be enriched in the L. proxima mycosphere and TTSS can be involved in some of the interactions with the fungal host.