Hitchhikers on the fungal highway: The helper effect for bacterial migration via fungal hyphae

Abstract

Previous work in our laboratory showed that several bacterial strains, either singly or in association with other bacteria (community migration), were capable of migrating together with the saprotrophic fungus Lyophyllum sp. strain Karsten through soil microcosms. A possible involvement of the type III secretion system (TTSS) in migration was indicated. In this study, we addressed the basis of the community migration, which might lie in a migration helper effect exerted by particular single-strain migrators on other members of the community. Different culturing (plating) as well as culture-independent (PCR-DGGE) methods were applied to assess the effects of putative bacterial helpers in the migration. We used, as a model, the migration-proficient Burkholderia terrae BS001 as the canonical helper strain. PCR-DGGE analysis of the soil system with or without added strain BS001, revealed that the latter consistently stimulated the migration of different bacterial species through the soil. This was observed both following introduction of the organism to a bacterial community from soil and on the basis of a similar organism that was naturally present. One strain, Dyella japonica BS003, was identified as an avid comigrator with B. terrae BS001, although it appeared to lag behind the latter strain in its migration speed. Further examination of the B. terrae BS001/D. japonica BS003 interaction at Lyophyllum sp. strain Karsten hyphae showed that the presence of the D. japonica strain did not negatively affect the growth and migration of B. terrae BS001 with the fungus. A biofilm of B. terrae BS001 was formed on the fungal hyphal front, and we postulate a role for this biofilm in the migration helper effect.