Chemotaxis and adherence to fungal surfaces are key components of the behavioral response ofBurkholderia terraeBS001 to two selected soil fungi

Abstract

Burkholderia terrae BS001 has previously been proposed to be a 'generalist' associate of soil fungi, but its strategies of interaction have been largely ignored. Here, we studied the chemotactic behavior of B. terrae BS001 towards Lyophyllum sp. strain Karsten and Trichoderma asperellum 302 and the role of fungal surface molecules in their physical interaction with the bacteria. To assess the involvement of the type 3 secretion system (T3SS), wild-type strain BS001 and T3SS mutant strain BS001-ΔsctD were used in the experiments. First, the two fungi showed divergent behavior when confronted with B. terrae BS001 on soil extract agar medium. Lyophyllum sp. strain Karsten revealed slow growth towards the bacterium, whereas T. asperellum 302 grew avidly over it. Both on soil extract and M9 agar, B. terrae BS001 and BS001-ΔsctD moved chemotactically towards the hyphae of both fungi, with a stronger response to Lyophyllum sp. strain Karsten than to T. asperellum 302. The presence of a progressively increasing glycerol level in the M9 agar enhanced the level of movement. Different oxalic acid concentrations exerted varied effects, with a significantly raised chemotactic response at lower, and a subdued response at higher concentrations. Testing of the adherence of B. terrae BS001 and BS001-ΔsctD to Lyophyllum sp. strain Karsten and to cell envelope-extracted ceramide monohexosides (CMHs) revealed that CMHs in both conidia and hyphae could bind strain BS001 cells. As BS001-ΔsctD adhered significantly less to the CMHs than BS001, the T3SS was presumed to have a role in the interaction. In contrast, such avid adherence was not detected with T. asperellum 302. Thus, B. terrae BS001 shows a behavior characterized by swimming towards Lyophyllum sp. strain Karsten and T. asperellum 302 and attachment to the CMH moiety in the cell envelope, in particular of the former.

The Role of Hydrophobicity and Surface Receptors at Hyphae of Lyophyllum sp. Strain Karsten in the Interaction with Burkholderia terrae BS001 – Implications for Interactions in Soil

“…In a recent paper, we provide the arguments for a focus on CMHs as dynamic molecular ‘rafts’ that can become surface-exposed in fungal hyphae (Haq et al, 2016). We thus questioned the putative role of such CMH ‘anchors’ as sites for attachment of mycosphere bacteria.…”

“…From the bacterial side, a role for the type 3 secretion system (T3SS) and/or for type 4 pili has been proposed (Yang et al, 2016). However, there has so far been no clue as to the existence of any dedicated ‘receptor’ site at the fungal cell surface, even though Haq et al (2016) recently found evidence for the presence of ceramide monohexosides (CMH) as molecular ‘rafts’ in the cell surface of Trichoderma asperellum 302. In previous studies, we have reported the finding of biofilm-like structures (cell agglomerates) consisting of B. terrae BS001 cells at the hyphae of Lyophyllum sp.…”

Gene mobility in microbiomes of the mycosphere and mycorrhizosphere –role of plasmids and bacteriophages

“…Figure 1 gives an impression of the stimulatory effects of soil fungi on hypha-associated bacteria. The end result is often an enhanced spatial proximity between the local cell populations, which may culminate in the formation of biofilms or similar aggregations (Berthold et al 2016;Haq et al 2016). In theory, such enhanced cell-to-cell proximity, equivalent to raised cell densities (expressed per unit soil volume) will enhance the frequencies of (genetic) interactions between the cells.…”

“…hospita cluster) have been encountered as prime and consistent colonizers of fungal tissue in soil. In a key laboratory study, Haq et al (2016) recently provided evidence for the high affinity and chemotactic movement of one such organism, P. terrae BS001, toward the hyphae of the soil saprotrophs Lyophyllum sp. strain Karsten and Trichoderma asperellum.…”

“…Interestingly, an attractant role for several small carbonaceous compounds present at fungal surfaces, e.g. glycerol and oxalic acid, was found (Haq et al 2016). Here, we will place a focus on mycosphere-derived Paraburkholderia as the model/paradigm organism, in order to critically examine the potential of HGT as a mechanism that enhances the adaptability of these organisms to conditions of the mycosphere.…”