The type three secretion system facilitates migration of Burkholderia terrae BS001 in the mycosphere of two soil-borne fungi

Abstract

The type three secretion system (T3SS) is known to play a critical role in several bacterial-eukaryotic cell interactions. Recent indirect evidence has also pointed to a role of this system in bacterial-fungal interactions in soil. In the current study, we examine if the T3SS of the fungal-interactive Burkholderia terrae strain BS001 can aid in the interaction of this bacterium with two soil fungi, i.e., Lyophyllum sp. strain Karsten and Trichoderma asperellum 302. We first analyzed the T3SS of strain BS001 and then constructed a knockout mutant of the essential sctD gene. The selected sctD mutant strain did not show any differences to the wild-type strain with respect to its growth and nutrient utilization behavior, excluding polar effects of the mutation. Then, the migration ability of the sctD mutant strain along with the hyphae of Lyophyllum sp. strain Karsten growing through presterilized soil was tested, revealing hampered comigration as compared to the wildtype strain. The effect was also observed with T. asperellum 302. However, the migration impairment was only noticed in mixed-inoculation experiments, whereas it remained unnoticed when the two strains were inoculated in separate. These data demonstrate that the T3SS assists B. terrae BS001 in its interaction with two soil fungi, without being essential for these interactions. As far as we know, this is the first time that the role of a T3SS in the comigration of bacteria along with soil-exploring fungi is verified directly.

Migration of Paraburkholderia terrae BS001 Along Old Fungal Hyphae in Soil at Various pH Levels

“…We used another soil (a loamy sand soil taken from an agricultural field denoted B, with native pH of 5.3), in microcosms. To understand the contribution of the flagellar, the T3SS, and the T4P systems to the survival and movement of strain BS001, we included, next to the wild type, the respective mutant strains of these systems (denoted BS001 ΔfliF , BS001 ΔsctD , and BS001 ΔpilN [4, 5]) in all experiments. The inclusion of the mutants was justified based on previous indications that pH affects the expression and/or function of the T3SS [15–17] and T4P/twitching motility [18, 19], whereas the flagellum is essential for strain BS001 migration in the mycosphere [4].…”

“…Three mutant strains were derived from it. These were P. terrae BS001 ΔsctD (mutant in the T3SS [5]), P. terrae BS001 ΔpilN (mutant in T4P [4]), and P. terrae BS001 ΔfliF (mutant in the flagellar system [4]). The fungal host used in this study was the aforementioned Lyophyllum sp .…”

“…Soil pH was measured in water (soil/water = 1:5, w / v ). Following these soil pH modulations, soil microcosms were prepared, using three-compartment petri dishes in accordance with previous studies [2, 4, 5]. Briefly, one of the compartments was filled with OFA, and the other two with either one of the pretreated B soil portions (adjusting soil moisture content to 12 or 17%, corresponding to 42 or 60% of water holding capacity [WHC], measured before the soil microcosm preparation to make the experimental conditions consistent).…”

“…In this context, we recently affirmed that P. terrae BS001 critically relies on flagellum-driven swimming motility for its “forward” dispersal along with fungal hyphae [4]. In contrast, the presence of a type 3 secretion system (T3SS) or of type 4 pili (T4P) was not critical to bacterial co-migration with the fungus, as these systems merely enhanced the flagellar-driven dispersion along fungal hyphae [4, 5]. …”

“…Therefore, we surmised that a critical balance exists between (positive) soil pH effects on proton-motive force-driven motility and those (negative) on bacterial fitness. We thus investigated the potential relationship of soil pH with bacterial cell motility in the mycosphere using a well-established three-compartment petri dish based soil microcosm [2, 4, 5]. …”