Mutations in Two Paraburkholderia phymatum Type VI Secretion Systems Cause Reduced Fitness in Interbacterial Competition

Abstract

Paraburkholderia phymatum is a highly effective microsymbiont of Mimosa spp. and has also been shown to nodulate papilionoid legumes. P. phymatum was found to be highly competitive both in a natural environment as well as under controlled test conditions and is more competitive for nodulation over other α- and β-rhizobial strains in a variety of different plant hosts. In order to elucidate the factors that make this bacterium highly competitive for legume infection, we here characterized the type VI secretion system (T6SS) clusters of P. phymatum. T6SSs have been shown to function as a contact-dependent injection system for both bacterial and eukaryotic cells. We identified two T6SS clusters in the genome, created respective mutant strains and showed that they are defective in biofilm formation and in interbacterial competition in vitro. While the T6SS mutants were as efficient as the wild-type in nodulating the non-cognate host Vigna unguiculata, the mutants were less competitive in in planta competition assays, suggesting that the T6SS is one of the factors responsible for the success of P. phymatum in infecting legumes by directly inhibiting competitors.

Burkholderia cenocepacia utilizes a type VI secretion system for bacterial competition

“…They are also consistent with recent observations in the related Paraburkholderia species P. phymatum, where two non-T6SS-1-type secretion systems (T6SS-3 and T6SS-b (T6SS-8)) were found to be responsible for interbacterial competition against β-rhizobia strains in vitro and as a consequence were less efficient in root nodulation (de Campos, Lardi, Gandolfi, Eberl, & Pessi, 2017).…”

“…The natural reservoir of B. cenocepacia is within the environment, particularly in the soil around plant root systems where many other bacteria compete to establish themselves. It is therefore unsurprising that B. cenocepacia has evolved a mechanism for competitive fitness against other bacteria, in a similar manner to other ubiquitous Burkholderia and Paraburkholderia species (de Campos et al, ; Schwarz et al, ). Future work will look to identify and characterize the secreted components responsible for the T6SS‐dependent antibacterial activity of B. cenocepacia .…”

Paraburkholderia phymatum STM815 σ54 Controls Utilization of Dicarboxylates, Motility, and T6SS-b Expression

“…A comparison of the phenotypic traits of several Paraburkholderia strains showed that P. phymatum STM815 produces large amounts of exopolysaccharides (EPS), is very motile, and able to outcompete other Paraburkholderia strains in vitro [20]. Our group has also shown that P. phymatum STM815 harbors two type VI secretion systems (T6SS) in its genome, which contribute to the competitive ability of this strain in vitro and in infecting plants [22]. These characteristics partly explain the success of this strain in competing with other rhizobia in the soil.…”

“…Our RNA-Seq data showed that the expression of one P. phymatum STM815 cluster coding for a T6SS (Bphy_5978-97, T6SS-b cluster) was dependent on σ 54 under nitrogen-limiting growth conditions (Figure 2C). Both P. phymatum STM815 T6SS systems were previously shown to be important for competition with other related Paraburkholderia strains such as P. diazotrophica [22]. In order to validate the RNA-Seq data, the promoters of Bphy_5978 (T6SS-b cluster) or Bphy_6115 (T6SS-3 cluster) were fused with gfp, and expression was measured in the P. phymatum STM815 wild-type and the σ 54 mutant background.…”

Identification of A Putative T6SS Immunity Islet in Salmonella Typhi

“…To investigate this hypothesis, we used conserved domain analysis and HMM homology searches to explore the function of the gene residing immediately upstream of the Typhi islet homologs identified in Figure 2 ( Table S4 , Table 2 ). For genes residing immediately upstream of t0729, t0730, t0732, and t0733 homologs, our analysis identified conserved enzymatic domains associated with established T6SS effector proteins [ 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ]. Additionally, genes residing immediately upstream of t0731 and t0735 homologs contain conserved PAAR and VgrG domains respectively with a C-terminal extension of unknown function.…”