Abstract
Burkholderia cenocepacia is an opportunistic bacterial pathogen that poses a significant threat to individuals with cystic fibrosis by provoking a strong inflammatory response within the lung. It possesses a type VI secretion system (T6SS), a secretory apparatus that can perforate the cellular membrane of other bacterial species and/or eukaryotic targets, to deliver an arsenal of effector proteins. The B. cenocepacia T6SS (T6SS‐1) has been shown to be implicated in virulence in rats and contributes toward actin rearrangements and inflammasome activation in B. cenocepacia‐infected macrophages. Here, we present bioinformatics evidence to suggest that T6SS‐1 is the archetype T6SS in the Burkholderia genus. We show that B. cenocepacia T6SS‐1 is active under normal laboratory growth conditions and displays antibacterial activity against other Gram‐negative bacterial species. Moreover, B. cenocepacia T6SS‐1 is not required for virulence in three eukaryotic infection models. Bioinformatics analysis identified several candidate T6SS‐dependent effectors that may play a role in the antibacterial activity of B. cenocepacia T6SS‐1. We conclude that B. cenocepacia T6SS‐1 plays an important role in bacterial competition for this organism, and probably in all Burkholderia species that possess this system, thereby broadening the range of species that utilize the T6SS for this purpose.
Highlights
Bacteria utilize many systems to establish a niche, including mech‐ anisms to exploit eukaryotic organisms and/or to compete effec‐ tively with other bacterial species colonizing the same ecosystem
The H111 and K56‐2 tssM mutants were sub‐ jected to a complementation analysis, whereby TssD secretion could be restored in both strains by introduction of a plasmid expressing tssM (Figure 3b). These results indicate that B. cenocepacia isolates H111 and K56‐2 have an active T6SS‐1 under standard lab‐ oratory conditions
As T6SS‐dependent effector genes in other species are often located within close proximity to tssI genes (Lien & Lai, 2017), we used the predicted amino acid sequences of protein products encoded within close proximity to the ten intact tssI genes and one disrupted tssI gene (BCAL2503) present within the B. cenocepacia J2315 genome as que‐ ries in BLASTP searches to identify putative functional domains and homology to proteins belonging to established T6SS effector super‐ families (Appendix Figure A6)
Summary
Bacteria utilize many systems to establish a niche, including mech‐ anisms to exploit eukaryotic organisms and/or to compete effec‐ tively with other bacterial species colonizing the same ecosystem. In many T6SS‐con‐ taining bacteria, these targets are other competing species of bac‐ teria, and so the system plays a major role in bacterial competition (Diniz & Coulthurst, 2015; Hood et al, 2010; MacIntyre, Miyata, Kitaoka, & Pukatzki, 2010; Schwarz et al, 2010). Such T6SS‐depen‐ dent competition can occur in a variety of environments, including plant hosts (Ma, Hachani, Lin, Filloux, & Lai, 2014) or the mamma‐ lian gut (Chassaing & Cascales, 2018; Sana et al, 2016; Zhao, Caro, Robins, & Mekalanos, 2018). The contribution of the T6SS‐1 to pathogenesis in three established eukaryotic models of B. cenoce‐ pacia infection was investigated, but our results indicated that the system does not contribute to pathogenesis in these models
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