From Welfare to Warfare: The Arbitration of Host-Microbiota Interplay by the Type VI Secretion System

Abstract

The health of mammals depends on a complex interplay with their microbial ecosystems. Compartments exposed to external environments such as the mucosal surfaces of the gastrointestinal tract accommodate the gut microbiota, composed by a wide range of bacteria. The gut microbiome confers benefits to the host, including expansion of metabolic potential and the development of an immune system that can robustly protect from external and internal insults. The cooperation between gut microbiome and host is enabled in part by the formation of partitioned niches that harbor diverse bacterial phyla. Bacterial secretion systems are commonly employed to manipulate the composition of these local environments. Here, we explore the roles of the bacterial type VI secretion system (T6SS), present in ∼25% of gram-negative bacteria, including many symbionts, in the establishment and perturbation of bacterial commensalism, and symbiosis in host mucosal sites. This versatile apparatus drives bacterial competition, although in some cases can also interfere directly with host cells and facilitate nutrient acquisition. In addition, some bacterial pathogens cause disease when their T6SS leads to dysbiosis and subverts host immune responses in defined animal models. This review explores our knowledge of the T6SS in the context of the "host-microbiota-pathogen" triumvirate and examines contexts in which the importance of this secretion system may be underappreciated.

Bioinformatic Analysis of the Campylobacter jejuni Type VI Secretion System and Effector Prediction

“…Genes encoding for the Type VI Secretion System (T6SS) are present in more than 25% of Proteobacteria (Bingle et al, 2008;Barret et al, 2013). The injection of a panel of T6SS effectors into competing bacteria promotes the fitness of T6SS-positive strains in polymicrobial environments, including the gut ecosystem (Coulthurst, 2019;Wood et al, 2020). However, the T6SS is not restricted to bacterial antagonism and can mediate host-pathogen interactions.…”

The cost of bacterial predation via type VI secretion system leads to predator extinction under environmental stress

“…The T6SS is present in nearly 25% of gram-negative pathogens, including Pseudomonas aeruginosa ( Mougous et al., 2006 ), Vibrio cholerae ( Basler et al., 2012 ; Fast et al., 2018 ), Burkholderia mallei ( Schwarz et al., 2010 ), Salmonella enterica ( Wang et al., 2011 ), and Campylobacter jejuni ( C. jejuni ) ( Noreen et al., 2018 ; Lertpiriyapong et al., 2012 ; Sha et al., 2013 ). A functional T6SS facilitates bacterial predation ( Cianfanelli et al., 2016 ), host cell adherence, and invasion ( Lertpiriyapong et al., 2012 ), conferring a selective advantage to the predator over other prey bacteria within the same environmental niche ( Dörr and Blokesch, 2018 ; Wood et al., 2020 ). However, an important question is how molecular activities of T6SS at a cellular level translate into ecological outcomes at a population level.…”

Lipopolysaccharide core type diversity in the Escherichia coli species in association with phylogeny, virulence gene repertoire and distribution of type VI secretion systems

“…T6SSs have recently been shown to contribute to the adaptation of strains to their niche, such as the intestinal microbiota. T6SS, thus, could be a major player for adaptation to different niches, each being colonized by different microbiota [ 39 ]. To that end, it is interesting to note that, based on strains isolated from different sources in Australia, Touchon et al showed strong associations between the phylogenetic structure of populations and the natural habitats of strains [ 32 ].…”