Type VI secretion system effectors: poisons with a purpose

Abstract

The type VI secretion system (T6SS) mediates interactions between a diverse range of Gram-negative bacterial species. Recent studies have led to a drastic increase in the number of characterized T6SS effector proteins and produced a more complete and nuanced view of the adaptive significance of the system. While the system is most often implicated in antagonism, in this review we consider the case for its involvement in both antagonistic and non-antagonistic behaviors. Clarifying the roles that T6S plays in microbial communities will contribute to broader efforts to understand the importance of microbial interactions in maintaining human and environmental health, and will inform efforts to manipulate these interactions for therapeutic or environmental benefit.

Human symbionts inject and neutralize antibacterial toxins to persist in the gut

“…3A; Dataset S1, Table S4). Three of these proteins are homologs of the secreted proteobacterial T6SS structural proteins Hcp [also observed in a recent study (16)] and VgrG, and the fourth contains a PAARrepeat domain typical of T6SS adaptors (9,10). Of the two remaining proteins, BF9343_1937 lacks any known T6S effector domains, and BF9343_1928 contains a MIX domain found in other T6SS effectors (17).…”

“…The gut environment is characterized by constant peristalsis and extensive niche heterogeneity, and factors previously implicated in shaping these communities (metabolites, vitamins, dietary polysaccharides, host IgA, bacteriocins) are freely diffusible (6)(7)(8). In this context, the recent identification of genes encoding type VI secretion systems (T6SSs) in the genomes of many prominent human gut symbionts was unexpected because the ability of these dynamic machines to inject toxic effectors into adjacent cells is strictly dependent on direct cell-to-cell contact (9)(10)(11)(12).…”

“…The T6SSs of Bacteroidetes share many subunits with those of Proteobacteria; these include the Hcp and TssB-TssC proteins, which interact and assemble into a contractile bacteriophage taillike structure that is required for effector translocation from donor to recipient cells (9,10). Bacteria with T6SSs lack a means for self-/non-self-discrimination; thus, sister cells inject one another with effectors.…”

Bacterial type VI secretion system facilitates niche domination

“…During this process, effector proteins are delivered, which can intoxicate the target organism. These effectors have different functions but frequently target conserved bacterial or eukaryotic cellular structures such as the cell wall, the membrane compartment, nucleic acids, or the actin cytoskeleton (9). This suggests that the system is multipurpose and might have evolved to cope with a wide variety of both bacterial and eukaryotic competitors.…”

A type 6 secretion system (T6SS) encoded gene within Salmonella enterica serovar Enteritidis contributes to virulence

“…The region of differences (ROD) 9, also referred to as SPI-19, was previously identified using a bioinformatic screen for type 6 secretion systems (T6SS) within Salmonella serovars [9]. T6SS are a protein secretion system that are directly involved in microbial competition [10]. ROD9 is intact in serovars Dublin, Weltevreden, Agona, and Gallinarum.…”