Type VI Secretion System Toxins Horizontally Shared between Marine Bacteria

Abstract

The type VI secretion system (T6SS) is a widespread protein secretion apparatus used by Gram-negative bacteria to deliver toxic effector proteins into adjacent bacterial or host cells. Here, we uncovered a role in interbacterial competition for the two T6SSs encoded by the marine pathogen Vibrio alginolyticus. Using comparative proteomics and genetics, we identified their effector repertoires. In addition to the previously described effector V12G01_02265, we identified three new effectors secreted by T6SS1, indicating that the T6SS1 secretes at least four antibacterial effectors, of which three are members of the MIX-effector class. We also showed that the T6SS2 secretes at least three antibacterial effectors. Our findings revealed that many MIX-effectors belonging to clan V are “orphan” effectors that neighbor mobile elements and are shared between marine bacteria via horizontal gene transfer. We demonstrated that a MIX V-effector from V. alginolyticus is a functional T6SS effector when ectopically expressed in another Vibrio species. We propose that mobile MIX V-effectors serve as an environmental reservoir of T6SS effectors that are shared and used to diversify antibacterial toxin repertoires in marine bacteria, resulting in enhanced competitive fitness.

Acute Hepatopancreatic Necrosis Disease-Causing Vibrio parahaemolyticus Strains Maintain an Antibacterial Type VI Secretion System with Versatile Effector Repertoires

“…First, consistent with our previous report on mobile MIX effectors that belong to the MIX V clan, these E/I pairs are flanked by transposable elements (Fig. 4) (42). Genes encoding transposase or resolvase, labeled in red in Fig.…”

“…In RIMD2210633, this module is found outside the T6SS1 cluster within the V. parahaemolyticus island 6 (vpa1254-vpa1270), a mobile element flanked by a transposase and an integrase located on chromosome 2 (42,43). Notably, a predicted toxin/antitoxin module containing a putative HNH nuclease toxin (vpa1260) and a HigA-like antitoxin (vpa1259) is also encoded within the same mobile element (as predicted by HHpred [44]).…”

“…Members of the MIX V clan were shown to be "orphan" effectors encoded outside T6SS clusters, usually in regions next to mobile genetic elements, and were thus suggested to be horizontally shared between bacteria (35, 42). One such example is vpa1263 in RIMD2210633, which is located on an apparent prophage flanked by a transposase and an integrase (35,42,43). Since none of the toxic AHPND strains seem to encode a VPA1263 homologue at the same synteny as in RIMD2210633 (site three), we tested whether any of the AHPND strains contains other mobile T6SS MIX effectors.…”

“…As a positive control for bacterial killing at 37°C, we used Vibrio alginolyticus, which employs an antibacterial T6SS2 at this temperature (42). Except for D4, none of the V. parahaemolyticus strains were able to kill V. cholerae at 37°C (see Fig.…”

Microbial Herd Protection Mediated by Antagonistic Interaction in Polymicrobial Communities

“…Our data suggest that antagonistic interactions are the driving force for the acquisition of additional effector modules, which presumably promotes fitness, expands the range of susceptible species, and reduces the chances of target cells developing resistance. Indeed, environmental isolates of the same species have been shown to carry distinct effector-immunity modules (36,44). The presence of multiple T6SS systems and associated effectors in T6SS organisms is likely reflective of the complex environments these species have encountered during evolution.…”