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Abstract
Vibrionaceae is a widespread family of aquatic bacteria that includes emerging pathogens and symbionts. Many Vibrionaceae harbor a type VI secretion system (T6SS), which is a secretion apparatus used to deliver toxins, termed effectors, into neighboring cells. T6SSs mediate both antibacterial and anti-eukaryotic activities. Notably, antibacterial effectors are encoded together with a gene that encodes a cognate immunity protein so as to antagonize the toxicity of the effector. The MIX (Marker for type sIX effectors) domain has been previously defined as a marker of T6SS effectors carrying polymorphic C-terminal toxins. Here, we set out to identify the Vibrionaceae MIX-effector repertoire and to analyze the various toxin domains they carry. We used a computational approach to search for the MIX-effectors in the Vibrionaceae genomes, and grouped them into clusters based on the C-terminal toxin domains. We classified MIX-effectors as either antibacterial or anti-eukaryotic, based on the presence or absence of adjacent putative immunity genes, respectively. Antibacterial MIX-effectors carrying pore-forming, phospholipase, nuclease, peptidoglycan hydrolase, and protease activities were found. Furthermore, we uncovered novel virulence MIX-effectors. These are encoded by “professional MIXologist” strains that employ a cocktail of antibacterial and anti-eukaryotic MIX-effectors. Our findings suggest that certain Vibrionaceae adapted their antibacterial T6SS to mediate interactions with eukaryotic hosts or predators.
Highlights
Vibrionaceae is a widespread family of aquatic Gram-negative bacteria, to which the genera Vibrio, Aliivibrio, Photobacterium, and others belong [1]
Our findings suggest that certain Vibrionaceae adapted their antibacterial T6SS to mediate interactions with eukaryotic hosts or predators
Whereas most MIX-effectors identified to date are predicted to mediate antibacterial toxicity [16,21,26], we recently found that a member of the MIX V clan that is encoded by V. proteolyticus, VPR01S_11_01570, carries a CNF1 toxin domain and modulates the actin cytoskeleton of eukaryotic phagocytic cells [16]
Summary
Vibrionaceae is a widespread family of aquatic Gram-negative bacteria, to which the genera Vibrio, Aliivibrio, Photobacterium, and others belong [1]. An increase in Vibrionaceae abundance and in the number of disease incidence caused by these pathogens was observed in the past half-century [6] This increase was linked to the world-wide rise in ocean water temperature, implying that a further rise in water temperature may intensify the spread of Vibrionaceae and disease occurrence [6]. Members of this family were shown to cause disease as individual clones, and as consortia [7].
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