Vibrio parahaemolyticus RhsP represents a widespread group of pro-effectors for type VI secretion systems

A Bacterial Phage Tail-like Structure Kills Eukaryotic Cells by Injecting a Nuclease Effector

Many bacteria interact with target organisms using syringe-like structures called contractile injection systems (CISs). CISs structurally resemble headless bacteriophages and share evolutionarily related proteins such as the tail tube, sheath, and baseplate complex. In many cases, CISs mediate trans-kingdom interactions between bacteria and eukaryotes by delivering effectors to target cells. However, the specific effectors and their modes of action are often unknown. Here, we establish an ex vivo model to study an extracellular CIS (eCIS) called metamorphosis-associated contractile structures (MACs) that target eukaryotic cells. MACs kill two eukaryotic cell lines, fall armyworm Sf9 cells and J774A.1 murine macrophage cells, by translocating an effector termed Pne1. Before the identification of Pne1, no CIS effector exhibiting nuclease activity against eukaryotic cells had been described. Our results define a new mechanism of CIS-mediated bacteria-eukaryote interaction and are a step toward developing CISs as novel delivery systems for eukaryotic hosts.

Manipulating the type VI secretion system spike to shuttle passenger proteins

The type VI secretion system (T6SS) is a contractile injection apparatus that translocates a spike loaded with various effectors directly into eukaryotic or prokaryotic target cells. Pseudomonas aeruginosa can load either one of its three T6SSs with a variety of toxic bullets using different but specific modes. The T6SS spike, which punctures the bacterial cell envelope allowing effector transport, consists of a torch-like VgrG trimer on which sits a PAAR protein sharpening the VgrG tip. VgrG itself sits on the Hcp tube and all elements, packed into a T6SS sheath, are propelled out of the cell and into target cells. On occasion, effectors are covalent extensions of VgrG, PAAR or Hcp proteins, which are then coined "evolved" components as opposed to canonical. Here, we show how various passenger domains could be fused to the C terminus of a canonical VgrG, VgrG1a from P. aeruginosa, and be sent into the bacterial culture supernatant. There is no restriction on the passenger type, although the efficacy may vary greatly, since we used either an unrelated T6SS protein, β-lactamase, a covalent extension of an "evolved" VgrG, VgrG2b, or a Hcp-dependent T6SS toxin, Tse2. Our data further highlights an exceptional modularity/flexibility for loading the T6SS nano-weapon. Refining the parameters to optimize delivery of passenger proteins of interest would have attractive medical and industrial applications. This may for example involve engineering the T6SS as a delivery system to shuttle toxins into either bacterial pathogens or tumour cells which would be an original approach in the fight against antimicrobial resistant bacteria or cancer.

PAAR proteins act as the 'sorting hat' of the type VI secretion system

Bacteria exist in polymicrobial environments and compete to prevail in a niche. The type VI secretion system (T6SS) is a nanomachine employed by Gram-negative bacteria to deliver effector proteins into target cells. Consequently, T6SS-positive bacteria produce a wealth of antibacterial effector proteins to promote their survival among a prokaryotic community. These toxins are loaded onto the VgrG–PAAR spike and Hcp tube of the T6SS apparatus and recent work has started to document the specificity of effectors for certain spike components. Pseudomonas aeruginosa encodes several PAAR proteins, whose roles have been poorly investigated. Here we describe a phospholipase family antibacterial effector immunity pair from Pseudomonas aeruginosa and demonstrate that a specific PAAR protein is necessary for the delivery of the effector and its cognate VgrG. Furthermore, the PAAR protein appears to restrict the delivery of other phospholipase effectors that utilise distinct VgrG proteins. We provide further evidence for competition for PAAR protein recruitment to the T6SS apparatus, which determines the identities of the delivered effectors.

Comparison of Extracellular Proteins from Virulent and Avirulent Vibrio parahaemolyticus Strains To Identify Potential Virulence Factors

Vibrio parahaemolyticus is a leading seafood-borne pathogen that causes gastroenteritis, septicemia, and serious wound infections due to the actions of virulence-associated proteins. We compared the extracellular proteins of nonvirulent JHY20 and virulent ATCC 33847 V. parahaemolyticus reference strains. Eighteen extracellular proteins were identified from secretory profiles, and 11 (68.75%) of the 16 proteins in ATCC 33847 are associated with virulence and/or protection against adverse conditions: trigger factor, chaperone SurA, aspartate-semialdehyde dehydrogenase, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase, glutamate 5-kinase, alanine dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, outer membrane protein OmpV, ribosome-associated inhibitor A, chaperone protein Skp, and universal stress protein. Two nontoxic-related proteins, amino acid ABC transporter substrate-binding protein and an uncharacterized protein, were identified in JHY20. The results provide a theoretical basis for supporting safety risk assessment of aquatic foods, illuminate the pathogenic mechanisms of V. parahaemolyticus, and assist the identification of novel vaccine candidates for foodborne pathogens.

Two DsbA Proteins Are Important for Vibrio parahaemolyticus Pathogenesis

Bacterial pathogens maintain disulfide bonds for protein stability and functions that are required for pathogenesis. Vibrio parahaemolyticus is a Gram-negative pathogen that causes food-borne gastroenteritis and is also an important opportunistic pathogen of aquatic animals. Two genes encoding the disulfide bond formation protein A, DsbA, are predicted to be encoded in the V. parahaemolyticus genome. DsbA plays an important role in Vibrio cholerae virulence but its role in V. parahaemolyticus is largely unknown. In this study, the activities and functions of the two V. parahaemolyticus DsbA proteins were characterized. The DsbAs affected virulence factor expression at the post-translational level. The protein levels of adhesion factor VpadF (VP1767) and the thermostable direct hemolysin (TDH) were significantly reduced in the dsbA deletion mutants. V. parahaemolyticus lacking dsbA also showed reduced attachment to Caco-2 cells, decreased β-hemolytic activity, and less toxicity to both zebrafish and HeLa cells. Our findings demonstrate that DsbAs contribute to V. parahaemolyticus pathogenesis.

Ecological implications of gene regulation by TfoX and TfoY among diverse Vibrio species

Bacteria of the genus Vibrio are common members of aquatic environments where they compete with other prokaryotes and defend themselves against grazing predators. A macromolecular protein complex called the type VI secretion system (T6SS) is used for both purposes. Previous research showed that the sole T6SS of the human pathogen V. cholerae is induced by extracellular (chitin) or intracellular (low c‐di‐GMP levels) cues and that these cues lead to distinctive signalling pathways for which the proteins TfoX and TfoY serve as master regulators. In this study, we tested whether the TfoX‐ and TfoY‐mediated regulation of T6SS, concomitantly with natural competence or motility, was conserved in non‐cholera Vibrio species, and if so, how these regulators affected the production of individual T6SSs in double‐armed vibrios. We show that, alongside representative competence genes, TfoX regulates at least one T6SS in all tested Vibrio species. TfoY, on the other hand, fostered motility in all vibrios but had a more versatile T6SS response in that it did not foster T6SS‐mediated killing in all tested vibrios. Collectively, our data provide evidence that the TfoX‐ and TfoY‐mediated signalling pathways are mostly conserved in diverse Vibrio species and important for signal‐specific T6SS induction.