Insights into the Contribution of Type VI Secretion Towards AHPND Pathogenesis

The Type VI Secretion System (T6SS) is a bacterial organelle that resembles a poison-tipped spear and is deployed by a wide range of Gram-negative species to intoxicate prokaryotic and eukaryotic prey. In this capacity, T6SSs are implicated in facilitating interbacterial antagonism and host-microbe interactions. Past reports have elucidated that many strains of Vibrio parahaemolyticus which cause Acute Hepatopancreatic Necrosis Disease (AHPND) in shrimp harbor two functional T6SSs (T6SS1 and T6SS2), leading to speculation about the contribution of these systems to disease progression. In the present study, we confirm the antibacterial functionality of T6SS1 and T6SS2 in the representative AHPND-causing V. parahaemolyticus (VPAHPND) strain 13-306/D4. Employing a small-scale shrimp infection model, we then demonstrate that T6SS1 enhances the lethality of this isolate against whiteleg shrimp (Litopenaeus vannamei) postlarvae under warm, marine-like conditions. We additionally describe a novel antibacterial toxin/immunity pair encoded on pVA-like plasmids that features a putative colicin/bacteriocin effector domain. Our findings provide empirical evidence that Type VI Secretion (T6S) contributes to AHPND pathogenesis under conditions that are relevant to commercial aquaculture, inviting further studies to clarify the role of T6S as a virulence determinant. Author summaryType VI Secretion Systems (T6SSs) have drawn interest as versatile organelles that augment virulence and competitive fitness in myriad bacterial species. Included among these species are members of the genus Vibrio, many of which have clinical and commercial importance as pathogens or aquaculture pests. Isolates of Vibrio parahaemolyticus that harbor the pirAvp and pirBvp toxin genes encoded on pVA1-like plasmids have been identified as a causative agent of Acute Hepatopancreatic Necrosis Disease (AHPND), an illness that results in mass mortality events in shrimp aquaculture systems. It has been noted that, aside from PirA/Bvp, many VPAHPND strains harbor two T6SSs (T6SS1 and T6SS2) which may be employed to outcompete and displace the native shrimp microbiome during colonization. In this study, we have directly assessed the effects of inactivating T6SS1 and T6SS2 on the lethality of a representative VPAHPND isolate against whiteleg shrimp (Litopenaeus vannamei) postlarvae using an immersion challenge infection assay. Our findings indicate that T6SS1 contributes to the virulence of VPAHPND against L. vannamei under warm, marine-like conditions, although further studies are needed to determine the underlying mechanisms. The present study helps to elucidate the factors involved in AHPND pathogenesis, thereby informing future efforts to develop countermeasures against this disease.