Insights into the Contribution of Type VI Secretion Towards AHPND Pathogenesis

Abstract

AbstractThe Type VI Secretion System (T6SS) is a bacterial organelle resembling a poison-tipped spear that can be deployed by a wide range of gram-negative bacteria to intoxicate prokaryotic and eukaryotic prey. In this capacity, Type VI Secretion Systems (T6SSs) play various roles in facilitating interbacterial competition and augmenting virulence towards animal and plant hosts. Prior reports have elucidated that many strains ofVibrio parahaemolyticusthat cause Acute Hepatopancreatic Necrosis Disease (AHPND) in shrimp harbor two functional T6SSs (T6SS1 and T6SS2), leading to speculations about the contribution of these systems to disease progression. In the present study, we demonstrate the antibacterial functionality of T6SS1 and T6SS2 in a representative VPAHPNDstrain, and provide direct evidence that T6SS1 augments the lethality of this isolate against whiteleg shrimp (Litopenaeus vannamei) postlarvae under conditions relevant to commercial aquaculture. Our findings provide early evidence that Type VI Secretion (T6S) plays a role in facilitating AHPND pathogenesis, and invites additional lines of inquiry aimed at uncovering the mechanisms responsible for this contribution.Author SummaryType VI Secretion Systems (T6SSs) have drawn interest as important determinants of virulence and fitness in the bacterial taxa that utilize them. Included among these species are members of the genusVibrio, many of which have clinical and commercial relevance as pathogens or aquaculture pests.Vibrio parahaemolyticusisolates that harbor thepirA/Btoxin genes encoded on pVA1-like plasmids have been identified as a causative agent of Acute Hepatopancreatic Necrosis Disease (AHPND), an illness which results in mass mortality events in shrimp aquaculture systems. It has previously been noted that, aside from the PirA/B binary toxin, many VPAHPNDstrains harbor two T6SSs (T6SS1 and T6SS2), which they may rely on 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 VPAHPNDisolate againstL. vannameipostlarvae (PL) using a simple infection assay. Our findings indicate that T6SS1 putatively contributes to the pathogenicity of VPAHPNDagainstL. vannameiPL under warm, marine-like conditions. These initial results help to provide insight into the factors which enable AHPND pathogenesis, and may inform future efforts to develop countermeasures against this disease.