Hubert C (2020). Characterising the role of Vibrio vulnificus type 6 secretion systems 1 and 2 in an in vivo oyster model.
Abstract: Characterising the role of Vibrio vulnificus type 6 secretion systems 1 and 2 in an in vivo oyster model
Vibrio vulnificus is a significant human pathogen commonly isolated from temperate marine environments, where it is particularly abundant within filter-feeding shellfish. V. vulnificus is currently increasing in prevalence, theorised to be due to climate change facilitating V. vulnificus growth in previously inhospitable environments. Infection of susceptible individuals with V. vulnificus typically results in either primary septicaemia or necrotic wound infection, depending upon the route of entry, and frequently results in death if not treated rapidly. Two type 6 secretion systems (T6SS) have been identified in V. vulnificus, termed the T6SS1 and the T6SS2. The T6SS is a molecular syringe utilised to inject cytotoxic effector proteins into neighbouring cells. Whilst the T6SS2 is present in all sequenced V. vulnificus strains, only a subset possesses the T6SS1. Previous bacterial co-culture killing assays between T6SS1+ and T6SS1- V. vulnificus strains demonstrated thermoregulated T6SS1-mediated killing of T6SS1- strains. This study further characterised the role of both the T6SS1 and the T6SS2 in vitro. In vitro co-culture assays demonstrated that both the T6SS1 and the T6SS2 have antibacterial killing activity at the environmentally representative temperature of 21 °C. This is the first characterised role for the T6SS2 of V. vulnificus. No anti-eukaryotic activity was observed following co-culture with the phagocytic amoeba, Dictyostelium discoideum, suggesting that T6SS activity is purely antibacterial. In vitro bacterial co-culture assays were replicated in vivo using an oyster model. To facilitate high-level uptake of bacterial strains of interest by oysters, an artificial marine snow model was developed where bacteria were incorporated into easily ingested phytoplankton aggregates. Uptake of bacteria from artificial marine snow was extremely successful, resulting in bacterial loads within oysters significantly greater than achieved by any study to date. Using this model, this study was able to demonstrate that V. vulnificus utilises both the T6SS1 and the T6SS2 to target and kill neighbouring bacteria, in both an intra and inter-species manner. This data suggests that the T6SSs of V. vulnificus play a key role in V. vulnificus ecology and the dynamics between bacterial populations in vivo.