VasH Contributes to Virulence of Aeromonas hydrophila and Is Necessary to the T6SS-mediated Bactericidal Effect.

Jihong Li,Yang Zhou,Zhihao Wu,Dan-Dan Chen,Yong-An Zhang,Changsong Wu

doi:10.3389/fvets.2021.793458

Abstract

Aeromonas hydrophila is a Gram-negative bacterium that is commonly distributed in aquatic surroundings and has been considered as a pathogen of fish, amphibians, reptiles, and mammals. In this study, a virulent strain A. hydrophila GD18, isolated from grass carp (Ctenopharyngodon idella), was characterized to belong to a new sequence type ST656. Whole-genome sequencing and phylogenetic analysis showed that GD18 was closer to environmental isolates, however distantly away from the epidemic ST251 clonal group. The type VI secretion system (T6SS) was known to target both eukaryotic and prokaryotic cells by delivering various effector proteins in diverse niches by Gram-negative bacteria. Genome-wide searching and hemolysin co-regulated protein (Hcp) expression test showed that GD18 possessed a functional T6SS and is conditionally regulated. Further analysis revealed that VasH, a σ54-transcriptional activator, was strictly required for the functionality of T6SS in A. hydrophila GD18. Mutation of vasH gene by homologous recombination significantly abolished the bactericidal property. Then the virulence contribution of VasH was characterized in both in vitro and in vivo models. The results supported that VasH not only contributed to the bacterial cytotoxicity and resistance against host immune cleaning, but also was required for virulence and systemic dissemination of A. hydrophila GD18. Taken together, these findings provide a perspective for understanding the VasH-mediated regulation mechanism and T6SS-mediated virulence and bactericidal effect of A. hydrophila.

Highlights

Aeromonas hydrophila is an opportunistic pathogen widespread in aquatic environments
Aeromonas hydrophila is ubiquitous in various aquatic environments and causes disease in fish, reptiles, amphibians, and humans [48]
Observation with the transmission electron microscope clearly showed that A. hydrophila GD18 possesses polar flagellum

Summary

Introduction

Aeromonas hydrophila is an opportunistic pathogen widespread in aquatic environments. This bacterium could cause multiple diseases in different animal species, such as fish, amphibians, reptiles, and humans [1]. A. hydrophila can cause outbreaks of motile Aeromonas septicemia (MAS) with symptoms including reddened fins, inflammation of the anus, diffuse hemorrhages on the skin, exophthalmia, and abdominal swelling [2]. This pathogen has frequently caused a high. MAS caused by A. hydrophila has hindered the rapid development of carp industry in China and catfish industry in the United States [4, 5]. Increased incidence of infection and the broad spectrum of antibiotic resistance has made A. hydrophila a severe threat to the aquaculture industry

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