Evidence for a type III secretion system in Aeromonas salmonicida subsp. salmonicida

Area of Science:

• Microbiology
• Fish Pathology
• Bacterial Pathogenesis

Background:

• Aeromonas salmonicida subsp. salmonicida is a significant fish pathogen causing furunculosis.
• Type III secretion systems (T3SS) are crucial virulence factors in many bacterial pathogens.
• The yscV gene encodes a key component of T3SS, conserved across various bacterial species.

Purpose of the Study:

• To investigate the presence and role of a T3SS in Aeromonas salmonicida subsp. salmonicida.
• To identify T3SS components and secreted effectors in this fish pathogen.
• To determine the contribution of the T3SS to the virulence of Aeromonas salmonicida subsp. salmonicida.

Main Methods:

• Screening for T3SS homologous genes using a yscV-directed probe.
• Identification and characterization of seven open reading frames encoding T3SS homologues.
• Inactivation of the ascV gene (A. salmonicida subsp. salmonicida yscV homologue) via marker replacement mutagenesis.
• Comparison of extracellular protein profiles between wild-type and ascV mutant strains.
• Assessment of cytotoxicity against RTG-2 rainbow trout gonad cells.

Main Results:

• Seven open reading frames homologous to Yersinia T3SS proteins were identified.
• Six of these genes form a virA operon.
• The ascV mutant showed altered extracellular protein profiles compared to the wild-type.
• The ADP-ribosylating toxin AexT was identified as a T3SS-secreted protein.
• The ascV mutant exhibited no toxicity to RTG-2 cells, unlike the wild-type strain.

Conclusions:

• Aeromonas salmonicida subsp. salmonicida possesses a functional type III secretion system.
• This T3SS is essential for the secretion of virulence factors like AexT.
• The identified T3SS plays a critical role in the virulence of Aeromonas salmonicida subsp. salmonicida against fish.