Aeromonas hydrophila AH-3 AexT is an ADP-ribosylating toxin secreted through the type III secretion system

We cloned and sequenced an ADP-ribosylating toxin (AexT) from a mesophilic Aeromonas hydrophila strain AH-3 with a type III secretion system (T3SS). This toxin only showed homology, in genes and proteins, with the first half of A. salmonicida AexT. The A. hydrophila AexT showed ADP-ribosyltransferase activity, translocation through the T3SS system, and this A. hydrophila T3SS system is inducible under calcium-depleted conditions. The A. hydrophila aexT mutant showed a slight reduction in their virulence assayed by several methods when compared to the wild-type strain, while an A. hydrophila T3SS mutant is highly reduced in virulence on the same assays. The A. hydrophila AexT is the first described and the smallest T3SS effector toxin found in mesophilic Aeromonas with a functional T3SS.