Aeromonas hydrophila is a Gram-negative pathogen that causes motile aeromonas septicemia (MAS) in farm-raised catfish, which is the largest aquaculture industry in the U.S. Although it is historically considered a secondary pathogen, MAS outbreaks caused by a clade of virulent A. hydrophila (vAh) since 2009 have resulted in significant loss of market-size catfish in Mississippi and Alabama. vAh isolates have unique biochemical and molecular phylogenetic features. In particular, comparative genomics revealed that the vAh isolates have approximately 55 unique and distinguishable genomic features compared to historical reference A. hydrophila . Further analysis revealed that v Ah is a distinct monophyletic clade within the A. hydrophila species, but it has at least two distinct subclades: one predominating in West Alabama and one occurring in the Mississippi Delta.
A nalysis of the 55 v Ah-unique genetic regions revealed 313 predicted v Ah-specific genes; 35% of the unique genes are located within predicted genomic islands, suggesting their acquisition through lateral gene transfer. The v Ah-associated regions also encode predicted prophage elements, and 34 of the genes encode potential virulence factors. Several of the unique genes encode predicted surface or secreted proteins. Th e coding regions of eight v Ah-specific surface proteins (fimbrial and outer membrane proteins) were amplified from A. hydrophila strain ML09-119 genomic DNA, and recombinant proteins were expressed. Vaccination with some of these recombinant proteins provided significant protection against MAS, and these proteins also demonstrate potential for use in a recombinant vaccine strategy.
Figure 1. Average nucleotide identities (ANI) of A. hydrophila genomes and phylogenetic tree based on core genome comparison. vAh strains share >99.8% ANI.
In this presentation, a summary of comparative genomics between the vAh clade and other A. hydrophila strains will be presented. An example of how comparative genomics can be leveraged for vaccine development will be discussed. Comparative genomics of another Aeromonas species, A. veronii, will also be presented, which revealed evidence for a virulent pathotype affecting aquaculture globally.