Classification of a hypervirulent Aeromonas hydrophila pathotype responsible for epidemic outbreaks in warm-water fishes

Authors: RASMUSSEN-IVEY, CODY - Auburn University; HOSSAIN, MOHAMMAD - Auburn University; ODOM, SARA - Auburn University; TERHUNE, JEFFERY - Auburn University; HEMSTREET, WILLIAM - Alabama Cooperative Extension Service; Shoemaker, Craig; Zhang, Dunhua; Xu, Dehai; GRIFFIN, MATT - Mississippi State University; LIU, YONG-JIE - Nanjing Agricultural University

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2016
Publication Date: 10/18/2016
Citation: Rasmussen-Ivey, C.R., Hossain, M.J., Odom, S.E., Terhune, J.S., Hemstreet, W.G., Shoemaker, C.A., Zhang, D., Xu, D., Griffin, M.J., Liu, Y. 2016. Classification of a hypervirulent Aeromonas hydrophila pathotype responsible for epidemic outbreaks in warm-water fishes. Frontiers in Microbiology. 7:1615.

Interpretive Summary: Aquaculture industries across the world have been impacted by virulent Aeromonas hydrophila (vAh). Aeromonas hydrophila a ubiquitous Gram negative bacteria present within warm-water environments with a diverse host range (i.e., amphibians, birds, fishes, reptiles, and mammals) that results in motile Aeromonas septicemia (MAS) disease of fish. MAS produces internal and external hemorrhage and exophthalmia (pop-eye), often followed by mortality within hours or days following manifestation of clinical disease. A core genome phylogenetic (i.e., study of the relationship among genetic material or DNA) analysis on 61 Aeromonas spp. genomes, of which 40 were affiliated with A. hydrophila with 26 identified as vAh was completed to address the genomic context for the putative intercontinental transfer and subsequent geographic spread of this pathogen. Phylogenetic analyses indicate all sequence type (ST) 251 strains formed a coherent lineage affiliated with A. hydrophila. Within this lineage, conserved genetic loci unique to A. hydrophila were identified, with some genes present in consistently higher copy numbers in vAh than in non-epidemic A. hydrophila isolates. In addition, results from analyses of representative ST251 isolates support the conclusion that multiple lineages are present within US vAh isolated from Mississippi, whereas vAh isolated from Alabama appear clonal. This is the first report of genomic heterogeneity within US vAh isolates, with some Mississippi isolates showing closer affiliation with an Asian grass carp isolate than other vAh isolated in the US. To evaluate the biological significance of the identified heterogeneity, comparative virulence studies were conducted with representatives of different vAh genotypes. These studies revealed that isolate ZC1 (Asian grass carp isolate) yielded significantly lower mortality (~ 27 %) in channel catfish, relative to Alabama and Mississippi vAh isolates (greater than or equal to 60% mortality). Pathology associated with vAh in channel catfish was also described. Like other Asian vAh isolates, the ZC1 lineage contains all core genes for a complete type VI secretion system (T6SS). In contrast, more virulent US isolates retain only remnants of the T6SS (clpB, hcp, vgrG, and vasH) which may have functional implications in disease. Future research should aim to elucidate the role of these T6SS components, and the effect of a reduced or rearranged T6SS, on vAh virulence in catfish.

Technical Abstract: Lineages of hypervirulent Aeromonas hydrophila (vAh) are the cause of persistent outbreaks of motile Aeromonas septicemia in warm-water fishes worldwide. Over the last decade, this virulent lineage of A. hydrophila has resulted in annual losses of millions of tons of farmed carp and catfish in the People’s Republic of China and the United States (US). Multiple lines of evidence indicate US catfish and Asian carp isolates of A. hydrophila affiliated with sequence type 251 (ST251) share a recent common ancestor. To address the genomic context for the putative intercontinental transfer and subsequent geographic spread of this pathogen, we conducted a core genome phylogenetic analysis on 61 Aeromonas spp. genomes, of which 40 were affiliated with A. hydrophila with 26 identified as epidemic strains. Phylogenetic analyses indicate all ST251 strains form a coherent lineage affiliated with A. hydrophila. Within this lineage, conserved genetic loci unique to A. hydrophila were identified, with some genes present in consistently higher copy numbers than in non-epidemic A. hydrophila isolates. In addition, results from analyses of representative ST251 isolates support the conclusion that multiple lineages are present within US vAh isolated from Mississippi, whereas vAh isolated from Alabama appear clonal. This is the first report of genomic heterogeneity within US vAh isolates, with some Mississippi isolates showing closer affiliation with the Asian grass carp isolate ZC1 than other vAh isolated in the US. To evaluate the biological significance of the identified heterogeneity, comparative disease challenges were conducted with representatives of different vAh genotypes. These studies revealed that isolate ZC1 yielded significantly lower mortality in channel catfish, relative to Alabama and Mississippi vAh isolates. Like other Asian vAh isolates, the ZC1 lineage contains all core genes for a complete type VI secretion system (T6SS). In contrast, more virulent US isolates retain only remnants of the T6SS (clpB, hcp, vgrG, and vasH) which may have functional implications. Collectively, these results characterize a hypervirulent A. hydrophila pathotype that affects farmed fish on multiple continents.