Outbreaks in fish of motile Aeromonad septicemia (MAS) caused by Aeromonas hydrophila have caused a great concern worldwide. Here, for the first time, we provide two complete genomes of epidemic A. hydrophila strains isolated in China. To gain an insight into the pathogenicity of epidemic A. hydrophila, we performed comparative genomic analyses of five epidemic strains belonging to sequence type (ST) 251, together with the environmental strain ATCC 7966^T. We found that the known virulence factors, including a type III secretion system, a type VI secretion system and lateral flagella, are not required for the high virulence of the ST251 clonal group. Additionally, our work identifies three utilization pathways for myo-inositol, sialic acid and L-fucose providing clues regarding the factors that underlie the epidemic and virulent nature of ST251 A. hydrophila. Based on the geographical distribution and biological resources of the ST251 clonal group, we conclude that ST251 is a high-risk clonal group of A. hydrophila which may be responsible for the MAS outbreaks in China and the southeastern United States.

Aims: To investigate the species distribution in Aeromonas isolates from diseased fish, healthy controls and water environment in China; to evaluate the frequency of the aerolysin (aer), cytotonic enterotoxin (alt), cytotoxic enterotoxin (act), temperature‐sensitive protease (eprCAI) and serine protease (ahp) genes in Aeromonas isolates; and to determine the potential pathogenicity of these isolates. Methods and Results: Two hundred and two Aeromonas isolates from diseased fish (n = 42), healthy fish (n = 120) and water environment (n = 40) in China were identified to species levels based on sequencing of the housekeeping gene gyrB, while the distribution of five virulence factors, including aer, alt, act, eprCAI and ahp, was investigated by PCR. Aeromonas veronii (25/42; 60%) and Aeromonas hydrophila (14/42; 33%) were the species most commonly isolated from diseased fish, while Aer. veronii was the most common species in healthy fish (90/120; 75%) and water samples (25/40; 62·5%). All the five virulence genes were present in 9% (19/202), among which 10 strains were from diseased fish and nine were identified as Aer. hydrophila. For the strains carrying five virulence genes, the average 50% lethal doses (LD_50s) of strains from diseased fish were lower when compared with the strains from healthy fish and water environment. Conclusions: Aeromonas veronii is the most common species, but no significant difference exists in the isolates obtained from diseased fish and from healthy fish. However, Aer. hydrophila isolates were significantly more frequent from diseased fish than from healthy fish. aer⁺alt⁺act⁺eprCAI⁺ahp⁺ was more frequent virulence genotype in Aeromonas isolates from diseased fish than from healthy fish and water environment, and the aer⁺alt⁺act⁺eprCAI⁺ahp⁺ isolates were more virulent to zebrafish comparing to the other genetic profiles. Significant and Impact of the Study: Aeromonas species in aquatic environments are various and have considerable virulence potential, and therefore, there is a need for more careful and intensive epidemiology studies.

Background: Aeromonas hydrophilais an important water-borne pathogen that leads to a great economic loss in aquaculture. Along with the abuse of antibiotics, drug-resistant strains rise rapidly. In addition, the biofilms formed by this bacterium limited the antibacterial effect of antibiotics. Bacteriophages have been attracting increasing attention as a potential alternative to antibiotics against bacterial infections.Results: Five phages against pathogenicA. hydrophila, named N21, W3, G65, Y71 and Y81, were isolated. Morphological analysis by transmission electron microscopy revealed that phages N21, W3 and G65 belong to the familyMyoviridae, while Y71 and Y81 belong to thePodoviridae. These phages were found to have broad host spectra, short latent periods and normal burst sizes. They were sensitive to high temperature but had a wide adaptability to the pH. In addition, the phages G65 and Y81 showed considerable bacterial killing effect and potential in preventing formation ofA. hydrophilabiofilm; and the phages G65, W3 and N21 were able to scavenge mature biofilm effectively. Phage treatments applied to the pathogenicA. hydrophilain mice model resulted in a significantly decreased bacterial loads in tissues.Conclusions: FiveA. hydrophilaphages were isolated with broad host ranges, low latent periods, and wide pH and thermal tolerance. And the phages exhibited varying abilities in controllingA. hydrophilainfection. This work presents promising data supporting the future use of phage therapy.