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 7966T. 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.

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Background Aeromonas hydrophila is 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 pathogenic A. 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 family Myoviridae, while Y71 and Y81 belong to the Podoviridae. 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 of A. hydrophila biofilm; and the phages G65, W3 and N21 were able to scavenge mature biofilm effectively. Phage treatments applied to the pathogenic A. hydrophila in mice model resulted in a significantly decreased bacterial loads in tissues. Conclusions Five A. hydrophila phages were isolated with broad host ranges, low latent periods, and wide pH and thermal tolerance. And the phages exhibited varying abilities in controlling A. hydrophila infection. This work presents promising data supporting the future use of phage therapy.

Digitaria sanguinalis is a widespread troublesome weed distributed all over the world. Curvularia eragrostidis QZ‐2000 is a potential candidate for biocontrol of D. sanguinalis. A phytotoxic metabolite from the culture filtrate of this fungus was extracted by ethyl acetate, isolated by bioassay‐guided column chromatography and thin layer chromatography on silica gel, characterised by ultra violet, infrared ray (IR), mass spectrometry (MS), 1H‐nuclear magnetic resonance (NMR) and 13C‐NMR spectral data analyses and identified as α,β‐dehydrocurvularin. The phytotoxin significantly inhibited seed germination of D. sanguinalis from 43 to 688 μM. The EC50 value of seed germination was 152 μM. The EC50 values of elongation of radicle and coleoptile were 102 and 172 μM, respectively. α,β‐dehydrocurvularin caused extensive necrosis on leaves of many notorious weeds at 688 μM, while maize and soybean were insensitive to it. Therefore, α,β‐dehydrocurvularin was regarded as a non‐host‐selective phytotoxin. At concentrations of 172–688 μM, α,β‐dehydrocurvularin caused a decrease in chlorophyll content. α,β‐dehydrocurvularin had stronger impacts on chlorophyll A fluorescence, photophosphorylation and Mg2+‐ATPase activity at higher concentrations. These results suggest that α,β‐dehydrocurvularin affected the photosynthetic capacity. In the present study, α,β‐dehydrocurvularin significantly inhibited mitosis of root tip cells. It supports that the α,β‐dehydrocurvularin has potential for development as a natural bioherbicide.

The growth-stimulating effects of catecholamine stress hormones have been demonstrated in many pathogens. However, catecholamine-induced growth and its underlying mechanisms remain poorly understood in Aeromonas hydrophila. The present study sought to demonstrate that norepinephrine (NE), epinephrine (Epi), dopamine (Dopa), and L-dopa stimulate the growth of A. hydrophila in iron-restricted media containing serum. NE exhibited the strongest growth stimulation, which could be blocked by adrenergic antagonists. Furthermore, it was demonstrated that NE could sequester iron from transferrin, thereby providing a more accessible iron source for utilization by A. hydrophila. The deletion of the amoA gene associated with amonabactin synthesis revealed that the amonabactin siderophore is not required for NE-stimulated growth. However, the deletion of the TonB2 energy transduction system resulted in the loss of growth promotion by NE, indicating that a specific TonB-dependent outer membrane receptor might be involved in the transport of iron from transferrin. Collectively, our data show that catecholamine sensing promotes the growth of A. hydrophila in a manner that is dependent on the TonB2 energy transduction system.