The severity of Motile Aeromonas septicemia caused by virulent Aeromonas hydrophila in channel catfish is influenced by nutrients and microbes in water

Authors: Zhang, Dunhua; Xu, Dehai; Shoemaker, Craig; Beck, Benjamin

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2019
Publication Date: 3/1/2020
Citation: Zhang, D., Xu, D., Shoemaker, C.A., Beck, B.H. 2020. The severity of Motile Aeromonas septicemia caused by virulent Aeromonas hydrophila in channel catfish is influenced by nutrients and microbes in water. Aquaculture. https://doi.org/10.1016/j.aquaculture.2019.734898.
DOI: https://doi.org/10.1016/j.aquaculture.2019.734898

Interpretive Summary: Since the first outbreak of motile Aeromonas septicemia (MAS) in farmed catfish in the southeastern United States in 2009, recurring outbreaks of MAS had resulted in the loss of millions of pounds of food-size fish annually. Fish mortalities in ponds ranged from 5 to 100%. A new pathotype of Aeromonas hydrophila, named virulent A. hydrophila (vAh), was identified and attributed to the cause of the outbreak. However, information is limited as for what nourished vAh population in pond water to reach a threshold level that caused acute outbreaks of MAS in fish. In this study, we found for the first time that vAh was able to proliferate rapidly in water and reach to a density high enough in a short period to kill a high percentage of fish when the warm water was supplied with commercial fish feeds. Findings of this study clearly suggest that, when fish were under environmental stress (such as high temperature) and had poor appetite for feed supplied in the pond, the unconsumed feed provided vAh with ample nutrients to propagate instantly and rapidly, which would result in outbreaks of MAS. Additionally, we evidently showed that a probiotic candidate, Pseudomonas mosselii, was able to protect fish against vAh infection by using the in situ culture method we developed.

Technical Abstract: Motile Aeromonas septicemia (MAS), caused by virulent Aeromonas hydrophila (vAh), has severely impacted catfish farming in the southeastern United States since 2009. Conditions that trigger the onset of MAS outbreaks remain largely unknown. In this study, the effects of nutrients and select microbes in water on proliferation of vAh and severity of MAS in channel catfish were assessed. Results of the study demonstrated that both nutrient-rich tryptic soy broth powder (TSBp, the microbiological growth medium) and the commercial fish feed supported vigorous growth of vAh in water. By addition of 6 g TSBp or 6 g fish feed to 15 L water (0.04%, w/v), vAh multiplied from 2.5 x 105 CFU/mL to approximately 4.7 x 108 CFU/mL and 2.0 x 108 CFU/mL, respectively, at 24 h post inoculation (hpi). Challenge of fish in the vAh-propagated water at 24 hpi resulted in approximately 96% and 73% mortality, respectively. Co-inoculation of vAh with a vAh-antagonistic Pseudomonas mosselii or a non-antagonistic Aeromonas veronii in water had no significant effects on vAh growth and infection; nevertheless, P. mosselii protected fish against vAh infection (with approximately 79% of relative percentage of survival) when fish were briefly pre-immersed in P. mosselii-propagated water prior to challenge. Culture of vAh in situ described in this study was shown to be a useful method for mimicking vAh growth dynamics in response to nutrients and probiotics in relatively natural environment. Findings of this study would help understand the mechanism of MAS outbreaks and facilitate research on control and prevention of MAS.