Prolonged low-salt immersion effectively controls Flavobacterium columnare infection in Murray cod Maccullochella peelii peelii.

A disease outbreak occurred in Murray cod Maccullochella peelii peelii in a recirculating aquaculture farm in Tianjin city, China, in 2019. Strain MRX-2019 was isolated and considered to be the etiological pathogen; it was identified as Flavobacterium columnare based on a 16S rDNA gene sequence analysis and physiological and biochemical tests. The effect of salinity on the growth of MRX-2019 was investigated in vitro. Salinity >4‰ (i.e. 6‰) inhibited MRX-2019 growth, whereas 8 and 10‰ salinity killed it. The effect of 4‰ salinity on F. columnare was not significant (p > 0.05). When MRX-2019-infected Murray cod were treated with 4, 6, or 8‰ salinity, the mortality rate was reduced by 8.9, 67.76, or 75.56%, respectively, compared with that of the control. However, the mortality rate increased by 7.77% at 10‰ salinity. In this study, we found that maintaining the fish in freshwater with 6-8‰ salinity effectively reduced the mortality of these fish when infected with F. columnare. The findings provide an environmentally friendly control strategy for columnaris disease in Murray cod.

Identification and Characterization of Four Autophagy-Related Genes That Are Expressed in Response to Hypoxia in the Brain of the Oriental River Prawn (Macrobrachium nipponense).

Autophagy is a cytoprotective mechanism triggered in response to adverse environmental conditions. Herein, we investigated the autophagy process in the oriental river prawn (Macrobrachium nipponense) following hypoxia. Full-length cDNAs encoding autophagy-related genes (ATGs) ATG3, ATG4B, ATG5, and ATG9A were cloned, and transcription following hypoxia was explored in different tissues and developmental stages. The ATG3, ATG4B, ATG5, and ATG9A cDNAs include open reading frames encoding proteins of 319, 264, 268, and 828 amino acids, respectively. The four M. nipponense proteins clustered separately from vertebrate homologs in phylogenetic analysis. All four mRNAs were expressed in various tissues, with highest levels in brain and hepatopancreas. Hypoxia up-regulated all four mRNAs in a time-dependent manner. Thus, these genes may contribute to autophagy-based responses against hypoxia in M. nipponense. Biochemical analysis revealed that hypoxia stimulated anaerobic metabolism in the brain tissue. Furthermore, in situ hybridization experiments revealed that ATG4B was mainly expressed in the secretory and astrocyte cells of the brain. Silencing of ATG4B down-regulated ATG8 and decreased cell viability in juvenile prawn brains following hypoxia. Thus, autophagy is an adaptive response protecting against hypoxia in M. nipponense and possibly other crustaceans. Recombinant MnATG4B could interact with recombinant MnATG8, but the GST protein could not bind to MnATG8. These findings provide us with a better understanding of the fundamental mechanisms of autophagy in prawns.