Moritella viscosus infected Atlantic salmon (Salmo salar) express genes during infection associated with quality changes after recovery

The gram-negative bacterium Moritella viscosus comprise a huge problem for farmed salmonid fish during cold periods and is the main aetiological agent of winter ulcers. Infection by M. viscosus causes excessive development of punctual lesions in the musculature of the fish, which can cover large parts of the body.

Infected fish are able to recover and regenerate new skin despite previously heavy wounding. Recovery usually occurs when the water temperature rises above 10-12 °C or the salinity of the water decrease below 12-15 ‰. Regardless of surface/skin recovery, the underlying musculature can become permanently affected and filets from formerly infected fish are usually downgraded in quality at slaughter.

The quality down grade is possibly related to deposition of connective tissue during tissue regeneration. Tissue responses following infection were examined in post smolt Atlantic salmon (Salmo salar) in a Moritella viscosus bath-challenge model. Mortality was observed two days post-challenge and wounds developed three to four days later observed as swelling of the skin.

Subsequently, muscle tissue was sampled from infected and non-infected control fish 4, 7 and 14 days post challenge. The muscle tissue samples were then subjected to real-time RT-PCR in order to measure the expression of genes involved in tissue regeneration. From infected fish, an additional muscle tissue sample was taken at a non-wounded site (internal control) in order to examine if the responses measured at the wounded site were local or systemic in the infected fish.

Healing and regeneration of tissue damage imply complex processes involving both physiological factors and immune components. Consequently, genes coding for immune factors (interleukin-1β, 8, 10) and muscle tissue growth, collagen synthesis and deposition of connective tissue were examined (transforming growth factor-β, matrix metalloproteinase-2, myostatin-1αβ, collagen-1α and connective tissue growth factor).

The results showed a highly inflammatory response at the wounded sites peaking 7 days post challenge, which decreased and was followed by up-regulation of genes coding for regenerative responses and deposition of collagen in the wounded area. Further, the results also showed that the tissue responses in infected fish were mainly localised to the wounded areas since the internal control samples were less affected by the infection.

In conclusion tissue damage have proven effects on parameters of relevance to quality.