28 December 2011 Immunological Self-Damage A second reason for textural changes of the meat after infection is the responses from the immune system against the bacteria during infection. When the tissue in the fish becomes infected, the immune apparatus is warned and activated by the presence of foreign bacteria. These are danger signals and attract cells from the immune system, which then attempt to combat the bacteria. During this battle between the immune system of the fish and the foreign bacteria, a high number of immune cells become present in the infected tissue, causing destruction of the bacteria. These immune cells do not only harm the bacteria, but also attack and kill infected host cells in the fish. This phenomenon is known as the inflammatory reaction and is essential to clear the presence of pathogenic bacteria. A successful clearance of the infection will result in a recovering stage. In this stage the fish will synthesize a high number of new tissue cells in the affected area, depending on how severe the infection and the following inflammatory reaction have been. Further, the immune response may trigger some cells to increase the synthesis of new collagen molecules, which, thereby, promotes the accumulation of collagen in the tissue and hence it will affect the texture of the fillet. Early Markers of Quality Changes Experimentally, we previously showed a link between genetic markers expressed early after infection and the textural changes observed subsequent to recovery from a M. viscosa infection (Ingerslev et al. 2010). In a laboratory trial, Atlantic salmon experimentally infected with M. viscosa, inFig. 2. Infection by Moritella viscosa generates deposition of scarring in regions of the fish, which were previously ulcerated. This is illustrated by the red filaments in the left lower site of the figure. creased expression of a number of different genes coding for proteins involved in the immunological processes and tissue regeneration were seen once the fish were infected. The observed reactions related only to the ulcers of the infected fish, while not to the neighboring tissues, which were not visibly apparent. In the ulcerated tissues, the machinery for production of collagen proteins and connective tissue was highly activated. About one week after the fish were experimentally infected the collagen-1α and connective tissue growth factor genes were activated and were expressed significantly higher in comparison to tissue without clinical signs. Hence, the textural changes were traceable in the tissue very rapidly after the infection by M. viscosa. Can Signs of Previous Disease Be Prevented? As seen in evolutionarily more advanced vertebrates such as humans, scars in fish seem to persist throughout the entire life of the fish after previous tissue damage or infection. Consequently, effects of a previous M. viscosa infection on the meat are found at slaughter even if the infection was overcome at an early life stage. However, fish are growing during their entire life, so scars obtained at a small size will seem relatively smaller when the fish has grown to a larger size. In that respect, the effects of a previous infection may be diminished over time, but a scar will never disappear completely. As pointed out earlier, vaccination protects to a high degree fish against infection caused by M. viscosa. By protecting the fish against infection the tissue is also saved from harmful bacteria and heavy immune responses leading to tissue damage. That is one way to prevent changes that the texture undergoes and, thus, decreased quality during farming. A second is antibiotic treatment, which usually will stop the infection and thus limit the tissue damage and textural influence on the meat. Conclusions The texture of the meat from salmonid species is influenced by previous infection with the bacterium M. viscosa. Collagen deposition and formation of scars in the skin and musculature of previously infected fish cause changes in texture, which, in turn, cause the meat to be downgraded at slaughter. Scarring is a consequence of the repair response in the tissue following tissue damage caused by the infecting bacteria. Regeneration of damaged and lost tissue implies synthesis of new tissue, but the scars cannot be avoided. The production of these components is detectable very rapidly following experimental infection on a genetic level. Thus, the textural changes can be predicted and have been observed in ongoing experiments. Notes 1Biological Quality Group, National Food Institute, Technical University of Denmark, Søltofts Plads, Building 221, DK-2800 Kgs Lyngby, Denmark. *Corresponding author: hain@food.dtu.dk References Bornø, G., C. Sviland, B. B. Jensen, A. Tarpai, Å. H. Garseth, H. (Continued on page 68)
RkJQdWJsaXNoZXIy MjExNDY=