Experimental infection of Flavobacterium psychrophilum in fins of Atlantic salmon Salmo salar revealed by scanning electron microscopy

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Abstract
Infections caused by Flavobacterium psychrophilum include 'bacterial coldwater disease' (BCWD) and 'rainbow trout fry syndrome' (RTFS), which are severe diseases that can cause high mortality and significant losses in hatchery-reared salmonids worldwide. Usually, these conditions start with necrosis along the edge of the fins. As the infection progresses, both the fish surface and the internal organs can be involved. The aetiological agent produces a Ca-dependent protease that can be responsible for some of the pathogenic responses, although the precise nature of the response remains to be elucidated. Atlantic salmon Salmo salar were experimentally infected by E psychrophilum in order to investigate the bacterial invasion in the fin tissues by scanning electron microscopy. The images showed numerous bacteria embedded in the mucous layer when this remained on the tegument. In other zones without mucus, it was observed that bacteria were present on the axis of fin rays, but not on the epidermal surface. The material on these axes was largely eroded by tubular boreholes, and bacterial rods could be seen in these perforations. EDX (Energy Dispersive X-ray) microanalysis of the axis of the fin rays showed significant amounts of P and Ca, revealing the ossification of the ray axis. The protease activity could explain the formation of the tubular boreholes, allowing the bacteria the necessary Ca for the activation of the enzyme. The erosion pattern suggests that the gliding motility of F psychrophilum could be involved in this burrowing ability.