Dedicated Protocol for Ultrastructural Analysis of Farmed Rainbow Trout (Oncorhynchus mykiss) Tissues with Red Mark Syndrome: The Skin—Part One

: The present study aims to provide a specific protocol for transmission electron microscopy of a sample of skin of rainbow trout affected by red mark syndrome (RMS). The red mark syndrome is a skin disease that affects the rainbow trout (Oncorhynchus mykiss). The disease, probably due to the Midichloria-like organism infection, is not lethal, but morbidity can reach up to 60%, leading to significant economic impact associated with the downgrading of the commercial product, increased labor, and susceptibility to secondary infections. The ultrastructure analyses allowed an earlier study to identify the presence of scattered microorganisms characterized by an oval shape, mainly in the cytoplasm of the cells. The protocol developed in this study will be instrumental in visualizing the ultrastructure of the microorganism, which is probably responsible for red mark syndrome infection.