In vitro adhesion and infectiveness of piscirickettsia salmonis on mussel shells mytilus chilensis

The salmon industry in Chile is seriously affected by piscirickettsiosis or rickettsial salmonid sep-ticemia (SRS) that produces annual losses of over 300 million dollars. Its etiological agent Piscir- ickettsia salmonis is an intracellular facultative pathogen that was initially isolated in Chile and has been reported from other salmon-producing countries. It has been demonstrated in vitro that the microorganism attaches to the eggs through the so-called Piscirickettsial Attachment Complex. In the present work, we studied the ability of P. salmonis to bind to the surface of mussel shells (Mytilus chilensis), an endemic bivalve blue mussel from southern Chile that is cultivated in the same areas where salmon is kept. The bacterium was cultured in the CHSE-214 cell line free of antibiotics and fungistats until a 100% cytopathic effect (CPE) was produced. To study attachment, pieces of mussel shells of approximately 0.5 cm2 that had been subjected to a cleaning and disinfection process were placed in a bacterial suspension for 1,24, or 48 h at 15 °C, fixed and processed for scanning electron microscopy. Additionally, two pieces of shells subjected to 48 h of exposure were rinsed in sterile PBS buffer (pH = 7.2) and placed in a culture bottle with a confluent CHSE-214 cell monolayer, where they yielded CPE around the edge starting three days after infection to eventually producing 100% CPE after 14 days of incubation. The results demonstrate adhesion to and subsequent release of infective P. salmonis from the surface of mussel shells, a phenomenon that may have epidemiological implications since P. salmonis-exposed mollusks may serve as reservoirs and/or vectors of the agent.