Pasteurella piscicida is the causative agent of pseudotuberculosis in cultured yellowtail,
Seriola quinqueradiata. Preservation of the bacteria without genetic changes, especially in pathogenicity, is essential not only for bacteriological and pathological studies but also for the production of highly effective bacterins for the control of the disease.
This report describes the survival and pathogendcity of
P. piscicida preserved by freeze-drying. Freeze-drying media, prefreezing methods, and storage temperatures were investigated. Freeze drying media with a serum base were assessed. Prefreezing was carried out in liquid N2, solid CO
2 with ethanol, solid CO
2, and in a freezer (-20°C). The freeze-dried bacteria were stored at 25, 4, -20 and -80°C. Virulence of the control bacteria was maintained using fish passages where the bacteria were injected into yellowtail and recovered on media. For the pathogenicity assessment, fish were infected by the waterborn route in sea water. Freeze-dried bacteria were recovered from the ampoules and cultured in media, then added to sea water.
The results showed mist desiccans was the most effective drying medium with high survival for 6 months after preservation. The mist desiccans contained 3 parts calf serum and 1 part nutrient broth with 7.5% galactose and 0.5% NaC1. Prefreezing method and storage temperature below 4°C did not affect the survival. However, storage at 25°C decreased survival at 6months after drying. Pathogenicity was maintained for at least 6 months after freeze-drying.
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