Virulence stability in Flavobacterium psychrophilum after storage and preservation according to different procedures

Experimental infections and lethal dose 50% (LD 50) evaluation were conducted in rainbow trout fingerlings, using a virulent strain of Flavobacterium psychrophilum processed and stored or maintained in different ways; lyophilisation, freezing at -80 degrees C, maintenance in enriched Anacker and Ordal (EAO) medium at 4 degrees C, revival and subsequent in vivo passages in fish. Experiments were performed 1, 8 and 23 months after storing the bacteria. Out of a total of 12 cultures revived for experimentation, one failed to grow and another was found to express modified properties including decreased virulence in spite of in vivo passages. In all other cases, whatever the conditions of preservation, virulence was fairly well maintained after 1 and 8 months of storage. In the last test, after 23 months, the bacteria maintained in the EAO medium at 4 degrees C were found significantly attenuated. Conversely, lyophilised and frozen bacteria only expressed a slight increase in LD0. It was concluded that virulent strains of F. psychrophilum were likely to retain their properties without special provisions within limited periods of time, and that both lyophilisation and freezing at -80 degrees C were reliable methods for long-term preservation of virulence.