DAO 103:111-119 (2013)  -  DOI: https://doi.org/10.3354/dao02573

Structure of Flavobacterium psychrophilum populations infecting farmed rainbow trout Oncorhynchus mykiss 

Krister Sundell1,*, Sirpa Heinikainen2, Tom Wiklund

1Laboratory of Aquatic Pathobiology, Environmental and Marine Biology, Department of Biosciences, Åbo Akademi University, Finland
2Veterinary Bacteriology Research Unit, Finnish Food Safety Authority Evira, Kuopio, Finland

ABSTRACT: Flavobacterium psychrophilum isolated from rainbow trout Oncorhynchus mykiss suffering from bacterial cold-water disease (BCWD) can dissociate into 2 morphological colony types, rough (R) and smooth (S). However, the presence of the 2 morphotypes in disease outbreaks has not yet been investigated thoroughly. We examined the occurrence of R and S morphotypes in rainbow trout from BCWD outbreaks and in unfertilized eggs from a hatchery. The isolated colony types were characterized by pulsed field gel electrophoresis (PFGE), plasmid analysis, and oxolinic acid susceptibility testing. From most outbreaks, both morphotypes were isolated, although the S type only was isolated from the majority of individual fish. PFGE analysis showed both diverse and indistinguishable genetic patterns among the concurrent morphotypes. While PFGE patterns common to both fish and egg isolates were identified, this was not always the case. Resistant and sensitive isolates of both colony types were isolated from individual disease outbreaks. The plasmid pattern was partly associated with the colony type, showing identical or completely different patterns for the R and S types isolated from the same outbreak. The study showed that within a BCWD outbreak, F. psychrophilum cells with different morphology, plasmid content, antibiotic susceptibility, and PFGE pattern can be isolated, suggesting that F. psychrophilum populations infecting rainbow trout in farm environments can be diverse and thus complicate the control of the disease.


KEY WORDS: Population structure · Phase variation · Morphotype · Bacterial cold-water disease · Oxolinic acid · Pulsed field gel electrophoresis · Plasmid profiling


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Cite this article as: Sundell K, Heinikainen S, Wiklund T (2013) Structure of Flavobacterium psychrophilum populations infecting farmed rainbow trout Oncorhynchus mykiss . Dis Aquat Org 103:111-119. https://doi.org/10.3354/dao02573

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