Columnaris disease caused by Flavobacterium columnare is a problem in fish farming worldwide. During the last 15 yr, outbreaks have started to emerge in Finland. Flavobacterium columnare Type Strain NCIMB 2248 T and 30 Finnish F. columnare isolates were studied using analysis of 16S rDNA by restriction-fragment length polymorphism (16S RFLP), length heterogeneity analysis of polymerase chain reaction (LH-PCR) products, automated ribosomal intergenic spacer analysis (ARISA), and 16S rDNA sequence analysis. All isolates fell into RFLP Genomovar I and had the same length in the LH-PCR analysis. Based on ARISA, 8 genetically different strains were selected for further analyses. The growth of these strains under different temperatures, NaCl concentrations, and pH values was tested. The Finnish F. columnare strains did not grow at NaCl concentrations >0.1% or at pH values ≤6.5, and they were susceptible to several antimicrobial agents, but not to Polymyxin B or neomycin. These findings may aid in development of methods for disease management at fish farms.KEY WORDS: Flavobacterium columnare · ARISA · RFLP · 16S rRNA gene sequencing Resale or republication not permitted without written consent of the publisherDis Aquat Org 70: [55][56][57][58][59][60][61] 2006 tives for the prevention and treatment of the disease. The use of oxytetracycline is the only method practically applied for the treatment of columnaris disease in Finland, so there is a real threat of development of resistant strains. Therefore, physiological testing and genetic typing of F. columnare isolates is of great importance for understanding the increasing severity of outbreaks and for designing disease management strategies. Tolerance to salinity is of special interest, since columnaris disease has not been reported from coastal areasof Finland, where the salinity of the brackish water is between 2 and 7 ‰. However, F. columnare Type Strain NCIMB 2248 T (National Collection of Industrial, Marine, and Food Bacteria) is known to grow in media with 0.5% NaCl (Bernardet & Grimont 1989).In the present study, we investigated 30 Finnish Flavobacterium columnare isolates obtained from disease outbreaks in northern and central Finland, in order to select representative strains for further analysis. The molecular diversity of these strains was studied using 16S rDNA RFLP, LH-PCR (length heterogeneity analysis of polymerase chain reaction products, Suzuki et al. 1998), and ARISA (automated ribosomal intergenic spacer analysis, Fisher & Triplett 1999). Using this information, 8 genetically different strains were selected for the physiological analysis. The growth patterns of 8 selected strains were studied under different temperatures, salt concentrations, and pH, as well as the susceptibility of the strains to various antibiotics, to gather information for developing potential disease management strategies. MATERIALS AND METHODSBacterial strains and DNA extraction. Thirty Flavobacterium columnare strains isolated from disease outbreaks at Finnish fis...

Aims:  Colony morphology variants of fish pathogenic Flavobacterium columnare were studied to clarify the role of colony morphology change in the virulence of the bacterium. Typical rhizoid colony (Rz) variants are virulent and moderately adherent, nonrhizoid rough (R) colony variants are nonvirulent and highly adherent, and soft colony (S) variants are nonvirulent and poorly adherent. Methods and Results:  Chondroitin AC lyase activity, adhesion to polystyrene at different temperatures and after modification of bacterial surface, and lipopolysaccharide (LPS) profiles of the variants were studied. The chondroitinase activity was significantly higher in the virulent, rhizoid variants than in the rough variants of the same strain. Temperature significantly increased the adhesion of rhizoid variants up to 20°C. Modification of bacterial surface suggested that adhesion molecules contain both carbohydrates and proteins. LPS did not differ between the variants of the same strain. Conclusions:  The results suggest that in Fl. columnare both rhizoid colony morphology and high chondroitinase activity are needed for virulence and that temperature may promote the adhesion of the virulent variants to surfaces at fish farms. Significance and Impact of the Study:  New information is produced on the virulence mechanisms of Fl. columnare and the reasons behind the survival of the bacterium at fish farms.

Intensive aquaculture conditions expose fish to bacterial infections, leading to significant financial losses, extensive antibiotic use and risk of antibiotic resistance in target bacteria. Flavobacterium columnare causes columnaris disease in aquaculture worldwide. To develop a bacteriophage-based control of columnaris disease, we isolated and characterized 126 F. columnare strains and 63 phages against F. columnare from Finland and Sweden in 2017. Bacterial isolates were virulent on rainbow trout (Oncorhynchus mykiss) and fell into four previously described genetic groups A, C, E and G, with genetic groups C and E being the most virulent. Phage host range studied against a collection of 227 bacterial isolates (from 2013 to 2017) demonstrated modular infection patterns based on host genetic group. Phages infected contemporary and previously isolated bacterial hosts, but bacteria isolated most recently were generally resistant to previously isolated phages. Despite large differences in geographical origin, isolation year or host range of the phages, whole-genome sequencing of 56 phages showed high level of genetic similarity to previously isolated F. columnare phages (Ficleduovirus, Myoviridae). Altogether, this phage collection demonstrates a potential for use in phage therapy.