Flavobacterium psychrophilum, causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), causes substantial economic losses worldwide, particularly in salmonid species (Family Salmonidae) such as rainbow trout (Oncorhynchus mykiss), coho salmon (O. kisutch), and Atlantic salmon (Salmo salar). Current challenges in managing, preventing, and controlling BCWD outbreaks may partially relate to the considerable intraspecific diversity present within this species, including that revealed via multilocus sequence typing (MLST). Indeed, MLST-based epidemiological studies and others suggest F. psychrophilum diversity may influence the ecology and behavior of some variants, affect their detection and diagnosis, and play a role in host specificity, transmission, and environmental persistence. However, controlled studies examining these aspects of BCWD ecology in relation to F. psychrophilum genetic diversity are lacking. My dissertation addressed these critical knowledge gaps, with the goal of contributing to enhanced strategies for the management, prevention, and control of BCWD. To improve F. psychrophilum recovery and detection, I initially compared colony yields of geographically, temporally, and genetically diverse F. psychrophilum isolates on three previously established F. psychrophilum culture media. The selected media included the current gold-standard medium, tryptone yeast extract salts (TYES) agar, which yielded the most colonies. With TYES as a foundation, I employed a Plackett-Burman experimental design, culminating in the development of two new culture media (F. psychrophilum medium-A and -B). These optimized media significantly improved F. psychrophilum recovery in the laboratory and from naturally infected salmonids when compared to TYES, and thus will enhance BCWD research and diagnostic efforts. To assess F. psychrophilum host specificity, variants belonging to MLST clonal complexes most associated with Atlantic salmon, coho salmon, or rainbow trout were cross challenged against each of these salmonid species via immersion. Resultingly, some variants were host specific, as evidenced by only causing disease and mortality in one species, whereas others caused disease and mortality in all three species, although to varying degrees. Variation in molecular serotype and proteolytic activity were also observed among variants. Collectively, findings highlighted the complexities of host-pathogen interactions and may guide the development of BCWD prevent strategies, such as vaccines. In a separate experiment, the shedding dynamics of live and dead Atlantic salmon, coho salmon, and rainbow trout were assessed, marking the first study to evaluate F. psychrophilum shedding dynamics in Atlantic salmon and coho salmon. Although both live and dead fish of all species shed F. psychrophilum, dead fish shed substantially more bacterial cells and for a longer duration. Furthermore, shedding dynamics varied by F. psychrophilum variant and/or host species, a matter that may complicate BCWD management. The persistence of predominating F. psychrophilum variants in microcosms composed of sterile well water only, sterile well water with commercial trout feed, and sterile well water with raceway detritus was measured via culture over 13 weeks. All variants remained culturable in each microcosm for at least eight weeks, with bacterial concentrations significantly higher in the presence of raceway detritus. However, significant differences in culturability were observed within and between microcosms, suggesting potential variability in environmental persistence strategies among specific variants. In total, the findings of my dissertation supported my overarching hypothesis that F. psychrophilum intraspecific diversity plays an important role in shaping our understanding of BCWD ecology.
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