by Piter Kehoma Boll
Bacteria are some of the most diverse organisms on Earth and they perform all sort of ecological roles, although they are more often associated with diseases by the average human being. This is, of course, due to the fact that most bacteria that have a direct and perceptible influence on human life are, in fact, pathogenic, often parasitic, bacteria. Today I am introducing one of those pathogenic bacteria, but not one that infects humans.
Currently known as Flavobacterium psychrophilum, this species is known to infect freshwater fish, causing a disease known as bacterial coldwater disease (CWD). As a result, I will call this species the coldwater disease bacterium, or CWD bacterium for short.
The CWD bacterium is a typical rod-shaped bacterium measuring up to 1 µm in width and 5 µm in length. It lacks any type of flagellum or other structure that helps it move, but it can move by gliding, although this movement is incredibly slow and very difficult to observe. When cultured in a growth medium, they produce small 3-mm-diameter yellow colonies with thin margins.
Living in freshwater, the CWD bacterium prefers cold waters, with temperatures of 16 °C or lower, with the optimal temperature being 13°C. They grow on all sort of tissues on the body of fish, such as the skin, gills, mucous and internal organs such as the brain, kidney, spleen and the sex organs. Its preferred hosts are salmonoid fishes, such as salmons and trouts, but it can be found in other species eventually.
The CWD bacterium is an aerobic bacterium but is unable to use carbohydrates as a source of carbon, feeding on peptides instead. Thus, it secretes enzymes on the host’s tissues to degrade its proteins into peptides, causing structural damage.
Infected fish show tissue erosion, which often begins on the caudal fin and eventually spread. Fins become dark, ragged, split or torn and may be completely lost. Ulcerations appear on the skin, especially around the jaw, and the fish present behavioral issues such as spiral swimming and lethargy. The infection often kills the fish but sometimes a milder chronic infection can occur, which, however, still causes considerable behavioral changes in the host.
The bacteria are often transmitted from fish to fish via direct fish contact, but infected adult fish can end up passing the infection directly to their offspring through infected eggs. The infection can be treated in early stages using the antibiotic oxytetracline or by adding quaternary ammonium cations to the water.
In natural environments the problems caused by this infection are rarely problematic and its damage is more often seen in fish farms, where the poor creatures are forced to live in higher densities, which increases the bacterium’s success. Apparently native to North America, where it was discovered in the 1940s, it was spread via fish farming across the whole world in the following decades. We humans, therefore, are once more the main reason why this species has become a worldwide problem.
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References:
Cipriano, R. C., & Holt, R. A. (2005). Flavobacterium psychrophilum, cause of bacterial cold-water disease and rainbow trout fry syndrome. Kearneysville, WV: US Department of the Interior, US Geological Survey, National Fish Health Research Laboratory.
Langevin, C., Blanco, M., Martin, S. A., Jouneau, L., Bernardet, J. F., Houel, A., … & Boudinot, P. (2012). Transcriptional responses of resistant and susceptible fish clones to the bacterial pathogen Flavobacterium psychrophilum. PLoS One, 7(6), e39126. https://doi.org/10.1371/journal.pone.0039126
Starliper, C. E. (2011). Bacterial coldwater disease of fishes caused by Flavobacterium psychrophilum. Journal of Advanced Research, 2(2), 97-108. https://doi.org/10.1016/j.jare.2010.04.001
Wikipedia. Flavobacterium psychrophilum. Available at < https://en.wikipedia.org/wiki/Flavobacterium_psychrophilum >. Access on 10 June 2020.
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