More than Mucus: The Hidden World of the Fish Microbiota
By Andrea M. Larsen and Cova R. Arias
All living organisms maintain a diverse community of microbes that live in association with mucus-covered surfaces including the digestive, respiratory, and urogenital tracts. These microbial communities are known as ‘microbiota’. We grow up viewing microbes as disease-causing organisms, but most of those associated with our surfaces are harmless or even beneficial to our health. Studies on the gut microbiota have indicated that microbes aid in proper development and function of the digestive tract and immune system, help obtain energy from food, and act as a barrier against potential disease-causing microbes. These roles have also been demonstrated in fishes, all of which have microbiota associated with the gills, skin, and gut: the primary entry routes of pathogenic organisms.
Under ordinary circumstances, a fish maintains a healthy, normal microbiota that forms a protective barrier on the mucosal surfaces, preventing infection from potential pathogens. Normal fluctuations in the microbial community structure are expected due to changes in environmental factors such as season, location, water quality (including temperature, salinity, and oxygen content), and presence of organic matter (for example carbohydrates and amino acids). However not all changes are harmless and deviations from the normal microbiota (also known as dysbiosis) may lead to higher susceptibility to infection. These more serious changes may arise from a combination of factors including unfavorable diet, dramatic changes in environmental factors, limited resources, pollution, and disease. Dysbiosis is often observable prior to physical signs of disease and thus may serve as a warning sign of disease-favoring conditions.
Photo credit: M. Klug/NOAA