We found that heavy oil exposure to fish changed microflora –a colony of microorganisms present in a specific location – in fish skin mucus, which led the risk of developing bacterial infection rising. This is the first study to show clear changes in the microflora profile in fish skin mucus after chemical exposure, and give new insight on how environmental pollution is related to infectious diseases.
For this experiment, we used Japanese flounder P. olivaceus –one of the well-known coastal fish– and evaluated the influence of heavy oil exposure. Three Japanese flounder (average body weight is 197 g) were exposed to heavy oil (bunker C) at a concentration of 3.75 g/L for 3 days, then they were reared in filtered seawater without heavy oil for 4 days and then collected for the analysis. We analyzed their microflora of skin mucus by a denaturing gradient gel electrophoresis (DGGE). The result of DGGE showed that two bands disappeared and 4 bands newly appeared in the heavy oil-exposed fish. Moreover, heavy oil exposure caused about 11-fold increase of bacteria in fish skin mucus, and about 2.5-fold increase of leukocytes in their peripheral blood.
Usually there are many kinds of resident bacteria in fish skin mucus and each colony is generally called as microflora. These resident bacteria normally lead to no disease. However, some stressors may cause immune suppression, which induce opportunistic infection disease caused by some kinds of resident bacteria or other infectious diseases caused by some pathogenic bacteria which newly invaded from outside and/or grew in the mucus. As just described, it is believed that the newly bacterial adhesion and the increase in the numbers of bacteria in the skin mucus of oil-exposed Japanese flounder were strongly related to the immune suppression caused by heavy oil exposure. Although it is unknown how infectious these bacteria are, we can say the fish were probably infected by some bacteria because the number of their leukocytes increased after the exposure. In addition, this study left intriguing question whether the disappeared bacteria due to heavy oil exposure played a role in the prevention of infectious diseases by other bacteria or not, since some of the resident bacteria are known to live in a symbiotic relationship with the fish and have this role.
In recent years, heavy oil spill accidents such as tanker accidents have occurred frequently, whose environmental influence should be of particular concern. Generally, once heavy oil spill occurs, expanding polluted area will be blocked by using OILFENCE etc., then heavy oil will be removed from the environment in a relatively short period. As this study showed, however, the risk of developing bacterial infection arising from the change of microflora in fish skin mucus, is predicted to grow continuously even after removing heavy oil.
Clarifying the whole picture of microflora including identification and characterization of the consisting bacteria is expected to provide helpful clues to the environmental assessment, to the application to fish’s health evaluation and control, and to understanding how the fish and the resident bacteria can live in a symbiotic relationship.
Original article (doi:10.1016/j.marpolbul.2008.01.024)