Summary of the published study: Exposure to 17α-ethinylestradiol results in differential susceptibility of largemouth bass (Micropterus salmoides) to bacterial infection (Leet et al. in review)
Fish kill events, disease outbreaks, skin lesions, and abnormalities in reproductive organs have been seen in wild populations of bass species throughout the United States. The timing of these events suggests influence from human activities. For example, heavy rains can cause increased runoff of pesticides and hormones from agricultural lands. The increased runoff and agricultural land use have been associated with poorer fish health.
This leads us to ask questions about how chemicals and hormones in the environment, stemming from human activity, possibly impact disease susceptibility in fish.
To investigate these connections, we tested if hormone exposure reduces a fish’s later ability to fight off bacterial infection.
To accomplish this, we first developed a reproducible laboratory method to induce bacterial infection in largemouth bass. We then tested how the bass responded to the laboratory bacterial infection after being exposed to a representative estrogen.
We used Edwardsiella piscicida as the bacteria for our laboratory infection model, as it is an emerging and virulent pathogen in warm water fish. We chose 17alpha-ethinylestradiol (EE2) as our representative estrogen because it is a potent estrogen and there have been many studies of its effects in fish. Additionally, because of its use in birth control pills it is commonly found at low levels in the environment near wastewater treatment plants.
To begin, we exposed juvenile largemouth bass (Micropterus salmoides) to 0, 0.9, or 9 ng/L EE2 for 4 weeks. After exposure these fish were injected with either a saline solution (control) or E. piscicida. Fish survival was monitored for 10 days. See Figure 1 for a schematic of the study design.
We hypothesized that the prior estrogenic exposure would make the fish more susceptible to mortality from the bacterial infection. Unexpectedly, fish with prior exposure to our high concentration of EE2 survived better than those fish with prior exposure to our low concentration of EE2 or no prior estrogen exposure (SC) (Figure 2).
To understand these findings better, we also looked at cellular structure in the liver with a microscope and measured gene expression in the liver and spleen. We observed altered gene expression in genes related to metabolism and immune function. For example, there was an increase in expression of genes associated with making proteins. This has been shown to help the immune system better fight microbial infections. We also observed decreased energy stores in livers of fish exposed to the high EE2 concentration that were similar to all treatment groups that had been infected with the bacteria. These data together indicate the fish with prior exposure to the high concentration of EE2 were better suited to adjust to fighting the bacterial infection than those with exposure to the low concentration of EE2 or no prior estrogen exposure. However, this is not an ideal strategy for the fish for longer-term survival, because the added protection from infection comes with the cost of depleting stored energy.
This work can help inform future studies investigating the specific ways bass respond to bacterial infections after contaminant exposure.
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- Overview
Summary of the published study: Exposure to 17α-ethinylestradiol results in differential susceptibility of largemouth bass (Micropterus salmoides) to bacterial infection (Leet et al. in review)
Fish kill events, disease outbreaks, skin lesions, and abnormalities in reproductive organs have been seen in wild populations of bass species throughout the United States. The timing of these events suggests influence from human activities. For example, heavy rains can cause increased runoff of pesticides and hormones from agricultural lands. The increased runoff and agricultural land use have been associated with poorer fish health.
Sources/Usage: Public Domain.Figure 1. Brief summary of the study design. The estrogen exposure experiment was conducted at the Columbia Environmental Research Center in Missouri. Then fish were transported overnight to the Western Fisheries Research Center in Seattle for the bacterial infection experiment. Edwardsiella piscicida was the bacterial species we used. This leads us to ask questions about how chemicals and hormones in the environment, stemming from human activity, possibly impact disease susceptibility in fish.
To investigate these connections, we tested if hormone exposure reduces a fish’s later ability to fight off bacterial infection.
To accomplish this, we first developed a reproducible laboratory method to induce bacterial infection in largemouth bass. We then tested how the bass responded to the laboratory bacterial infection after being exposed to a representative estrogen.
We used Edwardsiella piscicida as the bacteria for our laboratory infection model, as it is an emerging and virulent pathogen in warm water fish. We chose 17alpha-ethinylestradiol (EE2) as our representative estrogen because it is a potent estrogen and there have been many studies of its effects in fish. Additionally, because of its use in birth control pills it is commonly found at low levels in the environment near wastewater treatment plants.
To begin, we exposed juvenile largemouth bass (Micropterus salmoides) to 0, 0.9, or 9 ng/L EE2 for 4 weeks. After exposure these fish were injected with either a saline solution (control) or E. piscicida. Fish survival was monitored for 10 days. See Figure 1 for a schematic of the study design.
We hypothesized that the prior estrogenic exposure would make the fish more susceptible to mortality from the bacterial infection. Unexpectedly, fish with prior exposure to our high concentration of EE2 survived better than those fish with prior exposure to our low concentration of EE2 or no prior estrogen exposure (SC) (Figure 2).
Sources/Usage: Public Domain.Figure 2. Survival of largemouth bass during the 10 days after infection with Edwardsiella piscicida.Control fish were mock injected with saline. The bacterial part of the study followed a 4-week exposure of fish to solvent control (SC), 0.87 ng EE2/L (EE2Low), or 9.08 ng EE2/L (EE2High). For mock injected fish: n = 60; 5 fish x 4 replicates per EE2 exposure group. For fish infected with E. piscicida: n = 216; 18 fish x 4 replicates per EE2 exposure group. The blue line shows that all mock injected fish survived for all estrogen exposure groups. The asterisk indicates significantly higher survival in EE2High compared to solvent control or EE2Low (p<0.02). To understand these findings better, we also looked at cellular structure in the liver with a microscope and measured gene expression in the liver and spleen. We observed altered gene expression in genes related to metabolism and immune function. For example, there was an increase in expression of genes associated with making proteins. This has been shown to help the immune system better fight microbial infections. We also observed decreased energy stores in livers of fish exposed to the high EE2 concentration that were similar to all treatment groups that had been infected with the bacteria. These data together indicate the fish with prior exposure to the high concentration of EE2 were better suited to adjust to fighting the bacterial infection than those with exposure to the low concentration of EE2 or no prior estrogen exposure. However, this is not an ideal strategy for the fish for longer-term survival, because the added protection from infection comes with the cost of depleting stored energy.
This work can help inform future studies investigating the specific ways bass respond to bacterial infections after contaminant exposure.Return to Biochemistry and Physiology