How Oxytetracycline Affects Gene Expression and Tissue Health in Olive Flounder

Oxytetracycline (OTC) has been a workhorse antibiotic in aquaculture for a long time. In South Korea, where olive flounder is the top finfish species, farmers use it regularly to manage bacterial outbreaks. Everyone knows OTC works against bacteria. But what it actually does to the fish itself, especially at higher doses, has not been nearly as well understood.

A recent study took a closer look. Fish were exposed to OTC at 100, 200, and 400 ppm for just one hour. Then the researchers spent the next three days tracking what changed at the gene level and in the tissues. The results show that OTC does a lot more than kill bacteria.

Where the Action Happens: Gills and Liver

Fish take in chemicals from the water through their gills. From there, those compounds hit the bloodstream and end up in the liver, which handles most of the detox work. So it makes sense the study focused on both organs. The question was straightforward: what genes get turned on when OTC gets in?

The team tracked five drug metabolism genes (four from the cytochrome P450 family and one from the UGT family) plus four immune-related cytokine genes. These are the pieces of biological machinery that decide how a fish processes foreign substances and whether its immune system sounds the alarm.

Drug Metabolism Genes: Not Subtle

The drug metabolism genes did not stay quiet. Within hours of OTC exposure, they were firing.

Take PoCYP1A1. In the gills, expression jumped 3.4-fold at the highest OTC dose within 12 hours. In the liver, the same gene hit 7.6-fold at 400 ppm and stayed elevated past the 72-hour mark. That persistence is worth noting. It means the liver kept working to break down the antibiotic well after the exposure was over.

Then there is PoCYP27B1, which produced the most dramatic result in the whole study. In the liver of fish treated with 400 ppm OTC, expression shot up 58-fold at 12 hours. Three days later, it was still running 22 times above normal. That is not a routine metabolic response. The CYP27 family is involved in steroid hormone biosynthesis, and the connection to drug metabolism still needs sorting out. But a 58-fold jump demands attention.

PoUGT, the phase II metabolism gene, followed a similar arc. Expression peaked at 24 hours across all groups and then dropped, except in the 400 ppm group, where it stayed significantly elevated at 72 hours. The message is clear: higher doses mean the fish has to work harder and longer to clear the drug.

Immune Genes: The Alarm System Goes Off

The cytokine genes responded just as strongly. If the drug metabolism genes are the cleanup crew, the cytokines are the fire alarm.

PoIL-1β produced the biggest response in the gills. At 400 ppm, expression hit 22.7-fold at 24 hours and stayed above 14-fold even at 72 hours. That is a sustained inflammatory signal, and it points to real stress in the gill tissue.

In the liver, PoIL-8 jumped 19.2-fold at 12 hours in the 400 ppm group. PoTNF-α peaked at 8.6-fold in the same group at 24 hours.

Not everything followed a simple upward curve. The 100 and 200 ppm groups showed a typical spike-and-recover pattern, with expression rising at 12 to 24 hours and then dropping back near normal by 72 hours. The 400 ppm group behaved differently. Some genes stayed elevated longer. Others, like PoIL-6, stayed close to control levels at the highest dose. That variation suggests that when OTC hits a certain concentration, the fish’s immune regulation shifts into a different gear entirely.

What the Tissues Showed

Gene data only goes so far. The researchers also put gill and liver samples under a microscope.

At 100 and 200 ppm, the gills looked normal at every time point. No visible damage. At 400 ppm, things changed. By 72 hours, the gills showed epithelial cell edema and mild aneurysms. The tissue was swelling, and small blood vessels were starting to bulge. Those are early markers of real injury.

The liver showed a similar pattern. At 200 and 400 ppm, the 72-hour samples had increased macrophages with activated phagocytosis. Macrophages are the immune system’s first responders. When they show up in numbers and start actively clearing material, it means the body is dealing with damage. At 100 ppm, the liver was clean. At the higher doses, the cleanup crew was on the job.

Why This Matters for Fish Farmers

At the recommended dose range (around 100 ppm), olive flounder handled OTC without lasting problems. The metabolic and immune systems responded, did their job, and returned to baseline. The tissues stayed intact.

At 400 ppm, the picture was different. The metabolic machinery went into overdrive. The immune system lit up. The tissues started showing damage. Even at 200 ppm, the liver showed some tissue-level response, even though the gills were fine.

For farmers and aquaculture vets, the takeaway is practical. Using higher-than-recommended OTC concentrations does not just put more antibiotic in the water. It triggers a cascade of stress responses that can hurt fish health. And stressed fish are more vulnerable to disease, which works against the whole reason for treatment.

The Takeaway

The study demonstrates that at standard doses, OTC works without causing lasting harm. Push past those limits, and the fish pays for it at the molecular and tissue levels. Drug metabolism pathways get pushed into overdrive. Inflammatory responses stay activated. Tissues show damage.

For an industry built on healthy fish, the lesson is simple. Dose guidelines exist for a reason. More antibiotic does not mean better results. When drugs go into the water, their effects go far beyond the bacteria they target. They change the biology of the fish.

Next steps will involve connecting these gene-level findings to actual protein activity and long-term health outcomes. But for now, the evidence is straightforward. Oxytetracycline at high concentrations is not just a treatment. It is a physiological stressor.