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Probiotics modulates transcriptomic and gut microbiota responses to short-term thermal stress in American shad (Alosa sapidissima) fry

This study demonstrates that dietary supplementation with *Lactococcus lactis* alleviates short-term thermal stress in American shad fry by reshaping intestinal microbiota composition and modulating hepatic metabolic pathways, thereby providing a molecular basis for enhancing stress resistance in temperature-sensitive fish under climate warming.

Original authors: Chuwen Qiu, Minghui Zhang, Yonghai Shi

Published 2026-07-01

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Chuwen Qiu, Minghui Zhang, Yonghai Shi

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Heatwave in a Fish Bowl

Imagine you are running a fish farm for American Shad, a type of fish that loves warm water but gets very sick if it gets too hot. Just like humans, when these fish get overheated, their internal "machinery" starts to break down. Their stomachs get upset, their immune systems panic, and their livers (the body's main processing plant) get overwhelmed.

This study asked a simple question: Can we give these fish a "probiotic" (a good bacteria supplement) to help them survive a sudden, intense heatwave?

The researchers tested this by feeding baby shad (fry) a special diet containing Lactococcus lactis, a friendly bacteria, at two different strengths: a "low dose" and a "high dose." They also had a group with no bacteria (the control) and a group of fish that were so stressed they were dying (the "moribund" group).

The Experiment: A 10-Day Heat Challenge

The team put the fish in tanks where the water temperature was cranked up to a stressful 30.4°C–33.2°C (about 87°F–92°F) for 10 days. This is like putting the fish in a sauna for a week and a half.

They then looked at two main things to see how the fish were coping:

The Gut Garden: They checked the bacteria living inside the fish's intestines.
The Liver Blueprint: They looked at the fish's liver to see which genes (the instruction manuals for the body) were being turned on or off.

What They Found

1. The Gut Garden: Replanting the Soil

Think of the fish's intestine as a garden. When the heat stress hit, the garden started to get weedy and chaotic.

The Control Group (No Probiotics): The garden was messy. The "weeds" (potentially harmful bacteria) were taking over, and the variety of plants was low.
The Low Dose Group: This group had the most diverse garden. It was like having a wide variety of flowers and shrubs, which makes a garden resilient.
The High Dose Group: This group had the most abundant garden. There were just more plants overall, filling up the space and crowding out the bad stuff.
The Result: In both probiotic groups, the friendly Lactococcus bacteria increased, helping to keep the garden balanced even while the sun was scorching.

2. The Liver Blueprint: Different Strategies for Different Doses

The liver is like the factory where the fish processes food and manages stress. The researchers found that the "low dose" and "high dose" bacteria told the liver factory to do two completely different jobs to survive the heat.

The High Dose Strategy (The "Feeder"):
The high dose of bacteria told the liver: "Focus on eating and absorbing!"
The genes turned on were related to digestion and absorption. It was as if the factory switched all its workers to the assembly line to make sure the fish could still get nutrients from its food, even though it was stressed. This helped the fish keep growing and staying strong.
The Low Dose Strategy (The "Stabilizer"):
The low dose of bacteria told the liver: "Focus on hormones and stress management!"
The genes turned on were related to making steroids and terpenoids (chemicals that help regulate stress hormones like cortisol). It was as if the factory switched its workers to the "emergency response" team to keep the fish's internal temperature and stress levels from spiking out of control.

3. The "Dying" Group vs. The Protected Groups

The group of fish that was dying (the DD group) had a liver that was in total chaos. Their genetic instructions were screaming about protein breakdown and neurodegenerative issues (basically, their cells were falling apart and their nervous systems were failing).

However, the fish that got the probiotics (both low and high dose) looked much more like healthy fish. Their livers weren't in panic mode. The probiotics acted like a shield, stopping the heat from causing that total cellular collapse.

The Takeaway

This study shows that probiotics aren't just a "one-size-fits-all" cure.

If you give a high dose, you help the fish digest food better so they can keep growing during a heatwave.
If you give a low dose, you help the fish manage their stress hormones so they don't panic.

Both strategies worked to keep the fish alive and their internal "gardens" and "factories" running smoothly, whereas the fish without probiotics suffered severe damage to their guts and livers. The paper concludes that using these bacteria is a smart, natural way to help temperature-sensitive fish survive the increasingly hot summers caused by climate change.

Technical Summary: Probiotics Modulate Transcriptomic and Gut Microbiota Responses to Short-Term Thermal Stress in American Shad (Alosa sapidissima) Fry

Problem Statement
Global climate warming has led to frequent high-temperature events that severely threaten the aquaculture of thermosensitive species. American shad (Alosa sapidissima), a commercially valuable fish introduced to China, is particularly vulnerable during its fry stage, exhibiting significantly lower thermal tolerance than adults. High-temperature exposure disrupts intestinal microecological balance, impairs physiological functions, induces oxidative stress, and triggers hepatic damage, often leading to reduced survival rates and mass mortality. While probiotics have shown promise in alleviating heat-induced damage in adult fish, the regulatory mechanisms of probiotics against acute thermal stress in A. sapidissima fry remain poorly understood, limiting their targeted application in seedling cultivation.

Methodology
The study employed a controlled 10-day acute thermal stress trial (water temperature 30.4–33.2°C) using A. sapidissima fry. Four experimental groups were established:

LH Group: Fed a diet supplemented with Lactococcus lactis at $1 \times 10^8$ CFU/kg.
LL Group: Fed a diet supplemented with L. lactis at $1 \times 10^7$ CFU/kg.
CK Group: Fed a control diet with 0 CFU/kg of L. lactis.
DD Group: Consisted of moribund individuals from all treatments, serving as a severe stress control.

The researchers utilized a multi-omics approach to analyze regulatory mechanisms:

Gut Microbiota Analysis: 16S rRNA gene sequencing (V3-V4 region) was performed on intestinal samples to assess microbial diversity, community structure, and taxonomic composition.
Liver Transcriptomics: RNA-seq was conducted on liver tissues to identify differentially expressed genes (DEGs) and analyze functional pathways.
Validation: Key gene expression patterns were verified using quantitative real-time PCR (qRT-PCR).
Statistical Analysis: Data were analyzed using ANOVA, Kruskal–Wallis H tests, and bioinformatics tools (DESeq2, KEGG, GO enrichment).

Key Results

Gut Microbiota Remodeling: Dietary L. lactis significantly reshaped the intestinal microbiota. Alpha diversity followed the order LL > CK > LH, while community richness was LL > LH > CK. The dominant phyla remained Proteobacteria and Firmicutes across all groups. Notably, the relative abundance of Lactococcus was higher in probiotic-treated groups (LH and LL) compared to the control. The high-dose group (LH) showed a higher relative abundance of both beneficial and potentially pathogenic bacteria compared to the low-dose group, though differences were not statistically significant.
Transcriptomic Responses:
- Unique Genes: The number of unique genes varied by group: LH (741), LL (131), CK (177), and DD (603).
- DEG Enrichment (vs. CK): In the LH group, DEGs were enriched in protein digestion/absorption, mineral absorption, carbohydrate digestion/absorption, bile secretion, and sphingolipid metabolism. In the LL group, DEGs were enriched in protein digestion/absorption, steroid biosynthesis, and terpenoid backbone biosynthesis.
- Severe Stress Response (DD Group): The DD group exhibited a significantly higher number of DEGs (4,251) compared to other groups. These genes were primarily enriched in protein processing in the endoplasmic reticulum, proteasome function, and pathways related to neurodegenerative diseases (e.g., Parkinson's, Alzheimer's), indicating severe cellular and neurological disruption.
- Probiotic Mitigation: Compared to the DD group, probiotic-treated groups showed enrichment in pathways related to protein processing and proteasome function, suggesting a reversal of stress-induced transcriptional disorders.
Gene Expression Specifics: Specific genes related to immunity, stress response, and metabolism showed dose-dependent regulation. For instance, substance transport genes (slc6a19a.2) were significantly upregulated in probiotic groups compared to controls, with the LH group showing more prominent upregulation. Cell junction genes (nectin1b) were upregulated in probiotic groups compared to the DD group, aiding in maintaining intestinal structural integrity.

Key Contributions

Elucidation of Dose-Dependent Mechanisms: The study reveals that different concentrations of L. lactis trigger distinct regulatory pathways. High-dose supplementation ( $1 \times 10^8$ CFU/kg) primarily activates nutrient digestion and absorption pathways (bile secretion, mineral absorption) to support growth and metabolic efficiency under heat stress. In contrast, low-dose supplementation ( $1 \times 10^7$ CFU/kg) targets steroid and terpenoid biosynthesis pathways, likely aiding in the maintenance of stress hormone homeostasis (e.g., cortisol regulation).
Gut-Liver Crosstalk: The research provides evidence of a synergistic regulatory mechanism where probiotic-mediated gut microbiota modulation influences hepatic metabolic and immune responses to thermal stress.
Molecular Basis for Fry Survival: By identifying specific DEGs and pathways associated with thermal tolerance in fry, the study fills a gap in understanding acute heat stress responses in early developmental stages, which are more vulnerable than adults.

Significance and Claims
The authors claim that this study elucidates the synergistic regulatory mechanism of intestinal flora and hepatic metabolism in probiotic-mediated thermal stress tolerance. The findings provide a valuable molecular theoretical basis and technical support for:

Stress resistance regulation in temperature-sensitive fish.
Healthy breeding and sustainable culture of American shad.
High-temperature emergency management in outdoor pond aquaculture.

The paper emphasizes that probiotic intervention offers a feasible green strategy for thermal stress regulation, specifically by optimizing intestinal microbial composition and regulating host gene expression to alleviate thermal injury. The study concludes that precise application of probiotics, tailored to specific dosage effects, is critical for enhancing the survival and health of A. sapidissima fry under global climate warming scenarios.