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Our bodies are home to trillions of bacteria, especially in the gut. Scientists have long known that human cells and bacteria influence each other, but exactly how they communicate is still being uncovered. New research helps clarify this process by showing that extracellular vesicles (EVs) act as tiny messengers between human intestinal cells and gut bacteria.
The research team, which includes scientists from institutions in Germany such as Saarland University and collaborating research centers, explored how human colon cells and several common gut bacteria exchange molecular signals using EVs.
What Are Extracellular Vesicles?
Extracellular vesicles are small, membrane-bound particles released by cells. You can think of them as microscopic packages filled with biological cargo, such as RNA and proteins, that can be delivered to other cells. While EVs are well known for helping cells communicate within the same organism, their role in communication between humans and bacteria (called cross-kingdom communication) has been less clear.
Bacteria Send Messages to Human Cells
The researchers examined bacterial extracellular vesicles (BEVs) from three gut bacteria:
- Lacticaseibacillus casei
- Enterococcus faecalis
- Proteus mirabilis
They exposed human colon epithelial cells (Caco-2 cells, a common laboratory model) to these BEVs and analyzed how the human cells responded.
Interaction of L. casei, E. faecalis and P. mirabilis with Caco-2 EVs
(A and B) Characterization of Caco-2 EVs. (A) Typical size distribution of purified Caco-2 (-FCS) EVs measured by NTA. The size distribution is presented as mean ± standard error. (B) Representative cryo-TEM images of Caco-2 (-FCS) EVs showed round shaped particles. (C–E) Growth of L. casei (C), E. faecalis (D), and P. mirabilis (E) in the presence of different amounts of Caco-2 (-FCS) EVs or PBS as a control over a period of 48 h or 36 h at 37 °C. Results are presented as mean ± SD from 4 independent biological replicates. (F–H) Visualization of the interaction of L. casei (F), E. faecalis (G) or P. mirabilis (H) with Caco-2 (-FCS) EVs. Bacteria were incubated for 4 h, 8 h or 24 h with PKH26-labeled Caco-2 (-FCS) EVs. After incubation, bacteria were stained with SYTO™ 9 and fixed. SYTO™ 9 was visualized using a laser at 488 nm, and PKH26 with a laser at 561 nm. For better illustration, the bacteria are shown in blue, and EVs are displayed in yellow. The white arrows indicate areas where PKH26-labeled Caco-2 (-FCS) EVs were detected. Scale bar: 10 µm. Statistical significance was determined using a two-tailed unpaired t-test (p < 0.05*, p < 0.001***).
The results were striking. Depending on the bacterial species, up to 6,000 genes in the human cells changed their activity. Some of the affected genes, such as CCL20, CXCL8, and CXCL10, are involved in immune signaling and inflammation. This suggests that different bacterial species can trigger very different responses in human cells.
Importantly, when the researchers introduced only the RNA extracted from bacterial vesicles, they saw some of the same gene expression changes. This shows that the RNA cargo inside bacterial EVs plays a direct role in shaping how human cells respond.
Human Cells Talk Back
Communication wasn’t one-way. The team also showed that human cells release their own extracellular vesicles that interact with bacteria.
One small RNA molecule, miR-192-5p, was found at much higher levels in EVs released by colon cells grown in conditioned medium. When bacteria encountered these human-derived vesicles or this specific RNA molecule, their growth was modestly affected.
The researchers also discovered that how the RNA is packaged, particularly within lipid-based structures, changes how effectively it associates with bacteria. In other words, the delivery method matters.
Why This Is Important
These findings support a new model in which extracellular vesicles serve as bidirectional communication tools between humans and their microbiome. Rather than simply coexisting, gut bacteria and intestinal cells actively exchange RNA messages that influence gene activity on both sides.
This has important implications for understanding:
- Gut health and inflammation
- Microbiome-host balance
- Disease development
- Potential RNA-based therapeutic strategies
By showing that EV-mediated communication is species-specific and RNA-driven, the research highlights the need to study these interactions in context rather than assuming all bacteria behave the same way.
As scientists continue to explore how extracellular vesicles shape host-microbe relationships, new opportunities may emerge to fine-tune the microbiome or design therapies that harness these natural communication systems.
