A new study, led by a team from the Institute of Molecular Biology of Barcelona (IBMB – CSIC), located in the Barcelona Science Park, and IDIBELL, has succeeded in visualizing embryo cells eliminating bacterial infections before the formation of the immune system. The research describes a phagocytosis mechanism similar to that used by white blood cells and reveals that this mechanism is also present in human embryos. The work, published in the journal Cell Host and Microbe, highlights the increasingly important need to better understand the bacteria of the uterus and their effects on reproductive health.

The researchers saw how zebrafish embryos use cells present on their surface, known as epithelial cells, to ingest and destroy bacteria through a process called phagocytosis, similar to that carried out by white blood cells. Crucially, scientists could observe that this ability to eliminate bacteria is also present in human embryos.

Using state-of-the-art microscopy techniques, the research shows how cells capture Escherichia coli and Staphylococcus aureus bacteria through small protrusions of their membrane, in which the protein Actin is involved. “Our research shows that, at the beginning of development – before implantation in the uterus and before the formation of organs – embryos already have a defense system that allows them to eliminate bacterial infections,” says Esteban Hoijman, researcher at IBMB-CSIC and IDIBELL, leader of the study.

Zebrafish embryo with ingested E. coli bacteria (red and magenta) / Joan Roncero-Carol

This process, scientist explain, works as a phagocytosis mechanism, activates immunity genes in these cells, effectively eliminates bacteria and contributes to the correct embryonic development in case of infection. “This system could represent the origin of immunity. The study reveals the first interaction between the newly forming organism and its biological microenvironment,” adds Hoijman, who heads the Embryonic Cell Bioimaging laboratory at IBMB.

Preventing malformations and improving reproductive therapies

At the beginning of development, embryos are exposed to multiple changes in their environment which can pose a threat, since embryos have not yet developed the immune system to protect them.

In the uterus, infections have a high incidence and are associated with infertility. However, it has remained a mystery until now how an embryo reacts when it encounters a bacterium. This work reveals that immune capacities of an embryo begin long before the existence of white blood cells, and “could help us, in the future, to improve fertility, prevent embryonic malformations and develop new reproductive therapies,” explains Esteban Hoijman.

In this sense, the finding also highlights the important need to know in more detail the population of bacteria that can colonize the uterus, differentiating the invaders (and possible pathogens) from potential resident bacteria that could have beneficial effects on reproductive physiology.

Besides the CSIC and the IDIBELL, the research has involved scientists from CRG Barcelona, Pompeu Fabra University (UPF), the Institute for Bioengineering of Catalonia (IBEC), the University of Barcelona (UB), the Dexeus University Hospital and ICREA.

» Article of reference: Joan Roncero-Carol, June Olaizola-Muñoa, Begoña Arán, Loris Sebastiano Mularoni, Marta Miret Cuesta, Nuria Blanco-Cabra, Marc Casals, Mireia Rumbo, Miquel Sole Inarejos, Samuel Ojosnegros, Berta Alsina, Eduard Torrents, Anna Veiga, Manuel Irimia and Esteban Hoijman. Epithelial cells provide immunocompetence to the early embryo for bacterial clearance, Cell Host and Microbe. 2025. https://doi.org/10.1016/j.chom.2025.05.025

» Link to the news: CSIC website [+]