Scientists Find Active Pseudomonas Phages and Their Bacterial Host in Human Breast Milk

Human breast milk has long been celebrated as a nutritional and immunological powerhouse for newborns, but scientists are still mapping the full cast of microorganisms it carries. A team of Belgian, Portuguese, and Australian researchers has now reported something that had never been directly demonstrated before: intact, infectious bacteriophages specific to Pseudomonas aeruginosa, together with a live strain of that very bacterium, isolated directly from the breast milk of a single lactating woman. The study, published in npj Viruses, moves the field beyond DNA signatures on a sequencing readout and into the realm of functional virus particles that can kill bacteria in a laboratory assay.

Previous metagenomic surveys of human breast milk had revealed an abundance of viral genetic material, much of it phage DNA, hinting that these viruses might be active participants in the early-life microbiome. But metagenomics alone cannot distinguish between free-floating DNA fragments, prophage genomes quietly embedded in bacterial chromosomes, and fully formed virions capable of infection. Demonstrating the presence of actual phage particles requires a different approach: culturing a susceptible bacterial host and showing that something in the milk sample can lyse it, producing the clear plaques on a bacterial lawn that have been the calling card of phage hunters for more than a century.

That is precisely what the team, led by Steven De Soir of UCLouvain and the Queen Astrid Military Hospital in Brussels, together with colleagues at KU Leuven, the University of Minho, and Monash University, set out to do. Working with breast milk samples collected from a lactating donor under written informed consent, the researchers succeeded in isolating six distinct bacteriophages capable of infecting P. aeruginosa, along with a co-occurring strain of the bacterium itself. The identification of the Pseudomonas isolates was carried out with the support of the microbiology laboratory of the Cliniques universitaires Saint-Luc in Brussels, ensuring that the host bacterium recovered from the milk was unambiguously characterized.

The genomic characterization of the six phages revealed a striking diversity. Four of the isolates proved to be strictly lytic, meaning they replicate inside and destroy their bacterial hosts rather than integrating into the host genome. The remaining two were temperate phages, the kind that can choose between a lytic cycle and a lysogenic lifestyle in which their genome becomes a quiet passenger within the bacterium. Across the six isolates, the genomes spanned multiple genera, indicating that a single sample of human breast milk can harbor phages drawn from several branches of the viral tree of life, all targeting the same bacterial species.

Among the four lytic phages, three showed highly similar host ranges and produced comparable plaque morphologies, suggesting they may be closely related despite being recovered as separate isolates. This kind of redundancy is a familiar feature of phage ecology, where dominant viral lineages can appear repeatedly within a single ecological niche. Whether these three represent genuinely distinct entities or variants of a locally dominant phage is one of the questions the study leaves open for future work, but their coexistence in the same sample underscores how concentrated phage activity against a single host species can be within this body fluid.

Host-range profiling, the standard method for determining which bacteria a given phage can infect and kill, showed that the newly isolated phages display broad activity against P. aeruginosa. Critically, that activity included the co-occurring P. aeruginosa strain recovered from the very same milk sample. This observation is significant because it demonstrates that the phages and their bacterial host were not merely passive co-travelers; the viruses were capable of infecting the bacterium found alongside them, raising the possibility of active phage-host dynamics unfolding within the milk itself.

Perhaps the most intriguing laboratory finding concerns synergy. The researchers observed synergistic interactions between pairs of lytic phages, as well as between lytic phages and one of the temperate isolates. In practical terms, synergy means that combinations of phages can suppress bacterial growth more effectively than the sum of their individual effects, a property that underpins the design of phage cocktails in therapeutic applications. That such interactions should be detectable among phages recovered from breast milk suggests that the viral community in this fluid is not a random assortment but potentially a functionally coordinated ensemble shaped by coevolution with its bacterial targets.

The implications extend in two directions. The first concerns neonatal biology. P. aeruginosa is an opportunistic pathogen of considerable clinical importance, notorious for its antibiotic resistance and its role in hospital-acquired infections, particularly in immunocompromised patients and those with cystic fibrosis. Yet Pseudomonas species are also documented members of the early-life microbiome, and the presence of both the bacterium and its phages in breast milk raises the possibility that infants ingest a miniature predator-prey system with every feed. Such a system could influence which bacterial strains colonize the infant gut, modulate bacterial population sizes, and even shape the developing immune system’s encounter with microbial antigens. The authors note that these findings elicit questions toward phage-host dynamics in human breast milk and highlight implications for neonatal microbial colonization and immune modulation.

The second direction concerns phage therapy and biotechnology. Phages active against P. aeruginosa are of intense interest because of the pathogen’s inclusion on lists of priority antibiotic-resistant bacteria. Discovering that novel, genetically characterized lytic phages with broad host ranges can be recovered from an unexpected and easily accessible human source adds to the growing catalog of candidate therapeutic agents. The Belgian research community involved in this study, including the Laboratory for Molecular and Cellular Technology at the Queen Astrid Military Hospital, has a long-standing track record in phage therapy research, and the isolation methods demonstrated here could inform future searches for phages in other human-associated niches. The work was supported by Innoviris, the Brussels regional research funder, and by the Belgian FRS-FNRS, with fellowship support for early-career researchers.

There are, of course, important caveats. The study reports findings from breast milk of a single lactating woman, so the prevalence of active Pseudomonas phages in breast milk across the broader population remains unknown. It is also not yet clear whether the phages and the bacterium were produced locally within the mammary gland, transported there from elsewhere in the maternal body, or introduced through environmental contact. The entero-mammary pathway, by which maternal gut immune cells and microbes are thought to traffic to the breast, offers one plausible route, but confirming the origin of these phages will require further study. What the work establishes beyond doubt is that functional phage particles targeting a clinically significant pathogen exist in human breast milk, transforming what was once a metagenomic hint into a demonstrable biological reality. As researchers begin to probe the dynamics between these viruses, their bacterial hosts, and the infant recipients of both, breast milk may prove to be one of the most accessible windows into the hidden ecology of the human virome.

Subject of Research: Isolation of active Pseudomonas aeruginosa bacteriophages and their bacterial host from human breast milk

Article Title: Isolation of active Pseudomonas aeruginosa bacteriophages and of their host from human breast milk

Article References: De Soir, S., Sáez Moreno, D., Wagemans, J., Glorieux, A., Lavigne, R., Pirnay, J.-P., Merabishvilli, M., Barr, J. J., Van Bambeke, F., & De Vos, D. (2026). Isolation of active Pseudomonas aeruginosa bacteriophages and of their host from human breast milk. npj Viruses. https://doi.org/10.1038/s44298-026-00235-z

Image Credits: AI Generated

DOI: 10.1038/s44298-026-00235-z

Keywords: bacteriophages, Pseudomonas aeruginosa, human breast milk, virome, neonatal microbiome, phage therapy, lytic phages, temperate phages, host range, phage-host dynamics, microbiome development, antibiotic resistance