Invertebrates Can Discriminate Specific Bacteria, Study Finds

Researchers from Heinrich Heine University Düsseldorf (HHU) and Kiel University (CAU) have uncovered a surprising ability in an early-branching animal: a sea anemone can discriminate between different microbes and thereby protect beneficial bacteria while targeting harmful strangers. The work, published in Nature Communications, challenges a long-standing view that selective microbial recognition belongs only to vertebrates.

The study focuses on innate immunity in Nematostella vectensis, which lacks antibodies and adaptive immune memory. Instead, the researchers examined “nematosomes,” motile multicellular units inside the organism that patrol and respond to microbes in the surrounding environment. These structures function as cellular actors for the animal’s first-line defense.

Using functional experiments, the team showed that nematosomes preferentially engulf and break down non-native bacterial strains, while largely sparing bacteria that naturally form the anemone’s own microbiome. This selective phagocytosis supports a stable microbial community—one that benefits the host rather than disrupting it.

A central molecular regulator emerged from the experiments: the cJun gene. The researchers used CRISPR/Cas to switch off cJun, effectively disabling a key control mechanism for nematosome behavior. The modified animals produced fewer nematosomes than controls.

Most importantly, the cJun-deficient sea anemones lost reliable discrimination between foreign and self-associated bacteria. As a result, microbial balance shifted, and the animals became more susceptible to bacterial infections, linking gene-controlled cell behavior to ecosystem-level host protection.

Lead author Dr Nida Kaya emphasizes the evolutionary implication: “Targeted identification of microorganisms is not a privilege restricted to the adaptive immune system.” The findings indicate that invertebrates can still execute refined, microbiome-aware immune strategies despite relying on innate mechanisms alone.

Professor Sebastian Fraune further argues that selective recognition may be far older than previously assumed. By showing how early animals balanced beneficial microbes and pathogens for hundreds of millions of years, the study reframes the evolutionary origins of immune sophistication.

The results also revive interest in “trained immunity” or innate immune memory—processes by which prior microbial encounters can enhance later responses without adaptive antibodies. The nematosomes described here offer a tractable cellular system for probing the signaling pathways that underpin such responsiveness.

Beyond basic immunobiology, the work suggests practical research directions for understanding how selection at the cellular level maintains metaorganism stability—host plus microbiome as a functional unit. Future studies can map the molecular circuitry downstream of cJun to determine how closely related bacterial strains are distinguished.