Bioprospecting the Cultivable Bacteria Associated with Trididemnum maragogi from the Saint Peter and Saint Paul’s Archipelago (Brazil) for Bioactive Metabolites

Deep in the ocean, where sunlight fades and pressure rises, life finds a way to thrive in unexpected partnerships. Among the most successful of these partnerships are the relationships between marine animals and the microscopic bacteria that live on or inside them. For decades, scientists have known that these tiny neighbors are not just passengers; they are often the true factories behind the powerful chemical compounds found in sea creatures. Many of the medicines we use today, especially those that fight cancer, were originally discovered in marine animals like sea squirts, but further research has revealed that the bacteria living within them are frequently the ones actually making the drugs. This realization has shifted the focus of drug discovery toward the microscopic world, where researchers hunt for new chemical structures that could become the next generation of life-saving treatments.

In a remote corner of the Atlantic Ocean, far from the Brazilian mainland, lies a tiny cluster of volcanic islands known as the Saint Peter and Saint Paul's Archipelago. This isolated spot is home to a unique species of sea squirt, a small, stationary animal that filters water for food. A team of researchers traveled to this archipelago to collect samples of this specific sea squirt, which grows in large, flat patches on the rocky ocean floor. Their goal was not to study the animal itself, but to investigate the community of bacteria living on its surface. They wanted to see if these microbes could produce chemicals capable of stopping the growth of cancer cells, a property known as antiproliferative activity.

The scientists brought their samples back to the laboratory and began the slow work of separating the bacteria from the sea squirt tissue. They placed tiny amounts of the crushed animal material onto different types of nutrient-rich plates, waiting for the bacteria to grow into visible colonies. Over several months, this process yielded fifty distinct bacterial strains. The researchers then identified what kind of bacteria they had found by reading a specific genetic code common to all living things. They discovered a diverse mix of microbes, including groups known for their ability to produce complex medicines, such as Bacillus and several types of actinomycetes, which are bacteria famous for making antibiotics.

To test if these bacteria were useful, the team grew them in liquid cultures and extracted the chemicals they produced. They then exposed these extracts to two different types of human cancer cells in a dish: one from a melanoma skin cancer and another from a colon cancer. The results were promising. Several of the bacterial extracts stopped the cancer cells from growing, with some showing particularly strong effects. The most successful extracts came from bacteria belonging to the Micromonospora and Salinispora groups. These specific strains were able to kill or stop the growth of the cancer cells at very low concentrations, suggesting they were producing potent chemical weapons.

However, knowing that a bacteria kills cancer cells is only the first step. The researchers needed to understand what chemicals were responsible. They used a sophisticated technique that separates and identifies molecules based on their weight and structure. This method allowed them to create a map of the chemicals present in the extracts. They found that some of the active extracts contained known families of compounds, such as staurosporines and rifamycins, which are already known to have anti-cancer properties. Yet, the most striking finding was that the vast majority of the chemical signals they detected did not match anything in existing databases. Roughly ninety-five percent of the chemicals found were completely unknown to science, representing a hidden chemical diversity that had never been seen before.

To dig deeper into this mystery, the team selected one of the most active bacteria, a strain of Micromonospora, and sequenced its entire genetic code. This genomic map revealed that the bacterium possessed twenty distinct clusters of genes dedicated to making complex molecules. While some of these gene clusters matched known pathways for making antibiotics or other drugs, many others appeared to be entirely new. The genetic blueprint suggested the bacterium had the capacity to produce a wide variety of compounds, including some that had never been described. This confirmed that the bacteria were not just making the few known chemicals the researchers had spotted, but were likely synthesizing a much broader array of novel substances.

The study concludes that the sea squirts of the Saint Peter and Saint Paul's Archipelago, and the bacteria that live with them, are a rich and largely unexplored source of new medicines. While the researchers did not isolate a single new drug in this specific study, they demonstrated that these remote oceanic islands harbor microbial communities capable of producing a vast array of unique chemicals. The work highlights the value of looking beyond the animal itself to the microscopic partners that live within it. By combining the study of bacterial genetics with the analysis of their chemical output, scientists can better navigate the ocean's "dark matter"—the millions of unknown molecules waiting to be discovered. This approach offers a powerful path forward for finding the next generation of treatments for diseases like cancer, rooted in the biodiversity of the world's most isolated ecosystems.