Seagrass Microbes from Tanzania Hint at New Antibiotic Sources

Beneath the turquoise shallows off Mjimwema, south of Dar es Salaam, the serrated blades of the seagrass Oceana serrulata sway with the tide. To most observers, these meadows are simply nursery grounds for fish and feeding grounds for turtles and dugongs. To a team of microbiologists at the University of Dar es Salaam, however, they represented something far more tantalizing: a hidden reservoir of bacteria that could, in an era of mounting antibiotic resistance, yield the next generation of antimicrobial drugs. Their new study, published in Plant Biosystems, reports the isolation of endophytic Actinomycetota living inside the tissues of this seagrass, several of which appear to be entirely new to science and nearly all of which can kill or inhibit dangerous human pathogens.

Actinomycetota, the bacterial phylum that gave the world streptomycin, erythromycin, tetracycline and countless other antibiotics, have long been the workhorses of natural-product drug discovery. Yet as terrestrial soils have been sampled exhaustively over the past eight decades, the hit rate of genuinely novel compounds has declined, pushing researchers toward underexplored environments. Marine ecosystems, and marine plants in particular, have emerged as promising frontiers. Seagrasses occupy a biologically demanding niche, submerged in saline water, buffeted by tides and constantly exposed to diverse microorganisms, and their internal tissues harbor endophytes that may produce specialized chemistry to help the plant fend off pathogens. That chemical arms race is precisely what makes them attractive targets for bioprospecting.

In the new work, Lucy D. Mbusi, Jacqueline I. Chibona and Thomas J. Lyimo collected specimens of Oceana serrulata from the coastal waters at Mjimwema and set out to culture the actinomycetes residing within the plant’s leaves and roots. Endophytes are organisms that live at least part of their lives inside plant tissue without causing disease, and accessing them requires careful surface sterilization to eliminate microbes merely riding on the outside of the plant. From the processed tissues, the team succeeded in growing seven isolates: five from leaf material and two from roots. All proved to be Gram-positive, with cells ranging from rod-shaped to coccoid, and the colonies displayed a striking diversity of textures and pigments on the culture plates.

To place these organisms on the bacterial tree of life, the researchers sequenced a portion of the 16S rRNA gene, the standard molecular barcode used to identify bacteria and assess how closely an unknown isolate resembles known species. The analysis assigned the seven isolates to five genera: Diaminobutyricimonas, represented by two isolates; Microbacterium, also with two; and single isolates of Ornithinimicrobium, Frigoribacterium and Micrococcus. What caught the team’s attention was not just the breadth of genera but the genetic distances involved. Six of the seven isolates showed 16S sequence similarity to their closest reference strains of only 82.89 to 98.15 percent, well below the thresholds typically used to delineate bacterial species, marking them as putatively novel candidates that will require fuller taxonomic investigation, including genome sequencing and detailed phenotypic comparison, before they can be formally described as new species.

Two of the genera had never before been reported from any seagrass ecosystem anywhere in the world. Diaminobutyricimonas, first described from an air sample in Korea, and Frigoribacterium, a genus of cold-tolerant soil and plant-associated bacteria, had both remained absent from the seagrass literature until now. Their appearance inside Oceana serrulata suggests that the endophytic communities of tropical marine plants remain poorly charted, and that even a modest cultivation effort from a single site can surface lineages with no documented history in this habitat. The finding adds weight to a growing view that seagrass meadows, often overshadowed by coral reefs and mangroves in marine microbiology, deserve far more systematic attention as reservoirs of microbial novelty.

Identifying the microbes was only half of the study. The team extracted crude secondary metabolites from each isolate and tested them against a panel of six clinically significant test organisms: the bacterium Staphylococcus aureus, a common cause of skin and bloodstream infections; Escherichia coli and Salmonella enterica, both leading agents of foodborne and diarrheal disease; Klebsiella pneumoniae, an increasingly drug-resistant hospital pathogen; Pseudomonas aeruginosa, notorious for its resistance to antibiotics and its threat to patients with cystic fibrosis and burns; and the yeast Candida albicans, a frequent cause of fungal infections. Every one of the seven crude extracts showed activity against at least one of these six targets, a remarkably consistent hit rate for a small collection of isolates.

Two strains stood out from the pack. The Diaminobutyricimonas isolate designated ORE1 and the Microbacterium isolate designated ORE7 inhibited all six test microorganisms, spanning Gram-positive bacteria, Gram-negative bacteria and a fungus. That breadth of activity is significant because Gram-negative pathogens such as Pseudomonas and Klebsiella are wrapped in an outer membrane that excludes many conventional antibiotics, making them among the most difficult organisms to treat and a priority for the World Health Organization’s list of resistant bacteria in urgent need of new drugs. A crude extract that can suppress such a wide spectrum of organisms suggests the production of potent bioactive compounds, although the researchers emphasize that the active molecules themselves have not yet been isolated or chemically identified.

The authors are careful about what these results do and do not demonstrate. Crude extracts are complex mixtures, and inhibition in a laboratory assay does not guarantee that the responsible compounds will be chemically tractable, safe, or effective in a clinical setting. The path from a petri dish in Dar es Salaam to a pharmacy shelf is long, typically requiring compound purification, structure elucidation, optimization and years of preclinical and clinical testing. Moreover, the putatively novel status of most isolates rests on partial 16S sequences; confirming new species will demand the full polyphasic taxonomic treatment that modern bacteriology requires. Nevertheless, the combination of genetic novelty and broad antimicrobial activity in the same organisms is exactly the profile that natural-product scientists look for when prioritizing strains for deeper investigation.

The broader context makes the search urgent. Antimicrobial resistance is projected to contribute to millions of deaths annually in the coming decades, and the pipeline of new antibiotic classes has thinned dramatically since the golden age of discovery. Actinomycetes remain the single richest known source of antibiotics, and marine and endophytic lineages are among the least explored branches of the group. Previous work by the same research group has documented bioactive actinomycetes in Tanzanian seagrass sediments, on seagrass surfaces and in the leaves of other seagrass species, building a picture of the Western Indian Ocean’s seagrass ecosystems as a coherent and largely untapped bioprospecting landscape. The partial 16S sequences from the new isolates have been deposited in the NCBI GenBank database, making them available to other researchers.

For now, the serrated seagrass of Mjimwema has yielded seven microbial tenants, six of them possibly new to science, all of them chemically armed. Whether Diaminobutyricimonas ORE1, Microbacterium ORE7 or their relatives will one day contribute a new medicine remains an open question, but the study demonstrates that some of the planet’s most promising microbial chemistry may be hiding in plain sight, inside the leaves and roots of flowering plants that grow beneath the waves. As the authors note, the capacity of these strains to produce secondary metabolites with antimicrobial properties underscores their potential for pharmaceutical, agricultural and biotechnological applications, and it strengthens the case for protecting seagrass meadows not only as habitats for charismatic marine life but as living libraries of undiscovered biology.

Subject of Research: Endophytic marine Actinomycetota from Oceana serrulata seagrass and their antimicrobial activity

Article Title: Putative novel endophytic marine Actinomycetota from Oceana serrulata seagrass: molecular characterization and antimicrobial activity

Article References: Mbusi, L. D., Chibona, J. I., & Lyimo, T. J. (2026). Putative novel endophytic marine Actinomycetota from Oceana serrulata seagrass: molecular characterization and antimicrobial activity. Plant Biosystems, 160(5), Article 267. https://doi.org/10.1007/s44473-026-00282-7

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00282-7

Keywords: seagrass, Actinomycetota, endophytes, antimicrobial activity, marine microbiology, antibiotic resistance, bioprospecting, Tanzania, 16S rRNA sequencing, Microbacterium, Diaminobutyricimonas, natural products