Medicinal Herbs Enrich “Immune‐Boosting” Bacteria
Authors: Kalpana Kriti , Kawamura Akira 
Published: November 2023

Plant‐microbe interactions have been the focus of extensive research over decades. Several benefits of symbiotic bacteria to plants have been reported, for instance, as growth‐promoters and pest control agents. Recently, the presence of bacteria in association with medicinal plants, and the correlation with their immune‐boosting properties has been suggested. We call these bacteria as “immune‐boosting”. Pasco and co‐workers, successfully demonstrated not just bacteria being present in a herb, Echinacea purpurea, used for common cold treatment, but also correlated the total bacterial load with its immune‐boosting activity. A more recent study by Cech's group also found immune‐boosting compounds in Echinacea, contributed by endophytic bacteria.

Independently, we have demonstrated the presence of such bacteria in association with Juzen‐taiho‐to (JTT), an immune‐boosting formulation of ten medicinal plants used widely in East Asia. Immunostimulatory factors were purified from JTT and subjected to mass spectrometric analysis and the main active factors were found to be lipopolysachharides (LPS) of Gram‐negative bacteria. This led us to our hypothesis: bacteria are enriched in medicinal plants, and they contribute to the immunostimulatory activity of these plants. However, little is known about the identities of these immune‐boosting bacteria. To this end, we conducted metagenomic analysis of the 16S ribosomal RNA obtained from the dried roots of Angelica sinensis, the most potent herb among the ten JTT components. The 16S sequencing revealed a water and soil‐borne bacterium, Rahnella aquatilis as the most predominant species associated with A. sinensis. The abundance of Rahnella in different batches of A. sinensis tested also seemed to correlate with their corresponding immunostimulatory activities. This further strengthened our hypothesis as well as indicated Rahnella to be an immune‐boosting bacterial species.

Rahnella, being a Gram‐negative bacterium, presents LPS on the surface of its cell wall. LPS is a well‐known potent immunostimulant. The endotoxic or potent component of LPS is Lipid A (LA). We then sought to isolate the LPS and subsequently LA from Rahnella aquatilis. The different “LA‐like species” obtained were analyzed for their immunostimulatory activities and showed significant activities. The activities were estimated by quantitative Real Time‐Polymerase Chain Reaction (qRT‐PCR) of intercellular adhesion molecule 1 (ICAM‐1) in THP‐1 monocytes. The toxicity levels of these LA‐like species were estimated by the Limulus amebocyte lysate (LAL) assay, an endotoxin detection assay. The comparison of the immunostimatory activities of the Rahnella LA‐like factors and their endotoxin levels revealed some species with high activity but low endotoxicity. Taken together, these results corroborate the notion that bacteria are associated with medicinal herbs and their bacterial components like LPS and LA contribute immune‐boosting activities to us.

In conclusion, this work supports the possibility of immune‐boosting bacteria such as Rahnella being enriched in herbal plants. For centuries, people have consumed medicinal herbs for the treatment of several symptoms and ailments. We now believe that, in addition to the benefits derived from the herbs, they may be unknowingly consuming and benefiting from these immune‐boosting bacteria as well.

Support or Funding Information

NIGMS/NIH SC3 GM094070 (Research Support); NIMHD/NIH G12 MD007599‐27 (Hunter College CTBR)