In recent years, the aquaculture industry, particularly striped catfish farming in the Mekong Delta, has faced numerous challenges caused by disease outbreaks. One of the most dangerous pathogens is Aeromonas hydrophila, the causative agent of hemorrhagic septicemia, which can result in mortality rates of up to 90%. To control disease outbreaks, farmers often rely on antibiotics; however, excessive antibiotic use has led to several adverse consequences, including antibiotic residues in aquatic products and the emergence of antibiotic-resistant bacteria.

In this context, the search for biological alternatives to antibiotics has become an essential trend. One promising approach is the exploitation of endophytic bacteria from plants, particularly medicinal plants. Endophytic bacteria are microorganisms that inhabit plant tissues without causing harm to their host. These microorganisms are capable of producing a wide range of secondary metabolites, such as alkaloids, phenolic compounds, terpenoids, and natural antibiotics, which can inhibit pathogenic microorganisms.

Purple Adenosma (Adenosma bracteosum Bonati) is a valuable medicinal plant species commonly found in the Lo Go – Xa Mat National Park in Tay Ninh Province, Vietnam. This plant has been reported to possess various biological activities, including potential applications in the treatment of diabetes, cancer, and other diseases. However, previous studies have mainly focused on bioactive compounds extracted from the plant itself, while the endophytic bacteria residing within the plant have remained largely unexplored.

To address this research gap, a study was conducted to isolate endophytic bacteria from the roots, stems, and leaves of Adenosma bracteosum. The results showed that a total of 36 endophytic bacterial strains were successfully isolated. Among them, stem samples accounted for the highest proportion (39%), followed by root samples (33%) and leaf samples (28%). The isolated strains exhibited diverse morphological characteristics, predominantly rod-shaped forms, including both Gram-positive and Gram-negative bacteria.

Following isolation, the bacterial strains were evaluated for their antibacterial activity against Aeromonas hydrophila using the agar diffusion method. The results revealed that 8 out of the 36 isolates exhibited antibacterial activity. Notably, strain SB1R13.2 demonstrated the strongest inhibitory effect, producing an inhibition zone of approximately 18 mm, comparable to that of the antibiotic tetracycline. This finding highlights the significant potential of endophytic bacteria as alternatives or supplements to conventional antibiotics.

To further assess their antibacterial efficacy, co-culture experiments were conducted in liquid media containing both the endophytic bacteria and the pathogen. The results showed that strain SB1R13.2 completely inhibited the growth of Aeromonas hydrophila after 48 hours of incubation. In contrast, the control treatment without endophytic bacteria allowed the pathogen to maintain normal growth and population density. These findings confirmed the strong antibacterial activity of strain SB1R13.2 under conditions more closely resembling practical applications.

To identify this promising bacterial strain, the research team performed 16S rRNA gene sequencing analysis. The results demonstrated that strain SB1R13.2 shared 100% sequence similarity with Bacillus velezensis. This bacterial species is widely recognized for its ability to produce antimicrobial compounds and has been extensively applied in both agriculture and aquaculture. The discovery of an endophytic Bacillus velezensis strain within Adenosma bracteosum further enhances the biological significance of this medicinal plant.

The study's findings not only contribute valuable scientific knowledge but also offer considerable practical applications. The utilization of endophytic bacteria derived from medicinal plants for the production of biological products may help reduce antibiotic use in aquaculture, thereby mitigating antibiotic resistance and improving product quality. Furthermore, this approach aligns with the growing trend toward sustainable and environmentally friendly agricultural and aquacultural practices.

In conclusion, the study demonstrated that Adenosma bracteosum is a valuable biological resource harboring endophytic bacteria with strong antibacterial activity. In particular, Bacillus velezensis strain SB1R13.2 showed significant potential for the prevention and control of diseases caused by Aeromonas hydrophila. These findings provide an important scientific foundation for future research aimed at developing biological products for the aquaculture industry.