Abstract
Background:
Endophytic bacteria are emerging as promising microbial bioresources for producing biologically active compounds. These organisms live within plant tissues without causing obvious harm and may represent valuable resources for green biotechnology and sustainable drug discovery. Microbial production of bioactive compounds may also provide an alternative to the extensive harvesting of medicinal plants.

Aim:
This study aimed to explore endophytic bacteria associated with Catharanthus roseus as potential microbial sources of bioactive compounds, with particular emphasis on screening their extracts for an analytical profile consistent with vinblastine and evaluating the bacterial extracts’ in vitro cytotoxic activity of bacterial extracts against breast and lung cancer cell lines.

Methods:
Healthy C. roseus leaves were screened for the isolation of endophytic bacteria. The isolate Brevibacillus borstelensis AUMC B-448 was selected based on morphological and molecular identification and its analytical profile consistent with that of vinblastine. A compound consistent with vinblastine was evaluated using thin-layer chromatography, ultraviolet–visible (UV–Vis) spectrophotometry, and HPLC by comparison with an authentic vinblastine standard. An ethyl acetate crude extract of the bacterial culture was prepared, and its in vitro cytotoxic activity was evaluated against MCF-7 breast adenocarcinoma and A-549 lung cancer cells using the sulforhodamine B (SRB) assay. The 16S rRNA gene sequence of the strain was deposited in GenBank under accession number OR770212.

Results:
The crude ethyl acetate extract of B. borstelensis AUMC B-448 exhibited in vitro cytotoxic activity against both cancer cell lines. The IC50 values were 17.87 μg/ml against MCF-7 cells and 7.08 μg/ml against A-549 cells. The lower IC50 value indicated greater sensitivity of A-549 cells to the crude extract under the experimental conditions. Thin-layer chromatography, UV–Vis spectrophotometry, and HPLC analyses showed an analytical profile consistent with the authentic vinblastine standard, supporting the presence of a compound consistent with vinblastine in the bacterial crude extract.

Conclusion:
The findings highlight B. borstelensis AUMC B-448 as a promising endophytic bacterial resource for investigating bioactive compounds with potential cytotoxic activity. To the best of our knowledge, this is the first report describing the in vitro cytotoxic activity of an ethyl acetate crude extract obtained from an endophytic B. borstelensis isolate associated with C. roseus leaves. These findings support the potential of endophytic bacteria as microbial bioresources for pharmaceutical applications and discovery of natural products. However, the present findings represent preliminary in vitro evidence, and further studies are required to confirm the chemical identity and biosynthetic origin of the detected compound, as well as its cytotoxic selectivity and molecular mechanisms.

Key words: Bioactive compounds; Brevibacillus borstelensis; Endophytic bacteria; in vitro cytotoxic activity; Sustainable bioresources.