摘要Objective: To investigate antimicrobial activities of methanolic extract of leaves of Cleistocalyx operculatus L. (C. operculatus) grown in Vietnam. Methods: The methanolic extract of C. operculatus leaves was phytochemically screened and tested for its antimicrobial activity against six Gram-positive bacteria (three of which were antibiotic multiresistant Staphylococcus spp.), two Gram-negative bacteria, and one fungal species using an agar diffusion method. Anticaries activity was tested using pH drop and biofilm assays formed in 96-well plastic plates. Results: Phytochemical screening revealed the presence of flavonoids and terpenes, in which flavonoid content was 6.8 mg/g dry material. Antibacterial activity of the C. operculatus extract was shown only against Gram-positive bacteria Staphylococcus aureus, Bacillus subtilis and Streptococcus mutans GS-5 (S. mutans), and three multiresistant bacteria being Staphylococcus epidermidis 847, Staphylococcus haemolyticus 535 and Staphylococcus aureus North German epidemic strain. Interestingly, methanolic extract of C. operculatus leaves exhibited the anticaries activity against S. mutans in terms of inhibition of acid production and biofilm formation. Activity of two key enzymes responsible for acidogenicity of S. mutans, F-ATPase and phosphotransferase system were inhibited by the extract with IC50 of 51.0 and 98.0 mg/mL, respectively. Cytotoxicity of the extract against keratinocytes was found only for higher concentrations [IC50=(119.98 ± 4.63) mg/mL]. Conclusions: The methanolic extract of C. operculatus leaves has the potential for development of antimicrobial preparations, especially anticaries products.