Purpose: To investigate the antibacterial activity of extracts from Catunaregam tomentosa on Bacillus subtilis and Staphylococcus aureus, and the bacterial responses to the extracts.Methods: The antibacterial activity of fruit, leaf and stem bark extracts were evaluated against B. subtilis (ATCC6633) and S. aureus (ATCC25923). Using a disc diffusion method, extracts at concentrations ranging from 50 -1,000 µg/disc were tested. The minimum inhibitory (MIC) and minimum bactericidal concentrations (MBC) of the extracts against the test bacteria were determined. Fluorescent activated cell sorting (FACS) was used to assess the responses of both types of bacteria to the extracts. Results: The fruit and leaf extracts at 1,000 µg/disc showed optimum efficacy against B. subtilis and S. aureus with MIC of 1,000 µg/mL against both B. subtilis and S. aureus, for the fruit and the leaf extracts. With increasing doses of fruit and leaf extracts at 6 h of incubation, FACS profiles revealed that cell death for B. subtilis increased. The fruit and leaf extracts of C. tomentosa also exhibited antibacterial activity against S. aureus in a dose-and time-dependent manner. The bacteria initially lost their granularity, then lost membrane integrity, and consequently died. Conclusion: The fruit and leaf extracts of C. tomentosa exhibit significant antibacterial potential against Gram-positive bacteria by damaging bacterial granularity and membrane integrity. This is an Open Access article that uses a funding model which does not charge readers or their institutions for access and distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0) and the Budapest