Caulerpa (C.) sertularioides has many therapeutic uses in the practice of traditional medicine in Malaysia. Crude methanolic, diethyl ether extract, ethyl acetate extract and butanolic extract from C. sertularioides were subjected to antimicrobial screening including the three Gram-positive and three Gram-negative diarrhea-caused bacteria.The antimicrobial activities were studied by using disc diffusion method and broth dilution method. The effect of the extract on the growth profile of the bacteria was examined via time-kill assay. In addition to the bactericidal effects study, microscopic observations using Scanning electron microscopy (SEM) was done to determine the major alterations in the microstructure of Bacillus (B.) subtilis.Ethyl acetate extract demonstrated antibacterial activity towards all the tested bacteria and produced inhibition zone ranging from ≤ 9 mm - ≥ 15 mm. However, all the tested bacteria were resistant to the butanolic extract treatment. B. subtilis growth curve in the presence of the crude methanol extract at MIC showed bacteriostatic. The main abnormalities found from these microscopic observations were morphology alteration of the bacteria cells after exposure to the methanol extract.Data from this study revealed that C. sertularioides may be potential antimicrobial agents against foodborne Gram-positive and Gram-negative bacteria particularly cause diarrhea, and also food spoilage microorganisms.