119 ODONTO Dental Journal. Volume 8. Nomor 2. Desember 2021 ANTIBACTERIAL EFFECT OF NANNOCHLOROPSIS OCULATA AS ROOT CANAL STERILIZATION MATERIAL ON STREPTOCOCCUS MUTANS BIOFILM Diana Soesilo*, Sinta Puspita*, Phebe Fedora Christabel** ABSTRACT Background: Streptococcus mutans in the most frequent microbiota that causes pulp necrosis because of caries. The microorganism that is colonized and embedded in the biofilm matrix is resistant to antimicrobials compared to planktonic cells. Root canal sterilization materials must have good biocompatibility with tissues. Nannochloropsis oculata is an algae that contains various compounds such as terpenoids, alkaloids, and flavonoids that have potential as antibacterial and antioxidant and can be used as alternative to root canal sterilization. Method: This research was true experimental laboratory research with post- test only control group design. The antibacterial potential of Nannochloropsis oculata was tested using the biofilm method, divided into 5 groups. The control group was: K- (aquadest), K+ (calcium hydroxide), and the treatment group was given Nannochloropsis oculata: P1 (0.625%), P2 (1.25%), and P3 (2.5 %). Congo Red method test was to determine the formation of biofilm that shows black strains on agar. While biofilm test with Microtiter Plate Assay to measure the value of biofilm that were inhibited in Optical Density (OD) value in the ELISA Reader. The lower the value, the more biofilm inhibited, with OD value, inhibition percentage could counted Result: The result of all treatment groups were increasing in percentage inhibition value shows inhibition in biofilm growth (p <0.05). Conclusion: Nannochloropsis oculata had an antibacterial effect on the biofilm of Streptococcus mutans BACKGROUND Pulp necrosis requires root canal treatment, which is dental treatment by removing the entire pulp tissue, both in the pulp chamber and root canal 1 . Root canal treatment can be divided into three stages, biomechanical preparation of root canals, disinfection and obturation. The most frequent microbiota that causes pulp necrosis because of caries encountered is Streptococcus mutans (S. Mutans). This microorganism produces organic acids, especially lactic acid by fermenting carbohydrates on the surface of the teeth and make a decrease in salivary pH below 5.5 which will result demineralization of the tooth surface and dental caries will formed 2 . Biofilm are colonies of microorganisms that consist of cells from various kinds of bacteria that attach to one another on the surface of the teeth in irreversible form 3 . The formation of biofilm is a defense mechanism for bacteria to stay alive in environmental conditions in the oral cavity and is not easily attacked by the immune response of the host or drugs 4 . This formation process occurs through several phases, which are the formation of * Department of Conservative Dentistry, Dentistry Faculty, Universitas Hang Tuah ** Student of Dentistry Faculty, Universitas Hang Tuah Correspondence: diana.soesilo@hangtuah.ac.id Keywords: Streptococcus mutans biofilm, Nannochloropsis oculata, Antibacterial, Calcium Hydroxide