The oral microbiota is created through a symbiotic relationship between the host and the microorganisms in the oral cavity. One of the many roles the bacterium in the oral microbiota has is to act as a biofilm and protect the oral cavity. If changes in the oral microbiota occur, it can lead to dysbiosis that can result in oral diseases such as caries. One bacterium that plays an important role in the formation of caries is Streptococcus mutans. S. mutans can adapt to acidic environments by changing their physiology, also known as acid tolerance response (ATR). A well-studied probiotic bacterium with beneficial properties is Limosilactobacillus reuteri. A study has shown that L. reuteri can inhibit biofilm formation and proliferation in S. mutans. The aim of this project was to investigate if L. reuteri PTA 5289 has an effect on the gene expression of luxS, brpA, and ldh in S. mutans UA159 during ATR induction. L. reuteri PTA 5289 and S. mutans UA159 were first cultivated on blood agar, then grown to log-phase (OD₆₀₀=0,5–0,8) MRS broth respective TH broth. The bacteria were introduced to minimal media 4 (MM4) pH 7.5 to allow adherence to the surface, then they were exposed to MM4 pH 5.5 to induce the ATR. Method development was performed to obtain sufficient RNA after ATR induction. Initially, the bacteria were allowed to adhere and adapt first in Ibidi flow cells, then 6-well cell culture plates, and finally it was extended to petri dishes. The gene expression of luxS, brpA, and ldh was analyzed with Reversed-Transcription quantitative Polymerase Chain Reaction (RT-qPCR). The result showed that L. reuteri PTA 5289 has an inhibitory effect on luxS, brpA, and ldh gene expression in S. mutans UA159 in the course of ATR induction.