Background: According to the World Health Organization’s 2022 Global Oral Health Status Report, approximately 3.5 billion people worldwide - nearly half of the global population - suffer from some form of oral disease, among which dental caries is the most prevalent, affecting close to 2 billion people in permanent teeth (global average prevalence of 29%) and 514 million people in primary teeth (global average prevalence of 43%). In Mongolia, the prevalence of dental caries also remains high: a 2017 survey of 1,754 individuals from six provinces and five districts found caries prevalence of 82.9% among children under 5 years of age, 80.9% among those under 12, 97.8% among the population aged 35-44, and 99.1% among those aged 65-74. A 2010 study among Mongolian preschool and school-age children found a caries prevalence of 87.8-90%, with S. mutans identified alone in 75% of culture-positive plaque samples and together with S. sobrinus in the remaining 25%. Given the persistently high global burden of dental caries, preventing the disease rather than merely treating it - for example through “replacement therapy,” in which a less cariogenic strain of S. mutans is used to colonize the oral cavity - requires a reliable rat model in which standard and clinical strains of S. mutans can reproducibly induce dental caries, so that the efficacy of such preventive strategies can be properly evaluated. Untreated dental caries is also estimated to impose an annual global economic burden of approximately US$387 billion in direct costs and US$323 billion in indirect costs, underscoring the growing importance of research into new preventive approaches. Aim: Development of a dental caries model caused by Streptococcus mutans infection in rat teeth. Materials and Methods: This study used an experimental research design. It was conducted from March 30 to April 14, 2021, at the Core Laboratory and the Education Laboratory of the Department of Microbiology and Infection Prevention Control of the Mongolian National University of Medical Sciences (MNUMS). Ethical approval was obtained from the Medical Ethics Review Committee under the Ministry of Health (meeting No. 02, March 18, 2019). Five 8-week-old Wistar rats were divided into four experimental groups - differing by bacterial strain (S. mutans ATCC 25175 standard strain versus a clinical isolate) and by drinking-water composition (with or without sucrose) - plus one negative control group, with one rat assigned to each group. A bacterial suspension was applied to the rats’ molar teeth with a sterile cotton swab three times daily at two-hour intervals for 28 consecutive days. Rats were lightly sedated with midazolam during application, and the suspension was applied during the daytime, when salivary flow is naturally reduced in rodents, in order to promote bacterial colonization of the tooth surface. Result: On day 28, microscopic examination of each rat’s maxillary and mandibular teeth revealed carious spots and cavities in all groups except the sucrose-free negative control. Cavities were deeper in the groups that received sucrose-supplemented drinking water, whereas the sucrose-free groups showed comparatively milder lesions, indicating that bacterial infection alone was sufficient to induce caries. Conclusion Infection of 8-week-old Wistar rats with either the standard or a clinical strain of S. mutans for 28 days successfully induced dental caries, and sucrose intake increased the severity of the lesions. This baseline model can be reproduced within a relatively short time frame and at low cost, and may be applied in future studies evaluating the efficacy of replacement-therapy strains or probiotic products against dental caries.