| Summary: | Dental caries is one of the most common chronic diseases worldwide. <i>Streptococcus mutans</i> and <i>Candida albicans</i> are two major pathogens associated with dental caries. Several recent studies revealed that <i>Lactobacillus plantarum</i> inhibits <i>S. mutans</i> and <i>C. albicans</i> in biofilms and in a rodent model of dental caries. The aim of this study was to investigate the dose-dependent effect of <i>L. plantarum</i> against <i>S. mutans</i> and <i>C. albicans</i> in a planktonic model that simulated a high-caries-risk clinical condition. Mono-, dual-, and multi-species models were utilized, with five doses of <i>L. plantarum</i> (ranging from 1.0 × 10<sup>4</sup> to 1.0 × 10<sup>8</sup> CFU/mL). Real-time PCR was used to assess the expression of the virulence genes of <i>C. albicans</i> and <i>S. mutans</i> and the genes of <i>L. plantarum</i>. Student’s <i>t</i>-tests and one-way ANOVA, followed by post hoc tests, were employed to compare the cell viability and gene expression among groups. A dose-dependent inhibition on <i>C. albicans</i> and <i>S. mutans</i> was observed with increased dosages of <i>L. plantarum</i>. <i>L. plantarum</i> at 10<sup>8</sup> CFU/mL demonstrated the highest antibacterial and antifungal inhibitory effect in the dual- and multi-species models. Specifically, at 20 h, the growth of <i>C. albicans</i> and <i>S. mutans</i> was suppressed by 1.5 and 5 logs, respectively (<i>p</i> < 0.05). The antifungal and antibacterial effects were attenuated in lower doses of <i>L. plantarum</i> (10<sup>4</sup>–10<sup>7</sup> CFU/mL). The expression of <i>C. albicans HWP1</i> and <i>ECE 1</i> genes and <i>S. mutans lacC</i> and <i>lacG</i> genes were significantly downregulated with an added 10<sup>8</sup> CFU/mL of <i>L. plantarum</i> (<i>p</i> < 0.05). The addition of 10<sup>8</sup> CFU/mL <i>L. plantarum</i> further inhibited the hyphae or pseudohyphae formation of <i>C. albicans</i>. In summary, <i>L. plantarum</i> demonstrated dose-dependent antifungal and antibacterial effects against <i>C. albicans</i> and <i>S. mutans</i>. <i>L. plantarum</i> emerged as a promising candidate for the creation of novel antimicrobial probiotic products targeting dental caries prevention. Further research is warranted to identify the functional metabolites produced by <i>L. plantarum</i> at different dosages when interacting with <i>C. albicans</i> and <i>S. mutans</i>.
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