Streptococcus mutans is one of a small number of recognized pathogens that lives among the hundreds of other bacterial species which comprise the oral flora. The virulence of this organism is intimately associated with its ability to live as an attached biofilm community on the tooth surface, and consequently, there is a great interest in its biofilm lifestyle. Currently, there are no established protocols that facilitate drug screening against this organism while it is entrenched in the biofilm. Furthermore, greater complications arise when attempting to perform these experiments in a multi-species setting. In an effort to circumvent these problems, we developed a quick, real-time, and non-disruptive method to probe the metabolic status of S. mutans growing as either a planktonic culture or a biofilm community. This assay takes advantage of the proven utility of luciferase measurements for drug screening. We placed the luciferase gene under the control of the S. mutans lactate dehydrogenase promoter (ldh) and integrated the construct onto its native position on the chromosome. We found this construct to be both highly expressed (<10000 cells easily detectable) and insensitive to many different growth parameters. When testing this reporter in both planktonic and biofilm cultures receiving either bacteriostatic or bactericidal antibiotics, we found the ldh-luc reporter to be a very accurate measurement of cell viability. Furthermore, we also demonstrated that this assay can generate useful information about the characteristics of intoxication caused by antibiotic activity. In addition, we modified the biofilm assay into the 96-well format and demonstrated the feasibility of high throughput drug screening of biofilm embedded S. mutans.