Probiotics are microorganisms beneficial to gastrointestinal health. Although some strains are also known to possess positive effects on oral health, the effects of most intestinal probiotics on the oral microflora remain unknown. We assessed the ability of the intestinal probiotic Lactobacillus salivarius W24 to incorporate into and to affect the compositional stability and cariogenicity of oral microbial communities. Microtiter plates with hydroxyapatite discs were incubated with W24 ("+W24") or without W24 ("-W24") and saliva from four individuals in plain ("-sucrose") or sucrose-supplemented ("+sucrose") medium. Biofilms were subjected to community profiling by 16S rRNA gene-based Denaturing Gradient Gel Electrophoresis (DGGE) after 72 h growth. Diversity (Shannon-Weaver index) and similarities (Pearson correlation) between biofilm communities were calculated. Microcosms "+sucrose" were less diverse and more acidic than "-sucrose" microcosms (p < 0.001). The effects of W24 on the community profiles were pH dependent: at pH 4 ("+sucrose"), the respective "+W24" and "-W24" microcosms differed significantly more from each other than if the pH was ∼7 ("-sucrose"). The pH of "+W24/+sucrose" microcosms was lower (p < 0.05) than the pH of the microcosms supplemented with sucrose alone ("-W24/+sucrose"). Although not able to form a monospecies biofilm, L. salivarius W24 established itself into the oral community if inoculated simultaneously with the microcosm. In the presence of sucrose and low pH, W24 further lowered the pH and changed the community profiles of these microcosms. Screening of probiotics for their effects on oral microbial communities allows selecting strains without a potential for oral health hazards. © 2008 Elsevier Ltd. All rights reserved.