Bioactivity of essential oils from Eucalyptus globulus and Eucalyptus urograndis against planktonic cells and biofilms of Streptococcus mutans

Streptococcus mutans is the primary bacteria involved in caries. To prevent this disease, fluoride compounds are used; however, these compounds can become toxic at certain concentrations. The purpose of this study was to evaluate the antimicrobial activity of essential oils (Eos) from two eucalyptus species (Eucalyptus globulus and Eucalyptus urograndis) against planktonic and biofilm cells of S. mutans using the agar diffusion assay and by determining the minimum inhibitory concentrations, kinetic of action and ability to inhibit biofilm formation. The synergistic effect of the combined oils was also tested (1:1). Films constructed of biodegradable starch with the EOs were prepared, and their bioactivity was tested using the macrodilution technique. These essential oils were active against S. mutans, and they presented inhibition zones between 14.7 ± 1.2 mm and 35.3 ± 0.34 mm in diameter. It was observed that when the oils were used together, there was an increase in bioactivity. The kinetics of loss of cell viability test showed a reduction of 1 log cycle in the microorganism counts within less than 5 min of contact time. The essential oil from E. globulus gave the best results, requiring only 15 min of contact to cause microbial death; E. urograndis oil required 50 min to induce the same bactericidal effect. These results can be explained by the larger concentration of 1,8-cineole in the EO of E. globulus. Biofilm formation by S. mutans was also inhibited in the presence of the tested oils, which yielded more effective results when compared to 0.1% commercial NaF (sodium fluoride). The increased bioactivity of these oils suggests their technological potential.