The cytotoxicity assays of the EO and its fractions are shown in Table 3. A relationship between cytotoxicity and antimicrobial activity was established through the selectivity index (SI). The SI values were calculated by the logarithm of the ratio of the CC50 and the MIC value for microorganisms (SI = log [CC50]/[MIC]). Positive values represent elevated selectivity against microorganisms, whereas negative values indicate elevated toxicity towards HeLa cells (Case et al., 2006). The SI for the EO at inhibitory concentrations of 400 and 100 μg mL−1 were −0.46 and 0.14, respectively. The positive value found for the concentration of 100 μg mL−1 indicates that the EO is more selective against P. nigrescens and P. gingivalis than HeLa cells. On the other hand, the antimicrobial activity of the EO was moderated (400 μg mL−1), and the SI indicated higher toxicity to HeLa cells.