Methods: The Minimum Inhibitory Concentration (MIC) of the essential oil was performed by microdilution technique, in which was done the serial dilution, starting from the initial concentration of 8% to 0,0625%. Using a 96-well U-bottom microtiter plate, were inserted 100µL of RPMI-1640 broth, 100µL of the dilution of essential oil (at first well) and 10µL of the fungal suspension (1,5x106 organisms/mL). After incubation (37ºC for 48h), the MIC was considered the lowest concentration of the product under test capable of producing visible inhibition of the fungal growth. The absence of fungal growth was proven by the use of TTC dye. After evaluate the MIC of R. officinalis, the combined effect of the natural product and nystatin (100.000 UI/mL) was investigated. Using a 96-well U-bottom microtiter plate, were inserted: 100 µL of RPMI-1640 broth, 50µL of nystatin, 150µL of essential oil (concentrations ranging among MIC×4, MIC×2, MIC, MIC÷2 and MIC÷4); and 20µL of the fungal suspension. The nystatin alone was evaluated in concentration of clinical use (100.000 UI/mL). Fungal growth was evidenced through the use of TTC dye. The tests were performed in triplicate, and the microplates were incubated at 37°C for 48 hours.
Results: The R. officinalis essential oil presented MIC at concentration 18,0mg/mL (2%). Nystatin showed antifungal activity at evaluated concentration. When associated to nystatin, the essential oil inhibited the fungal growth at all evaluated concentrations.
Conclusions: The R. officinalis essential oil had antifungal activity against C. albicans (ATCC 289065). When combined with nystatin, the natural product did not interfered reducing the antifungal activity of the synthetic drug.