Purpose: The antifungal activity in vapor-phase of two Mexican oregano EOs (Lippia berlandieri Schauer or Poliomintha longiflora) on the growth of Aspergillus flavus was assessed in a model system, at selected temperatures and concentrations.
Methods: EOs were obtained by steam distillation, characterized by determining physical properties such as density, refractive index, and color, and their chemical composition by GC-MS. Potato-dextrose agar plates were center inoculated pouring 2μL of A. flavus spore suspension. Inoculated plates were incubated at 25, 30, or 35 °C in 1.7 l airtight chambers, in which plates without lids were placed with selected amounts of each studied EO to achieve concentrations of 14.7, 29.4, 58.8, or 117.6 μl / l of air, as well as a control (without EO). During incubation, colony diameters were measured. A. flavus radial growth rate and lag phase in the presence of vapors of each EO at studied temperatures were calculated. The effect of EO concentration and temperature on growth parameters was evaluated by polynomial equations. Further, minimum inhibitory concentrations (MIC) of EOs at different temperatures in which the effect was fungistatic or fungicidal were determined.
Results: Incubation temperature as well as EO concentration exerted significant effects (P <0.05) on growth rate and lag phase of A. flavus; at higher concentration of EOs and lower temperatures, growth rate decreased and lag phase increased, suggesting that tested EOs act by delaying or inhibiting mold germination stage. P. longiflora EO was significantly (P <0.05) more inhibitory than L. berlandieri EO, which can be attributed to differences in composition of volatile compounds with antimicrobial activity. Carvacrol: thymol ratio in L. berlandieri EO was 12:1 while in P. longiflora EO was 1:3.
Significance: Mexican oregano EOs applied in vapor-phase exert significant antifungal activity.