Antimicrobial Evaluation of Decoctions and Essential Oils against Cutaneous Propionibacterium acnes

Bacteria cell suspensions were prepared from pure cultures on MHA/24 h/3 days for P. acnes, in a sterile saline solution (API ampoules; BioMérieux, Marcy L’Etoile, France). The density was adjusted to obtain a MacFarland standard of 0.5 at 530 nm, corresponding to 1–5 × 10⁶ cells/mL (spectrophotometer, BioMérieux, Marcy L’Etoile, France). This suspension was diluted in MHB2 to obtain an inoculum suspension of 1–5 × 10⁴ CFU/mL. Equal volumes of cell suspension (50 µL) and decoctions/EOs dilutions (50 µL) were added in each well. The plates were incubated aerobically at 35 °C for 24 h and for 3 days in anaerobiosis for P. acnes. MIC was recorded as the lowest concentration at which no visible growth was observed. The MLC was assessed by spreading 20 µL of culture collected from wells showing no visible growth on MHA plates. The MLC was determined as the lowest concentration at which no colonies grew after 16–18 h incubation at 35 °C (3 days in anaerobiosis for P. acnes).