Microbial resistance has a major impact on infection control. In this context, the antimicrobial activity of essential oils has gained prominence in modern research, being fundamental to guide their use and to combat microbial resistance. Thus, this study aimed to determine the minimum inhibitory concentration (MIC) and minimum microbicidal concentration (MMC) of four essential oils – Ocimum gratissimum (clove basil), Lantana trifolia (capiçoba), Schinus terebinthifolius (Brazilian peppertree), and Siparuna guianensis (negramina) — against bacterial and fungal strains (Staphylococcus aureus, Streptococcus sobrinus, Candida tropicalis, Cutibacterium acnes).

Antimicrobial activity was evaluated by the broth microdilution method in Mueller Hinton broth, according to the Clinical and Laboratory Standards Institute (CLSI, 2022) protocol. Tested concentrations ranged from 30 to 0.23 mg/mL. Dimethyl sulfoxide (DMSO) was used as a solvent control and azithromycin (40 µg/mL) as a positive control. Microbial inocula were prepared from the 0.5 McFarland scale and serial dilutions, resulting in concentrations of 0.5 × 10⁵ CFU/mL (bacteria) and 0.5 × 10³ CFU/mL (fungi). After incubation at 37°C for 24 h, the MIC was defined as the lowest concentration that visually inhibited growth. The MMC was determined by plating inhibited wells on BHI agar followed by re-incubation.

The four essential oils tested showed variations in antimicrobial activity. Clove basil was the most effective, with MICs between 3.75 and 7.5 mg/mL for all microbial strains. Oils from Brazilian peppertree and negramina showed less expressive activity against this group of microorganisms, with MICs often above 30 mg/mL. Capiçoba oil had variable performance, with good responses against some strains. Overall, the comparison between MIC and MMC indicated a predominant microbicidal effect in the most active oils.

The essential oils tested demonstrated in vitro antimicrobial activity, with emphasis on O. gratissimum. Further studies, including in vivo models, are recommended to better evaluate the potential of these natural compounds.