Abstract

(Objective)  This study aimed to evaluate the antimicrobial efficacy of essential oil (EO) from diverse chemotypes of Lippia graveolens against oxytetracycline-resistant Aeromonas hydrophila, which primarily affects the tilapia aquaculture (O. niloticus) in Guatemala.

(Methodology)  L. graveolens were collected in three departments in Guatemala. The EO was obtained by hydrodistillation and characterized using gas chromatography and mass spectrometry (GC/MS). Subsequently, an antimicrobial assay was conducted using disk and dilution susceptibility tests and evaluation of synergistic interactions among the chemotypes. Each test was performed in triplicate.

(Results)  The analysis revealed the presence of twenty-seven compounds in the EO obtained from the chemotypes, with the main class being monoterpenes. The major constituents identified were cis-Dihydro-β-terpineol (8.84%) in chemotype I, carvacrol (51.82%) in chemotype II, and thymol (79.62%) in chemotype III. All EO chemotypes of L. graveolens demonstrated the ability to inhibit the A. hydrophila growth. Thymol chemotype exhibited the strongest inhibitory effect against bacterial growth, with a minimum inhibitory concentration (MIC) of 92.4 µg/mL and a minimum bactericidal concentration (MBC) of 184.8 µg/mL. Furthermore, the results suggest that there is no synergistic or additive effect when combining different chemotypes of L. graveolens.

(Conclusions)  This is the first report of L. graveolens chemotypes exhibiting antimicrobial activity against oxytetracycline-resistant A. hydrophila. The findings suggest that the chemotype thymol could be a potential treatment for infections in the tilapia aquaculture in Guatemala.

Keywords: essential oil; oregano; tilapia; aquaculture; thymol