In vitro EVALUATION OF THE ANTIMICROBIAL ACTIVITY OF PHENOLIC ESSENTIAL OILS AGAINST STREPTOMYCIN-RESISTANT Erwinia amylovora (Burrill)

Evaluación de aceites esenciales contra E. amylovora

Abstract

Fire blight, caused by Erwinia amylovora (Burrill), represents a major threat to apple production in Mexico, particularly due to the emergence of antibiotic-resistant strains. The objective of this study was to evaluate the antimicrobial activity of essential oils (EOs) rich in phenolic compounds, such as thymol and carvacrol, against a streptomycin-resistant E. amylovora strain (K43R mutation in rpsL, MIC ? 1,000 ?g mL?¹), and to determine its antibiotic susceptibility profile. Nine EOs obtained by steam distillation and characterized using the MicroScan autoSCAN-4 system were evaluated. Antimicrobial activity was assessed using the disk diffusion method on nutrient agar at concentrations of 25, 50, 75, and 100% (v/v). Thymus vulgaris at 100% showed the largest inhibition halo (36.5 ± 0.62 mm), followed by Origanum vulgare (35.8 ± 0.45 mm), cultivated Lippia graveolens (34.9 ± 0.37 mm), and wild L. graveolens (30.6 ± 0.41 mm). In contrast, EOs from Mentha piperita, Lavandula angustifolia, Ocimum basilicum, Eucalyptus globulus, and Citrus sinensis exhibited inhibition zones ? 18 mm. The most active EOs corresponded to those with higher thymol and carvacrol content. The antibiogram revealed resistance to penicillin, ampicillin, and amoxicillin, and sensitivity to gentamicin and tetracycline. These results indicate that phenolic essential oils have significant potential to inhibit resistant strains of E. amylovora and suggest that their integration with conventional antibiotics could contribute to the development of more effective management strategies for controlling fire blight.