Antibacterial Effects of Essential Oils of Seven Medicinal-Aromatic Plants Against the Fish Pathogen Aeromonas veronii bv. sobria: To Blend or Not to Blend?

Abstract

Despite progress achieved, there is limited available information about the antibacterial activity of constituents of essential oils (EOs) from different medicinal-aromatic plants (MAPs) against fish pathogens and the complex interactions of blended EOs thereof. The present study aimed to investigate possible synergistic antimicrobial effects of EOs from seven Greek MAPs with strong potential against Aeromonas veronii bv. sobria, a fish pathogen associated with aquaculture disease outbreaks. The main objective was to evaluate whether blending of these EOs can lead to increased antimicrobial activity against the specific microorganism. A total of 127 combinations of EOs were prepared and their effect on A. veronii bv. sobria growth was tested in vitro. We examined both the inhibitory and bactericidal activities of the individual EOs and compared them to those of the blended EOs. The vast majority of the investigated combinations exhibited significant synergistic and additive effects, while antagonistic effects were evident only in a few cases, such as the mixtures containing EOs from rosemary, lemon balm and pennyroyal. The combination of EOs from Greek oregano and wild carrot, as well as the combinations of those two with Spanish oregano or savoury were the most promising ones. Overall, Greek oregano, savoury and Spanish oregano EOs were the most effective ones when applied either in pure form or blended with other EOs.

Keywords: Greek native MAPs; antimicrobial activity; aquaculture; bacterial pathogens; essential oil combinations; natural products; synergistic effects.

Figure 1

Inhibitory concentrations (IC₅₀ in μg mL⁻¹) of 127 blends of EOs from seven Greek aromatic-medicinal plants against the fish pathogen Aeromonas veronii bv. sobria. The inset highlights the IC₅₀ values of the top-10, most effective blends. The composition of the seven EOs in each blend is shown in Table S2 (Supplementary Materials).

Figure 2

(a) Inhibitory Concentration (IC₅₀ in μg mL⁻¹; upper panel) and (b) Minimum Bactericidal Concentration (MBC in μg mL⁻¹; lower panel) for the top-10 most effective blends of essential oils against the fish pathogen Aeromonas veronii bv. sobria. The exact composition of the seven essential oils in each blend is shown in Table S2.

Figure 3

Principal component analysis scores plot (a) and loadings plot (b) derived from the compositional data of the 127 essential oils’ blends examined (see Table S1 Supplementary Materials). The blue dots reflect the differences in the chemical composition of individual blends, while the red vectors represent the 36 chemical compounds of the essential oils that were used as active variables. The half maximal inhibitory concentration (IC₅₀) of the blends against Aeromonas veronii bv. sobria is also projected on the loading plot (blue vector; supplementary variable).

Figure 4

Ratios between theoretical and measured 50% inhibitory concentration (IC_{50_T/M}) of the investigated 127 essential oils (EOs) blends. The datapoints lying on the dashed line (IC_{50_T/M} = 1) reflect an additive antibacterial activity of EOs against Aeromonas veronii bv. sobria, while the datapoints above and below the dashed line represent synergistic and antagonistic interactions of EOs, respectively.