Synergistic Antibacterial Activity of Ecklonia cava (Phaeophyceae: Laminariales) against Listeria monocytogenes (Bacillales: Listeriaceae)

Abstract

In an effort to discover alternative antimicrobials against Listeria monocytogenes, several marine algae were screened. The methanolic extract of Ecklonia cava exhibited the highest antibacterial activity against L. monocytogenes, with the ethyl acetate (EtOAc) soluble fraction of E. cava methanolic extract having a MIC value of <TEX>$256{\mu}g/mL$</TEX> and a MBC value of <TEX>${\mu}g/mL$</TEX>. The MIC values of streptomycin in combination with the EtOAc fraction were markedly reduced up to 64-fold, suggesting that the antibacterial activity of the antibiotic was restored when combined with the EtOAc fraction. The interaction between streptomycin and the EtOAc fraction was assessed by fractional inhibitory concentration (FIC) indices. The combination of streptomycin and the EtOAc fraction against L. monocytogenes resulted in <TEX>${\Sigma}FIC_{min}$</TEX> range of 0.141 to 0.266 and <TEX>${\Sigma}FIC_{max}$</TEX> of 0.531 for all strains. The median <TEX>${\Sigma}FIC$</TEX> against L. monocytogenes strains ranged from 0.172 to 0.344. Thus, synergistic ranges of FIC <1 were observed for all combinations of streptomycin and the EtOAc fraction against L. monocytogenes strains. Indeed, the median <TEX>${\Sigma}FIC$</TEX> of streptomycin-EtOAc fraction ranged from 0.172 to 0.344, suggesting a marked synergy.