The object of this study was to discover an alternative therapeutic agent with fewer side effects against acne vulgaris, which is one of the most common skin diseases. Acne vulgaris often associates with acne-related bacteria such as Propionibacterium acnes, Staphylococcus epidermidis, Staphylococcus aureus and Pseudomonas aeruginosa, some of which exhibit a resistant against commercial antibiotics used in the treatment of acne vulgaris (tetracycline, erythromycin, and lincomycin). In the current study, we evaluated in vitro antibacterial activity of chitosan-phytochemical conjugates against acne-related bacteria. Three of chitosan-phytochemical conjugates used in this study showed stronger antibacterial activity than that of chitosan (unmodified control). Chitosan-caffeic acid conjugate (CCA) exhibited the highest antibacterial activity against acne-related bacteria with minimum inhibitory concentration values of 8 μg/mL to 256 μg/mL. In addition, the MICs of antibiotics against antibiotic resistant P. acnes and P. aeruginosa strains were dramatically reduced in the combination with CCA, suggesting that CCA would restore the antibacterial activity of the antibiotics. The analysis of fractional inhibitory concentration indices clearly revealed a synergistic antibacterial effect between CCA and the antibiotics. Thus, the median ∑FIC values against the antibiotic resistant bacterial strains were ranged from 0.375 to 0.533 in the combination mode of CCA and antibiotics.
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 256 µg/mL and a MBC value of 512 µg/mL. 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 ∑FIC min range of 0.141 to 0.266 and ∑FIC max of 0.531 for all strains. The median ∑FIC 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 ∑FIC of streptomycin-EtOAc fraction ranged from 0.172 to 0.344, suggesting a marked synergy.
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