Antibacterial activity of plant extracts highly depends on extraction solvent

As an alternative to antibiotic growth promoters in animal nutrition, that have been banned in the EU in 2006, the demand for plant derived substances (phytogenics) is emerging to counteract bacterial infections in swine and poultry. In contrast to antibiotics, phytogenics are expected to refrain from causing transmissible bacterial resistances and leaving critical residues in animal tissue.

Looking for potential phytogenics, five different plant raw materials (Berberis aristata DC. root, Sophora flavescens Aiton root, Holarrhena antidysenterica (L.) Wall. bark, Bridelia ferruginea Benth. bark and leaves) were selected. Dry extracts were produced of each material using different extraction solvents (ethanol abs., water and 50/50 (v/v) ethanol/water). The antibacterial activity of the extracts on two pathogenic bacteria, Salmonella typhimurium and Clostridium perfringens Type C, was examined with a turbidimetric microdilution method. The bacterial cultures with defined microbial count were incubated together with different concentrations of the extracts. The change in optical density of the bacterial culture led to a quantitative result, indicated as the MIC₅₀ value.

The lowest MIC₅₀ values were reached by the ethanol extracts of B. aristata (78mg/l) and S. flavescens (156mg/l) against C. perfringens. The ethanol and ethanol/water extract of H. antidysenterica showed higher activity against S. typhimurium. In fact, the ethanol extracts of all plant materials were most effective, except for the extracts of B. ferruginea bark, whereof the water extract was most effective against C. perfringens (MIC₅₀ value 156–625mg/l). Based on these findings about extraction solvent-dependent activity, further investigations towards active substance identification will be accomplished.