Identification and determination of antibacterial constituents in residue discharged from garlic-processing plant

Activity-guided fractionation based on in vitro assays was used to identify antibacterial components in the residue wasted in the manufacturing process of aged garlic extract (AGE). Antimicrobial activity of the hexane extract of aged garlic-derived residue (AGR) was evaluated against clinically isolated Gram-positive and -negative bacteria including levofloxacin (LVX)-resistant strains. Among antimicrobial fractions separated from the hexane extract of AGR, several polysulfides, such as diallyl-tetrasulfide (DAS4), -pentasulfide (DAS5) and -hexasulfide (DAS6), were identified as active components, whose chemical structures were determined by means of LC–MS and NMR spectroscopy. We found that those sulfides exhibited antimicrobial activities against Gram-positive bacteria at minimum inhibitory concentrations (MICs) ranging from 32 to 80 μg/mL. Although these sulfides were less active than LVX against most of the bacteria examined, they were more potent against Staphylococcus aureus (S. aureus) including methicillin-resistant S. aureus (MRSA). Surprisingly, DAS5 significantly suppressed the biofilm formation of S. aureus by 60% at a concentration of 1.25 μg/mL, which is extremely lower than its MIC value. These results show that AGR contains several antimicrobial diallyl-polysulfides which have the potential to inhibit the growth and biofilm formation of bacteria and AGR may be a useful bioresource as a natural additive for antibiotic drugs or foods.