A Syringa microphylla seed extract includes the following components by weight based on a total weight of the Syringa microphylla seed extract: 0.4944-0.7142 mg/g of echinacoside, 6.624-7.617 mg/g of oleuropein, 0.4276-0.6309 mg/g of verbascoside, 3.927-4.684 mg/g of syringin, and 4.505-5.250 mg/g of forsythiaside B. A method of preparing the Syringa microphylla seed extract is disclosed. A composition for treating antibiotic-resistant infections that includes the Syringa microphylla seed extract is also disclosed.