A method of preparing probiotic nanoparticles can include dissolving formulated probiotics in methanol, spraying the methanol solution into boiling water under ultrasonic conditions to provide a sonicated solution, and stirring the sonicated solution to obtain probiotic nanoparticles. The probiotic nanoparticles may be cluster or rod-shaped. The probiotic nanoparticles may be administered to a subject to reduce oxidative stress or to treat diseases associated with oxidative stress.