Antimicrobial effect of lipoic acid against Cronobacter sakazakii

Lipoic acid (LA) has been reported to be an antioxidant and an essential mitochondrial cofactor, but its inhibitory effect on bacterial pathogens has rarely been studied. In this study, antibacterial activity of LA was tested against several Cronobacter sakazakii strains. Minimum Inhibitory Concentration (MIC) of LA against C. sakazakii strains was determined using agar dilution method. Changes in intracellular pH (pHin), membrane potential, intracellular ATP concentration were measured to elucidate the possible antibacterial mechanism. Moreover, cell morphology changes were observed under field emission scanning electron microscope. The MICs of LA against C. sakazakii strains ranged from 2.5 to 5.0 mg/mL. Addition of LA exerted an immediate and sustained inhibition of C. sakazakii proliferation. LA affected the membrane integrity of C. sakazakii, as evidenced by intracellular ATP concentration decrease. Moreover, reduction of pHin and cell membrane depolarization were detected in C. sakazakii after exposure to LA. Electronic microscopy observations confirmed the cell membrane damage of C. sakazakii by LA. Our results demonstrate that LA has moderate antimicrobial activity against C. sakazakii. It exerts its antimicrobial action partly through causing cell membrane dysfunction and changes in cellular morphology. Considering its antimicrobial properties, together with its well-known nutritional functions, LA has potential for development as a supplement in infant formula or other foods.