Abstract: The objective of the present study was to evaluate the effectiveness of thymoquinone (TQ) against Cronobacter sakazakii strains. The minimum inhibitory concentration (MIC) of TQ against C. sakazakii and its effect on microbial growth kinetics were determined. Changes in intracellular ATP concentration, intracellular pH (pHin), membrane potential and membrane integrity were measured to elucidate the possible antimicrobial mechanism. Cell morphology changes were also observed under a field emission scanning electron microscope. The results showed that the MICs of TQ against C. sakazakii strains ranged from 0.3 to 0.6 mg/mL, and TQ exposure resulted in a longer lag phase and lower specific growth rate compared to the control. TQ affected the cell membrane permeability of C. sakazakii, as evidenced by a reduction in intracellular ATP concentration from 6.52 to 0.27 and 0.17 μmol/L and a decrease in pHin from 5.69 to 5.22 and 4.99 at MIC and 2 × MIC, respectively. TQ at both tested concentrations caused 80% and 22% reduction in cell integrity, respectively. In addition, cells treated with TQ showed hyperpolarized cell membrane. Field emission scanning electron microscopy illustrated that C. sakazakii cells exposed to TQ exhibited a more wrinkled surface compared with the smooth surface of untreated cells. In conclusion, TQ exerts its antimicrobial action partly by causing excessive permeability of the cell membrane and changing cell morphology. These findings suggest that TQ has potential application in controlling C. sakazakii in foods.

Key words: Cronobacter sakazakii, thymoquinone, cell membrane, membrane potential