Purpose: In this study, we evaluated the antimicrobial activity of kefir supernatant against Cronobacter sakazakii in powdered infant formula (PIF).
Methods: Antibacterial activity was assessed by the spot-on-lawn method, agar well diffusion assay, measurement of optical density of culture broths. The antimicrobial activity of kefir supernatant to experimentally contaminated PIF was also investigated by plate count method.
Results: In a spot-on-lawn test, 20 C. sakazakii strains, including 10 clinical isolates and 10 food isolates, were completely inhibited by kefir supernatant. In an agar well diffusion test, significant differences were observed between the diameters of inhibition zones obtained by kefir supernatant and culture supernatants obtained from Lactobacillus kefiri and Candida kefyr, as well as solutions of lactic acid, acetic acid, and ethyl alcohol (P < 0.05). In addition, the growth of C. sakazakii in nutrient broth containing different volumes of kefir supernatant was evaluated by spectrophotometry. The addition of 100 μl of kefir supernatant into 1 ml of nutrient broth completely inhibited the growth of and killed C. sakazakii. Finally, we applied the antimicrobial activity of kefir supernatant to experimentally contaminated PIF. Remarkably, no viable C. sakazakii remained in the PIF rehydrated with 30% kefir supernatant solution.
Significance: In conclusion, kefir could be a great option to control C. sakazakii in PIF owing to its advantages of reduction in antibiotic use and its well-established safety for oral consumption.