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, the growth of 20 C. sakazakii strains—including 10 clinical and 10 food isolates—was completely inhibited in the presence of kefir supernatant. Significant differences in the diameters of inhibition zones were observed upon treatment with kefir as compared to Lactobacillus kefiri and Candida kefyr culture supernatants or solutions of lactic and acetic acid and ethyl alcohol in the agar well diffusion test (P < 0.05). The addition of 100-μl kefir supernatant to 1 ml of nutrient broth completely inhibited the growth of C. sakazakii, as evaluated by spectrophotometry. The antimicrobial activity of kefir supernatant in experimentally contaminated PIF was also tested; we found no viable C. sakazakii cells remaining in PIF rehydrated with 30% kefir supernatant solution for 1 h.
Significance: This is the first study demonstrating the antimicrobial effects of kefir against C. sakazakii. Kefir has antimicrobial effects against both clinical and food-isolated C. sakazakii strains and could kill viable C. sakazakii in real PIF samples. In conclusion, the antimicrobial activity of kefir supernatant against C. sakazakii could be applied in real food samples.