Growth Inhibition of Cronobacter sakazakii in Experimentally Contaminated Powdered Infant Formula by Kefir Supernatant

Kefir is a type of fermented milk containing lactic and acetic acid bacteria and yeast. In this study, we evaluated the antimicrobial activity of kefir supernatant against Cronobacter sakazakii in powdered infant formula (PIF). 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 compared with the results for 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 mu l of 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, demonstrating that the antimicrobial activity of kefir supernatant against C. sakazakii could be applied in real food samples.