Purpose: On the basis of these results, we applied the antimicrobial activity of kefir supernatant to controlling Cronobacter sakazakii in powdered infant formula (PIF). In addition, we tested the antimicrobial activity of culture supernatants derived from individual lactic acid bacteria in kefir to identify key microorganisms that mediate these effects.
Methods: The spot on lawn method and growth curve analysis were employed for the screening of the antimicrobial activity of kefir against C. sakazakii. In addition, individual kefir isolates were isolated and tested for their antimicrobial activity. To address the mechanisms, their metabolites were analyzed by using HPLC.
Results: We found no viable C. sakazakii cells remaining in PIF rehydrated with 30% kefir supernatant solution for 1 h, demonstrating the antimicrobial activity of kefir supernatant against C. sakazakii could be applied in real food samples. In addition, a total of 20 C. sakazakiistrains—including 10 clinical and 10 food isolates—was completely inhibited in the presence of kefir supernatant. We found that L. kefiri exerted strongest antimicrobial effects against C. sakazakii among all isolates although the supernatant of L. kefiri has higher pH and lower titrable acidity. Considering that the organic acid spectrum and pH neutralization reduced the L. kefiri-dependent growth suppression, it is inferred that this activity is mainly due to synergistic effect of various organic acids produced by heterofermentation of the strain.
Significance: Our results highlight the applicability of kefir and its individual isolates for preventing C. sakazakii contamination in the food industry.