Previously two antimicrobial peptides, IKHQGLPQE (caseicin A) and VLNENLLR (caseicin B), were identified following the fermentation of sodium caseinate with the proteolytic strain Lactobacillus acidophilus DPC 6026. This study evaluated the ability of these peptides to kill Enterobacter sakazakii ATCC 12868 spiked in reconstituted infant formula. The survival of E. sakazakii populations in reconstituted infant formula containing a sodium caseinate fermentate was compared with survival in formula containing positive (monocaprylin) and negative controls. The L. acidophilus DPC 6026 sodium caseinate fermentate reduced pathogen numbers by >4 log CFU/ml at 37 degrees C, comparing favorably with the activity of monocaprylin. Additionally, E. sakazakii NCTC 8155 was inoculated into pasteurized, reconstituted infant formula (6 log CFUlml) followed by the addition of increasing concentrations of the fermentate (0.21 to 6.7% [wt/vol]). At a concentration of 0.21% (wt/vol), pathogen viability was maintained over 4 h at 6.0 log CFU/ml. In contrast, pathogen numbers increased approximately 100-fold in the control formula in the same time frame. At higher final fermentate concentrations (-3.33% [wt/vol]), numbers were reduced to 0 log CFU/ml over 60 min. The spectrum of activity of the fermentate against other foodborne pathogens was also determined and shown to be effective against Escherichia coli O157:H7 and Listeria innocua. Results indicate the potential of this fermentate as a built-in protection mechanism against E. sakazakii strains in reconstituted infant formula.