Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2009
Publication Date: 6/1/2010
Citation: Somkuti, G.A., Paul, M. 2010. Enzymatic fractionation of the antimicrobial peptides casocidin and isracidin by Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. Applied Microbiology and Biotechnology. 87:235-242. Interpretive Summary: The basic requirements for protecting the nutritional and health-promoting properties of manufactured foods include the prevention of bacterial contamination. Several fragments of bovine milk proteins are known to have antimicrobial activities and may be valuable in applications as biodefensive agents in food preservation. The research evaluated the stability of casocidin and isracidin, two antimicrobial peptides found in bovine casein, during exposure to bacteria that are customarily used in the preparation of yogurt and other fermented dairy products. Exposure to yogurt bacteria at low levels of acidity resulted in more extensive breakdown of casocidin than isracidin. The breakdown was caused by protein degrading enzymes present in the cell envelopes of yogurt starter cultures. At high levels of acidity that would be typical at the end point of yogurt fermentation and unfavorable for protease activity, both casocidin and isracidin remained relatively stable. This research shows that casocidin and isracidin can be used as natural protective agents in fermented dairy foods against undesirable bacteria. However, the addition of these food grade antimicrobial agents must be timed at or near the end of yogurt fermentation process for maximum food protection.
Technical Abstract: The cumulative effect of peptidase and protease activities associated with cells of Streptococcus thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB) was evaluated on the milk-protein based antimicrobial peptides casocidin and isracidin. Reaction mixtures of casocidin or isracidin and nonproliferating mid-log cells of these essential yogurt starter cultures were individually incubated for up to 4 h at pH 4.5 and 7.0, and samples removed at various time points were analyzed by reverse phase-high performance liquid chromatography (RP-HPLC) and MALDI-TOF-MS/MS. Both casocidin and isracidin remained largely unchanged following exposure to cell suspensions of ST or LB strains at pH 4.5. Casocidin was extensively degraded by both ST and LB strains at pH 7.0, whereas isracidin apparently remained largely intact after incubation for 4 h with ST strains but was degraded by exposure to LB strains. The results showed the feasibility of using the bovine-casein based peptides casocidin and isracidin as food grade antimicrobial supplements to impart fermented dairy foods additional protection against bacterial contamination. The structural integrity and efficacy of these biodefensive peptides may be preserved by timing their addition near the end of the fermentation of yogurt-like dairy foods (at or below pH 4.5), when conditions for bacterial proteolytic activity become unfavorable.