Inhibition of foodborne bacteria by native and modified protamine: importance of electrostatic interactions

Protamine is a naturally occurring cationic antimicrobial peptide (CAP) that has shown some promise for control of microorganisms in food. It was hypothesized that the antibacterial effect is partially due to protamine's electrostatic affinity to the negatively charged cell envelopes of actively growing bacteria. However, nonspecific binding of the CAPs to negatively charged food particles may reduce the effect in food systems. To test the hypothesis, the antibacterial efficacies of native and reduced charge protamines (chemically modified by randomly blocking 10 to 71% of the guanido groups of the arginine residues) were compared in model and food systems. In Tryptic Soy Broth, moderate reductions of charge (<26%) resulted in either a similar or slightly improved antimicrobial efficacy, measured as the minimum inhibitory concentration (MIC) toward 21 food-related bacteria. Further reductions in positive charge led to lower antimicrobial activity. Compared to protamine, the affinity of reduced charge protamines (10 and 20%) for binding to Listeria monocytogenes cells was higher at pH 7 and 8. As perhaps would be expected, L. monocytogenes is most sensitive to modified protamines in this pH range. Protamine with reduced charge (14 and 23%) inhibited growth of L. monocytogenes in milk as well as total bacteria and coliforms in ground beef significantly (P<0.05) better than native protamine, demonstrating that the reduced charge peptides were more inhibitory in these high protein food matrices. Electrophoretic analysis of the 21 bacteria revealed a statistically significant (P<0.01) relationship with antimicrobial activity, where the most negatively charged bacteria were also the most susceptible to protamine. In conclusion, components of food matrices interfered with the antibacterial effects of the peptides, however; these undesirable interferences were reduced by altering the electrostatic properties of protamine.