Antibacterial effect of protamine in combination with EDTA and refrigeration

The antimicrobial effect of protamine (clupeine) on a range of Gram-positive and Gram-negative foodborne pathogens and spoilage bacteria, was evaluated using an agar dilution assay and a broth dilution assay with Alamar Blue™ as growth indicator. Protamine was tested alone at concentrations from 0 to 10 000 μg/ml, and in combination with EDTA (0.9 mM). Assays were performed at 5°C, 10°C, 18°C and 30°C to test the effect of temperature. Minimum inhibitory concentration (MIC) values ranged from 10 μg/ml for Brochothrix thermosphacta to no inhibition at 10 000 μg/ml for bacteria such as Aeromonas hydrophila, proteolytic strains of Clostridium botulinum, Hafnia alvei and Morganella morganii. The minimum bactericidal concentrations (MBCs) were generally higher than MICs. In combination with EDTA, MICs of protamine decreased for Gram-negative test strains, whereas EDTA alone inhibited Gram-positive strains. The effect of assay incubation temperature was variable and not clear for most strains. Concentrations of 100–750 μg/ml protamine inhibited the five non-proteolytic C. botulinum strains, while none of the eight proteolytic strains was inhibited, indicating the possible role of proteolytic enzymes in protecting cells from protamine. Clearing zones, indicative of proteolytic activity, were observed in the opaque TSB–agarose around colonies of some but not all protamine-resistant bacteria, suggesting that this is not the only resistance mechanism. Addition of 5% (w/v) gelatin to study the effect of an increased protein concentration in the agar dilution assay showed that electrostatic interactions between protamine and the protein decreased the antimicrobial efficacy of the peptide.