Antibacterial effect of protamine assayed by impedimetry

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Abstract
Impedimetric measurements were used to assay the antibacterial effect of protamine. A good linear correlation between the impedance detection time and the initial cell counts was obtained (r= 0.99, n= 2). As basic peptides may cause clumping of cells, this correlation curve was used when estimating the cell number after protamine treatment, rather than colony counts. Protamine from salmon killed growing Gram‐positive bacteria and significantly inhibited growth of Gram‐negative bacteria in Tryptone Soy Broth (TSB) at 25°C. In general Gram‐positive bacteria were more sensitive to protamine than Gram‐negative bacteria; the minimum inhibitory concentrations (MIC) determined for Gram‐positive strains varied from 20 to 1000 μ ml‐1 and for Gram‐negative strains from 500 μ ml‐1 to more than 4000 μ ml‐1. The effect of protamine on non‐growing Listeria monocytogenes Scott A suspended in buffer was not lethal as was the effect on growing cells; however, protamine (50–500 μg ml‐1) killed the Gram‐negative fish spoilage bacteria Shewanella putrefaciens when the live cells were suspended in buffer.