Specificity and dose-dependent bactericidal activity of P128. (a) P128 (50 μg/ml) was tested on log phase cells of gram positive and gram negative bacterial species by the turbidity reduction assay. P128 showed activity only against Staphylococcus species, which lysed rapidly after addition of the protein. No activity was observed against gram-negative bacteria or the other gram-positive bacteria tested. (b) The bactericidal effects of P128 are dose-dependent and 0.5 μg/ml was sufficient to reduce viable cell numbers by 90%. Reduction in viable cell numbers of over three orders of magnitude was observed in the concentration range of 2.5 - 25 μg/ml.