Solubility and antimicrobial efficacy of protamine on Listeria monocytogenes and Escherichia coli as influenced by pH

The antimicrobial efficacy of protamine on Listeria monocytogenes and Esche richia coli was evaluated at concentrations from 50 to 10 000 mu g ml(-1) a nd pH levels from 5.5 to 8.0. The minimum inhibitory concentrations decreas ed with increasing pH. Protamine inhibited E. coli at all pH values while L . monocytogenes was inhibited at pH 6.5 and above. The antimicrobial effica cy of protamine decreased in the presence of negatively charged gelatine B but remained almost unchanged with addition of the positively charged gelat ine A. Binding studies showed that the amount of protamine adsorbed to cult ure media components in tryptic soy broth and bacterial cells increased wit h increasing pH values. The increased efficacy of protamine at alkaline pH may be explained on the basis of an increase in electrostatic affinity for the cell surface of target cells. E. coli produced a protamine-degrading en zyme, however, was still susceptible to protamine.