Partially Unfolded Lysozyme at Neutral pH Agglutinates and Kills Gram-Negative and Gram-Positive Bacteria through Membrane Damage Mechanism

The antimicrobial mechanism and structural changes of hen egg white lysozyme irreversibly inactivated at 80 °C and at different pHs were investigated. We found that heat denaturation of lysozyme at increasing temperatures for 20 min at pH 6.0 results in progressive loss of enzyme activity while greatly promotes its antimicrobial action to Gram-negative bacteria. Interestingly, lysozyme devoid of enzyme activity (heated at 80 °C and pH 7.0 or at pH 6.0 over 90 °C) exhibited strong bactericidal activity against Gram-negative and -positive bacteria, suggesting action independent of catalytic function. The most potent antimicrobial lysozyme to either Gram-negative or -positive bacteria was that heated at 80 °C and pH 6.0 (HLz80/6), retaining 50% of the native enzymatic activity, which exhibited a 14-fold increase in surface hydrophobicity, with two exposed thiol groups. HLz80/6-induced agglutination coincided with severe reduction in colony-forming ability of the susceptible bacteria in a dose-dependent manner. Denatured lysozyme HLz80/6 showed promoted binding capacity to peptidoglycan of Staphylococcus aureus and lipopolysaccharide of Escherichia coli as assessed by ELISA. Addition of HLz80/6 to E. coli phospholipid vesicles resulted in a blue shift in the intrinsic tryptophan fluorescence accompanied by an increase in the size of the vesicles, indicating enhanced protein-membrane binding and subsequent fusion of liposomes. Direct membrane damage of E. coli membrane by HLz80/6 was revealed by electron microscopy observation. Thus, the results introduce an interesting finding that partial unfolding of lysozyme with the proper acquisition of the hydrophobic pocket to the surface can switch its antimicrobial activity to include Gram-negative bacteria without a detrimental effect on the inherent bactericidal effect against Gram-positive ones. The data suggest that the unique antimicrobial action of unfolded lysozyme attributes to membrane binding and subsequent perturbation of its functions.