Native lysozyme and dry-heated lysozyme interactions with membrane lipid monolayers: Lateral reorganization of LPS monolayer, model of the Escherichia coli outer membrane

• Very strong lysozyme affinity for LPS makes its insertion possible into a LPS monolayer.
• The polysaccharide moieties of LPS are essential for lysozyme insertion.
• Lysozyme interaction with LPS causes a slight reorganization of the LPS monolayer.
• Dry-heated lysozyme has a stronger affinity for LPS than native lysozyme.
• Dry-heated lysozyme causes more radical reorganization of the LPS monolayer.

Lysozyme is mainly described active against Gram-positive bacteria, but is also efficient against some Gram-negative species. Especially, it was recently demonstrated that lysozyme disrupts Escherichia coli membranes. Moreover, dry-heating changes the physicochemical properties of the protein and increases the membrane activity of lysozyme. In order to elucidate the mode of insertion of lysozyme into the bacterial membrane, the interaction between lysozyme and a LPS monolayer mimicking the E. coli outer membrane has been investigated by tensiometry, ellipsometry, Brewster angle microscopy and atomic force microscopy. It was thus established that lysozyme has a high affinity for the LPS monolayer, and is able to insert into the latter as long as polysaccharide moieties are present, causing reorganization of the LPS monolayer. Dry-heating increases the lysozyme affinity for the LPS monolayer and its insertion capacity; the resulting reorganization of the LPS monolayer is different and more drastic than with the native protein.