Over the two last decades, Enterococcus faecalis has emerged as major cause of nosocomial infections. E. faecalis is also a member of the few bacteria that are almost completely resistant to one of the most important and widespread compounds of the constitutive defence system: the lysozyme. Therefore, the bases of this high resistance to lysozyme were investigated within this thesis work. We identified two genes referred as EF0783 and EF1843, potentially involved in lysozyme resistance. Proteins encoded by these genes share homology with Staphylococcus aureus peptidoglycan O-acetyltransferase (OatA) and Streptococcus pneumoniae Nacetylglucosamine deacetylase (PgdA), respectively. We constructed the corresponding mutants (DEF0783 and DEF1843) and the double mutant DEF0783-DEF1843. We showed that EF0783 mutation leads to the loss of O-acetyl groups from the peptidoglycan and to a decrease of the lysozyme resistance. On the other hand, no effect on lysozyme sensitivity could be associated to the EF1843 gene. Moreover, the EF0783 and/or EF1843 deletions significantly affect the ability of E. faecalis to survive within murine macrophages. While EF0783 is involved in the lysozyme resistance, the peptidoglycan O-acetylation and de-N-acetylation are not the main mechanisms conferring high levels of lysozyme resistance to E. faecalis. Experiments aiming to identify additional factors explaining this lysozyme resistance have to be considered.
