The autolysin Ami contributes to the adhesion of Listeria monocytogenes toeukaryotic cells via its cell wall anchor

Adherence of pathogenic microorganisms to the cell surface is a key event d uring infection. We have previously reported the characterization of Lister ia monocytogenes transposon mutants defective in adhesion to eukaryotic cel ls. One of these mutants had lost the ability to produce Ami, a 102 kDa aut olytic amidase with an N-terminal catalytic domain and a C-terminal cell wa ll-anchoring domain made up of repeated modules containing the dipeptide GW ('GW modules'). We generated ami null mutations by plasmid insertion into L. monocytogenes strains lacking the invasion proteins InlA (EGD Delta inlA ), InlB (EGD Delta inlB) or both (EGD Delta inlAB). These mutants were 5-10 times less adherent than their parental strains in various cell types. The adhesion capacity of the mutants was restored by complementation with a DN A fragment encoding the Ami cell wall-anchoring domain fused to the Ami sig nal peptide. The cell-binding activity of the Ami cell wall-anchoring domai n was further demonstrated using the purified polypeptide. Growth of the am i null mutants constructed in EGD and EGD Delta inlAB backgrounds was atten uated in the livers of mice inoculated intravenously, indicating a role for Ami in L. monocytogenes virulence. Adhesive properties have recently been reported in the non-catalytic domain of two other autolysins, Staphylococcu s epidermidis AtlE and Staphylococcus saprophyticus Aas. Interestingly, we found that these domains were also composed of repeated GW modules. Thus, c ertain autolysins appear to promote bacterial attachment by means of their GW repeat domains. These molecules may contribute to the colonization of ho st tissues by Gram-positive bacteria.