Aggregation substance promotes adherence, phagocytosis, and intracellular survival of Enterococcus faecalis within human macrophages and suppresses respiratory burst

The aggregation substance (AS) of Enterococcus faecalis, encoded on sex phe romone plasmids, is a surface-bound glycoprotein that mediates aggregation between bacteria thereby facilitating plasmid transfer. Sequencing of the p AD1-encoded Asa1 revealed that this surface protein contains two RGD motifs which are known to ligate integrins. Therefore, we investigated the influe nce of AS on the interaction of E. faecalis with human monocyte-derived mac rophages which constitutively express beta(2) integrins (e.g., CD18). AS wa s found to cause a greater-than-fivefold increase in enterococcal adherence to macrophages and a greater-than-sevenfold increase in phagocytosis. Adhe rence was mediated by an interaction between the RGD motif and the integrin CD11b/CD18 (complement receptor type 3) as demonstrated by inhibition stud ies with monoclonal antibodies and RGD peptide. AS-bearing enterococci were significantly more resistant to macrophage killing during the first 3 h po stinfection, probably due to inhibition of the respiratory burst as indicat ed by reduced concentrations of superoxide anion.