Expression of S. aureus protein A (SPA) on the cell surface of L. monocytogenes strain Δ trpS,aroA,inlA/B,int ::P hly - spa × pFlo- trpS (Lm-spa+). (a) Western blot analysis with polyclonal goat-anti-Protein A antibody of protein extracts from ΔtrpS,aroA,inlA/B × pFlo-trpS (Lm-spa-, lanes 1 and 2) and Lm-spa+ (lanes 3 and 4); lanes 1 and 3: cell surface protein extracts; lanes 2 and 4: internal protein extracts. The arrow indicates the position of SPA in the SDS-PAGE. (b) Immunofluorescence micrographs showing specific binding of antibody Fc-part to SPA on the surface of Lm-spa+. Lm-spa+ were incubated with polyclonal anti-OVA antibody and stained with OVA-FITC protein (vii-ix). Lm-spa- stained with antibody and OVA-FITC (i-iii) and Lm-spa+ stained without antibody but with OVA-FITC protein (iv-vi) were used as negative controls. Phase contrast pictures are shown in the left column; FITC-stained images in middle column; picture overlays in the right column. (c) Flow cytometry quantifying the specific Fc-mediated antibody binding to SPA on the surface of L. monocytogenes strains. Mid-logarithmic grown bacteria were stained with polyclonal FITC-conjugated rabbit-anti-goat immunoglobulin G (H+L). Grey area indicates strain Lm-spa-, while the white area indicates strain Lm-spa+. (d) Western blot analysis was used for indirect quantitation of protein A on the surface of Lm-spa+. 5 × 108 bacteria were incubated simultaneously with antibody directed against native albumin and an excess of albumin. After incubation bacteria were washed and the amount of albumin bound to the bacteria via antibody was quantified by Western blot analysis with a primary antibody directed against denatured albumin. In the right lane 10 ng of pure serum albumin was applied as control.