Effect of serum incubation on antibody-mediated internalization of Lm-spa+. The bacteria were incubated with PBS (-mAb), Cetuximab or Trastuzumab and the antibodies were covalently bound to protein A by crosslinking with DMP. Subsequently the bacteria were incubated with murine serum prior to infection of 4T1-HER2 cells. Intracellular CFU was determined after gentamicin treatment by plating serial dilutions. The relative internalization rate in comparison to uncoated bacteria was calculated and is shown.