Prevention of false positive binding during immunofluorescence of Staphylococcus aureus infected tissue biopsies

Immunofluorescence is a fundamental tool used to analyse tissue and cell samples with a wide variety of available antibodies targeting specific proteins or molecules. Staphylococcal surface protein A is used both in clinical, research and industrial settings for its ability to bind mammalian immunoglobulin G. Spurious binding between protein A and IgG antibodies can lead to false-positive fluorescence and misleading results. Here we demonstrate this occurring in formalin-fixed patient samples that harbour Staphylococcus aureus infection, and characterise methods to overcome this issue. Specifically the use of F(ab') fragment antibodies or blocking with human IgG is shown to prevent antibody-protein A interaction in formalin-fixed S. aureus smears, biopsies obtained from infected patients, and experimentally infected tissue samples.