Immunofluorescence of Ser33-phosphorylated RPA (p-RPA) and γH2AX showed nuclear foci of both markers in HeLa cells 4 h after infection with Nissle 1917 or following treatment with the cross-linking drug cisplatin but not after infection with the clbA or clbP mutants (Fig. 3a). This evidence concerns the gene RPA1 and infection.