Figure 4

MR images of a subcutaneous infection induced by iron-labeled bacteria investigated 24 h post infection. Axial T2*-weighted (A) and UTE (B) images of the mouse flank, revealing an ulcerous abscess (red circle) with bacterial colonization (yellow arrows). (C) Axial T2*-weighted MRI of the subcutaneous infection with unlabeled bacteria, following intravenous co-injection of 5-nm IONP 2 h and 5 h post infection. (D-G) Axial T2*-weighted MRI of the mouse flank revealing an ulcerous abscess (red circle) with bacterial colonization (yellow arrows). The local skin infection was induced by different numbers of iron-labeled bacteria (D) 10⁷ CFU, (E) 10⁶ CFU, (F) 10⁵ CFU, (G) 10⁴ CFU. Bacteria were observed as hypointensities (yellow arrow) when iron-labeled S. aureus at a number ≥10⁵ CFU were applied. (H) Electron microscopy, (I) Prussian blue staining and (J) Gram staining of tissue sections of the inflammatory lesion showing iron-labeled bacteria. (K) CFU counts after magnetic separation of homogenized tissue infected with either iron-labeled or non-labeled S. aureus bacteria. (L and M) Fluorescence microscopy of homogenized infected tissue visualized in the green (1), red (2) and merged (3) fluorescence channel. Either unlabeled (L) S. aureus bacteria or bacteria labeled with rhodamine-coated IONPs (red, M) were used to induce the infection. For co-registration all bacteria were labeled with Syto 9 (green). (N) Gram (N1) and Prussian blue (N2) staining of homogenized tissue infected with iron-labeled bacteria. (O) Immunohistology of S. aureus infected skin tissue showing intracellular (yellow arrowhead, O1) and extracellular (O2) bacteria. Bacteria are stained green, macrophages red and nuclei blue. (P and Q) Hematoxylin/Eosin (P) and Prussian blue (Q) staining of tissue section of a lesion after subcutaneous infection with unlabeled S. aureus, followed by intravenous co-injection of 5-nm IONP 2 h and 5 h post infection. (R) Prussian blue staining of inflamed tissue with macrophage invasion, after infection with iron-labeled S. aureus bacteria.