Sodium cacodylate buffer

The bacteriophages on the bacterial cells were visualized using transmission electron microscopy (TEM) (Fig. S3). A 0.5 McFarland bacterial suspension of ATCC 700603 was mixed with 10⁷ PFU/mL bacteriophage cocktail of UZG4 and UZG13. The suspension was incubated at 37 °C at 160 rpm with shaking for 4 h. After the incubation, the suspension was centrifuged at 10,000 RCF for 1 h. The pellet was fixed with freshly prepared 2.5% (v/v) glutaraldehyde using 2.5% glutaraldehyde (SERVA Electrophoresis GmbH, Heidelberg, Germany) mixed with 0.1 M sodium cacodylate buffer (pH 7.4, SERVA Electrophoresis GmbH, Heidelberg, Germany) for 1 h at room temperature. After washing 3 times for 15 min each with 0.1 M sodium cacodylate buffer (pH 7.4), the pellets were post-fixed with 2% (w/v) osmium tetroxide for 1 h at room temperature. During the following dehydration in an ascending ethanol series, post staining with 1% w/v uranyl acetate for 1 h was performed. Afterward, the pellets were embedded in epoxy resin (Araldite) and sectioned using a Leica Ultracut S (Leica, Wetzlar, Germany). Finally, ultrathin sections were mounted on filmed Cu grids, post stained with lead citrate, and studied under a transmission electron microscope (EM 900, Zeiss, Oberkochen, Germany) at 80 kV and a magnification of 20,000×. For image recording, a 2K slow-scan CCD camera (TRS, Moorenweis, Germany) was used.

To prepare semi-thin and ultrathin sections, EBs were fixed by 2.5% glutaraldehyde (Sigma-Aldrich) in 100 mM sodium cacodylate buffer (Serva, Germany), pH 7.0 for 24 h at 4°C and then post-fixed with 1% osmium tetroxide (Serva, Germany) in 100 mM cacodylate buffer, pH 7.0 (all from Sigma-Aldrich) within 1 h at 4°C. After dehydration procedures in serial ethanol and acetone solutions and impregnation in resin, the specimens were embedded into the Spurr’s resin (Spurr kit, Sigma) or Araldite (Fluka). The EB sections were prepared using Ultrotome 3 (LKB, Germany). Semi-thin sections were stained with toluidine blue in 1% sodium tetraborate (Sigma-Aldrich). Ultra-thin sections were placed on the grids and stained with aqueous uranyl acetate (Merck, Germany) and lead citrate (Serva, Germany). The sections were examined under a JEM 100CXII electron microscope (Jeol, Japan).

Matrigel‐coated beads covered with cells were harvested, rinsed with PBS and fixed with 2.5% glutaraldehyde (Serva, Heidelberg, Germany) in 0.1 M sodium cacodylate buffer (Serva, Heidelberg, Germany) for 30 min at RT and stored at 4°C. The samples were postfixed with 1% osmium tetroxide (Electron Microscopy Sciences, Hatfield, USA) and 0.8% potassium ferrocyanide II (Roth, Karlsruhe, Germany) in 0.1 M cacodylate buffer for 1.5 h and embedded in agarose overnight. After cutting the agarose in smaller blocks, the samples were dehydrated in a graded ethanol series and transferred to Epon resin (Roth, Karlsruhe, Germany). Finally, ultrathin sections of the samples (70nm) were stained with uranyl acetate, and lead citrate. The examination was carried out with a Zeiss EM 906 electron microscope at 80kV acceleration voltage (Carl Zeiss, Oberkochen, Germany).