Omega filter

After phage purification, samples were subjected to negative staining transmission electron microscopy (TEM) as described previously by [43 (link)]. After application of 5 μl of sample to glow-discharged and carbon-coated copper grids, the samples were blotted on filter paper and washed twice with double-distilled water. After this, they were negatively stained with 2% uranyl acetate for 20 seconds, blotted again and left for air-drying.
For TEM, a Zeiss EM912 with an integrated OMEGA-filter (Zeiss) and a 2 k × 2 k CCD camera (TRS) was used. The microscope was operated at 80 kV in the zero-loss mode.

To prepare samples for cryoTEM measurement, 4 μL of each sample were transferred to a glow discharged lacey formvar carbon coated copper grid (PELCO NetMesh, Ted Pella, Redding, California, USA) and vitrified in liquid ethane using an automatic plunge freezer (Leica EM GP, Wetzlar, Germany). The grids were stored in liquid nitrogen until use. A Fischione model 2550 cryo transfer tomography holder was used to transfer the samples into the transmission electron microscope, a JEM-2200FS (JEOL, Peabody, Massachusetts, USA), equipped with an in-column energy filter (Zeiss omega filter, Oberkochen, Germany), which allows for zero-loss imaging. The accelerating voltage was 200 kV and images were recorded with a TVIPS TemCam-F416 digital camera using Serial EM under low-dose conditions. Multiple images were acquired for all cryo-TEM samples at all compositions.