Why do you think you would need to ultracentrifuge your samples? You should easily be able to get a high enough titer for your needs. If you want to purify then you should probably not pellet in an ultracentrifuge but rather do some gradient ultracentrifugation to remove any other macromolecules.

To prepare bacteriophage for TEM sample, it is recommended to first concentrate the phages by centrifugation at 25,000 × g for 120 min. After centrifugation, discard the supernatant and resuspend the pellet in Saline Magnesium Buffer (SM Buffer). This step ensures that the phages are properly concentrated and suspended in a suitable buffer for further processing and TEM analysis.

Thank you very much, Umut Yilmaz.

I assume that the pellet is not visible after centrifugation, right?

What do you think about adding glutaraldehyde to fix the sample? The TEM lab has recommended that I add it after centrifugation, but I have read that it can interfere with the TEM visualization.

Regarding what you say, Michael J. Benedik , I needed to know how to prepare a sample of phages (viable or non-viable) with a concentration of between E6-E7 PFU/mL for TEM observation. I think Umut's answer will help me.

Thank you for your answers, best regards.

Thank you for your message Víctor Pertegal Pérez. Yes, I assume that the pellet is not visible after centrifugation, but we can assume that it is present. I have not used any fixative such as glutaraldehyde in my protocol so far. However, if the TEM lab requires fixation, a fixative such as glutaraldehyde (final concentration 2-2.5%) can be added before shipping. I hope this approach will be sufficient to visualize your high titer phage samples. I hope your experiment yields excellent results.

Víctor Pertegal Pérez my point is that if you generate a high titer stock of MS2, your titer should be around E10. So you don't need to concentrate your phage but rather dilute them into any buffer you need to get E6-E7.

You can easily concentrate or change buffer by precipitating with PEG (10% PEG 6000-8000, chill >1 hour on ice and centrifuge 10-15k x g for a 10 minutes. But I'm not sure that is necessary for your applications if you have removed all the cellular crud and debris.

Umut Yilmaz are you sure about what you wrote? I have never heard of being able to pellet phage at 25,000 x g. Usually you need ultracentrifuge speeds which are much much higher (unless you are adding a precipitant like PEG)

Thank you for your valuable feedback, Michael J. Benedik. You are absolutely right that ultracentrifugation is often necessary for efficient phage pelleting. I did not mean to imply that a visible pellet is formed at 25,000 × g; instead, I wanted to share an alternative approach that I use to concentrate phages in my lab, which does not have access to ultracentrifugation. Additionally, a fixative such as glutaraldehyde (final concentration: 2–2.5%) can be added prior to fixation. This method is usually sufficient to visualize high-titer phage samples (at least E7 PFU/mL).