Thermus TB++ medium

Autoclave for 15 min at 121 °C. After autoclaving add sterile CaCl₂ and MgSO₄ solutions to a final concentration 1 mM of each.

Option with gellan gum instead agar:

Immediately before pouring plates, add the salts to the hot medium (right after autoclaving or right after melting in a microwave) to final concentrations of 7 mM CaCl₂ and 1 mM MgSO₄, mix, and pour the plates without delay.

Gellan gum possesses several advantages over agar:

1) Apparently, Thermus bacteria do not like agar, displaying the delayed growth on the agar plates. In addition, agar varies between manufacturers, and some types are completely unsuitable for cultivating Thermus due to the toxicity. See also these two papers: 10.1128/aem.47.2.427-429.1984 and 10.1007/s00203-024-04139-5;

2) At high temperatures (>65 °C), agar gels undergo syneresis, expelling water onto their surface, which causes separate colonies to spread;

3) Gellan gels are as transparent as glass, which is great if you need to take a picture of plates.

However, these gels have some limitations:

1) It is unsuitable for phage experiments using the standard double-layer approach (or, at least, I did not managed how to prepare semi-solid gellan gel);

2) Since it requires Ca²⁺ or Mg²⁺ salts to solidify, gellan gum cannot be used with phosphate-containing media;

Cost could be an issue, but in practice food-grade gellan gum performs almost identically to Sigma Gelzan (at least as the media solidifying agent) - I use this one, it works quite well.