How do you make NGM plates that don't precipitate?

I am a bacteriologist and have been working with C. elegans the last year as a host for bacterial virulence. My worm husbandry skills are fine, but I still haven't mastered making NGM plates that don't precipitate. I don't understand if it's expected/known that NGM plates eventually precipitate or form crystals, or if I'm doing something wrong. I've done a lot of googling, but realized someone here might have advice!

I have tried:

Making fresh pH 6 phosphate buffer

Allowing the autoclaved agar base (NaCl, bacto agar, bacto peptone) to cool to 55C before adding the following filter-sterilized components in the order listed, waiting 5 minutes between each addition

• phosphate buffer

• 1M CaCl2

• 1M MgSO4

• 5 mg/ml cholesterol in EtOH

I let the plates solidify at RT a minimum of 24 h before inoculating with OP50 E. coli, and at this point the plates are usually clear with no crystalization visible by eye.

I store plates inoculated with OP50 at 4C after letting the bacteria grow for 2 days at RT and at this point the plates are usually still clear. NGM plates I poured and plan to inoculate later I have tried leaving sealed in a sleeve at RT for up to a month (usually no precipitation), or stored at 4C. The 4C plates will definitely precipitate if cold and inoculated with OP50 culture that's been growing at 37C and often also precipitates if allowed to come to RT before inoculating with 37C culture.

Is there any way to prevent the formation of crystals in my agar? It seems like these plates are pretty temperature-sensitive and maybe I'm just supposed to make them as needed? The worms and bacteria are unaffected by the crystallization, so this is mostly just a frustrated curiosity. When I have received worm strains from other labs, their plates are beautiful and clear and I am jealous!

Thanks for any insight! 🪱🦠🧫🔬💗