I am checking Salmonella typhimurium TA98 strains for the Ames test. On GM plates (excess histidine, no biotin) (H plate). I did not observe any colonies (as expected), but I noticed irregular white patches/spots on the agar surface. On the control BHA plates (biotin + histidine), normal bacterial colonies grew well.

My questions are:

What exactly are these white patches on the GM plates?

How do such artifacts form during plate preparation or incubation (any issue)?

Are they something I need to worry about for the strain check, or can they be ignored as common artifacts?

Any insights, references, or similar experiences would be very helpful.