Final answer:
Mycobacterium does not retain the Gram stain due to its waxy mycolic acid coat and requires acid-fast staining to appear red, while Mycoplasma would be invisible in a Gram stain as it lacks a cell wall.
Step-by-step explanation:
When conducting a Gram stain on Mycobacterium, it would not retain the primary stain due to its waxy mycolic acid coat, which prevents the penetration of Gram stain reagents. Instead, a specialized acid-fast staining procedure, such as the Ziehl-Neelsen stain, is used to visualize these bacteria. In this procedure, Mycobacterium appears red because they are acid-fast, and the red-colored carbolfuschin dye is retained. In contrast, Mycoplasma would remain colorless after a Gram stain since it lacks a cell wall to retain any of the dyes used in this staining technique.
As for the specifics in the question, if one were to use a Gram stain on Mycobacterium, the cells would be invisible or would not take up the stain, and if one attempted to Gram stain Mycoplasma, it would also be invisible because they lack the specific structures the stain targets.