Final answer:
Gram-negative bacteria are unable to grow on Mannitol Salt Agar due to their cell wall structure and function, which does not tolerate the high salt content of MSA, and because most do not ferment mannitol.
Step-by-step explanation:
The inability of Gram-negative bacteria to grow on Mannitol Salt Agar (MSA) can be attributed to MSA's selective properties, which are hostile to Gram-negative bacteria due to their cell wall structure and function. MSA contains a high concentration of salt (about 7.5-10%), which is inhospitable to most bacteria that do not have an innate tolerance to such osmotic stress. Gram-negative bacteria have an outer membrane that is disrupted by high salt conditions, preventing them from withstanding the harsh environment of the MSA.
Additionally, the thin peptidoglycan layer of the Gram-negative cell wall does not provide sufficient structural support to withstand these osmotic conditions, unlike the thick peptidoglycan layer of Gram-positive bacteria. Since MSA is also a differential medium containing mannitol and a pH indicator, only organisms that can ferment mannitol and tolerate the high salt concentration will grow and produce a color change. Gram-negative bacteria usually fail to grow due to their inability to withstand high saline conditions and because most do not ferment mannitol, which is another reason they are unable to grow on MSA.