The mannitol fermentation test is used to test if a microbe can ferment the carbohydrate mannitol as a carbon source. If mannitol has been used by the microbe, acidic byproducts will be produced and the pH of the medium in which the microbe sits will drop.
Mannitol is also known as mannite, manna sugar and osmitrol. It is a white crystalline solid that looks and tastes like sugar. Its name is derived from its resemblance to the Biblical food manna. Its molecular formula is C6H14O6. In the mannitol fermentation test, a microbe is often grown in mannitol salt agar (MSA) plates, which contain both mannitol and salt. In these places, most microbes will be unable to grow unless they are able to metabolize mannitol.