Mannitol salt medium

In media selective for enterobacteria a surface-active agent is the main selector, whereas in staphylococcal medium sodium and lithium chlorides are the selectors staphylococci are tolerant of salt concentrations to around 7.5%. Mannitol salt, Baird-Parker (BP) and Vogel-Johnson (VJ) media are three examples of selective staphyloccocal media. Beside salt concentration the other principles are the use of a selective carbon source, mannitol or sodium pyruvate together with a buffer plus acid-base indicator for visualizing metabolic activity and, by inference, growth. BP medium also contains egg yolk the lecithin (phospholipid) in this is hydrolysed by staphylococcal (esterase) activity so that organisms are surrounded by a cleared zone in the otherwise opaque medium. The United States Pharmacopeia (1990) includes a test for staphylococci in pharmaceutical products, whereas the British Pharmacopoeia (1993) does not. 

Free living S. meliloti 41 accumulates up to 50% of its weight as poly- p-hydroxybutyrate (PHB) in yeast salts medium with mannitol as carbon source. When other carbon sources are used, S. meliloti 41 still produces high amounts of PHB under the same growth conditions (Table 1). This strain can also accumulate a copolymer when propionate or n-valerate were supplied to the culture (data not shown). 

The formation of mannitol is detected by growing the bacterium in the fructose-enriched medium described by Pilone etal. (1991). Formation of mannitol salt crystals in the dried medium are detected visually as large rosettes without further magnification. It is essential to allow a few days for crystal formation at 25°C/77°F after evaporating the water at 37°C/99 F because crystals are often not seen prior to the final drying time. 

Although most of the focus on solute effects on macromolecular systems has involved proteins, there is evidence that some of the patterns of solute accumulation reflect the dangers that high salt concentrations pose for the covalent structure of DNA. Using cultured mammalian kidney cells Kiiltz and Chakravarty (2001) showed that hyperosmotic stress could cause double strand breaks (dsb) in DNA. Hyperosmolality due to elevated [NaCl] in the culture medium caused the most dsb. Potassium chloride and mannitol led to less damage and, interestingly, no damage to DNA was found in cells exposed to elevated levels of urea. 

There are numerous variations on this medium using mannitol instead of lactose, teepol or sodium lauryl sulfate instead of bile salts, and alternative indicators such as Phenol Red. However, all the tests are based on the ability of coliforms to grow in the presence of some inhibitor, and ferment lactose or mannitol to acid and gas, the acid changing the colour of the indicator. 

---