Media Recipes

You'll want to include some of the following information in your original
research reports, as you see fit:

XLD agar = xylose lysine desoxycholate agar
dissolved in 1 L distilled water:
3.5 g xylose
5.0 g lysine
7.5 g lactose
7.5 g sucrose
5.0 g sodium chloride
3.0 g yeast extract
2.5 g sodium desoxycholate
6.8 g sodium thiosulfate
0.8 g ferric ammonium citrate
0.08 g phenol red
13.5 g agar

Xylose, lactose, and sucrose are sugars that can be fermented by E. coli and
a few other bacteria, producing an acidic byproduct. Salmonella can't
ferment these sugars, though. Phenol red is a color indicator, that is
yellow at low pH and red at neutral pH. Lysine is an amino acid. "Sodium
desoxycholate" is a synonym for "bile." Many bacteria are dissolved in
bile, but not ones that live in vertebrate guts (obviously, or else they'd
be dissolved.) Thus, bile is the selective agent in this medium that
prevents growth of non-gut bacteria. Sodium thiosulfate can sometimes be
broken down into H2S by Salmonella species, resulting in a black

Mannitol Salt Agar
dissolved in 1 L distilled water:
5.0 g digest of casein
5.0 g digest of animal tissue
1.0 g beef extract
75.0 g sodium chloride
10.0 g mannitol
0.025 g phenol red
15.0 g agar

The first three ingredients on this list are just to provide nutrients on
which many kinds of bacteria can grow. Sodium chloride is the selecting
agent: obviously, bacteria that cannot grow at high salt concentrations
cannot grow on this agar. Staphylococci, since they often grow on your
sweaty, salty skin, can grow at high salt concentrations. Phenol red is a
pH indicator, as described above. S. aureus can ferment mannitol, producing
an acidic byproduct, while other staphylococci cannot.

Eosin-Methylene Blue agar

Dissolved in 1 L distilled water:
10 g peptone
10 g lactose
2 g dipotassium phosphate
0.4 g eosin
0.065 g methylene blue
15 g agar

Peptone is a source of amino acids and other nutrients for bacterial growth.
Lactose is a sugar that coliforms (like E. coli) but not many other bacteria
can ferment, producing an acidic byproduct. Eosin and methylene blue
prevent gram-positive bacteria from growing on this agar. Also, under
acidic conditions, these two dyes interact producing a metallic green color.

Mueller-Hinton agar

Dissolved in 1 L distilled water:
2.0 g beef extract
17.5 g digest of casein
1.5 g starch
17.0 g agar

The first two ingredients on the list are to provide amino acids and other
nutrients needed for bacterial growth. These ingredients support the growth
of a very wide range of bacterial species, so practically everything will
grow on MH agar. The starch and the increased agar concentration (1.7%
instead of 1.5% or less as in the other media in this list) keeps
antibiotics from diffusing too rapidly through the medium, so that a smooth
gradient of antibiotic is created in the agar surrounding the antibiotic

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