Bacteriological Culture Media

All Prepared Media have their recipes listed on their labels.

The salient points about these media in agar forms are given.
For best results in all cases, use tap-water for its minerals.

Nutrient Agar

    Contain broths of chicken or beef. Good for growing many bacteria of medical interest. A small amount of yeast extract ("Marmite" from supermarket) is added to provide essential micronutrients. Other "complex" media are TSA (tryptic soy agar), PEA (potato extract agar).

EMB Agar (Eosin Y + Methylene blue dyes)

    This agar usually contains lactose so that it tests for Gram-negative lactose fermenters. Gram-positive bacteria cannot grow in this medium. Facultative bacteria are forced to grow anaerobically and thus, if possible, ferment the lactose into either alcohols or acids. If acids are formed in great amount, it over-rides the phosphate buffer in the medium causing the pH to drop and for eosin Y to precipitate as dark red to black with frequently irredescence. Most commonly used to discriminate for E. coli which are among the few G(-) lactose fermenters (mixed-acid fermentation).

MacConkey Agar

    This agar has all the facility of EMB agar with the added benefit that its dyes are less staining to the preparers. Furthermore, the means for preventing G(+) growth by the addition of a detergent (bile salts), also allows the growth of colonies of highly motile bacteria such as Proteus spp, as the bile salts poison flagella movement. Mixed-acid fermentation of lactose yields red colonies; others give light pink colonies.

MS agar (minimal salts)

    This is essentially nothing more than a small amount of the salts of phosphate, ammonium and sulfate added to tap water, which contains all the other trace elements needed. Only photosynthetic organisms can grow on this MS agar unless the bacteria possess agarase capability. MS agar is usually supplemented with one or another sugar to test for simple metabolism. E.coli can grow slowly on MSG (MS + glucose). Better growth is afforded if a trace of thiamine (vitamin B1) is added as E. coli are laggard in thiamine production. Even with B1, E. coli's generation time is about 3-times longer than on nutrient agar.

Tomato Juice Agar

    Many brands of nutrient broth/agar do not support the growth of lactic acid bacteria due to lack of some essential nutrient, which can be found in tomato juice or V8. 6.5 mL of juice + 93.5 mL tap water + requisite amount of nutrient broth/agar powder to make 100 mL. (Note: many lactic acid bacteria require every amino acid, every human vitamin, a purine and a pyrimidine, certain fatty acids, + who knows what else to grow. Very persnickety!