| MadSci Network: Microbiology |
What bacterial results can grow out of TSA, CNA & MacConkey
TSA is tryptic soy agar. It is a general purpose growth medium which will grow most types of bacteria. TSA is a typical medium used for cultivation of samples where many types of microorganisms might be expected. While TSA will not grow every type of bacterium, it will grow the vast majority of them.
TSA is also known as Soybean-Casein Digest agar, which is a good description of the medium. TSA has the following formulation:
tryptone (digest of casein) 15 grams soytone (digest of soybeans) 5 grams sodium chloride 5 grams agar 15 grams into 1 liter of waterThe casein (milk protein) and soybean digests are sources of nitrogen, amino acids and vitamins. The soybean digest also contains carbohydrates to help growth of the microorganisms. The sodium chloride maintains the osmotic balance of the medium, to promote cell growth. Agar cuases the liquid medium to solidify when the temperature cools below 40C.
CNA Agar: Also known as Columbia CNA agar, CNA agar (often with 5% added blood) is a selective medium which grows primarily gram positive cocci. The CNA stands for "Colistin Naladixic Acid agar". CNA is used to select for and grow bacteria of potential medical importance, such as Streptococcus pyogenes (causes "Strep" throat) and Staphylococcus aureus (a cause of food poisoning and skin infections).
CNA has the following formulation:
Pantone 10 grams Bitone 10 grams Beef Heart Digest 3 grams Corn Starch 1 gram Sodium Chloride 5 grams Colistin sulfate 10 milligrams Naladixic acid 15 milligrams agar 15 grams into 1 liter of water.The pantone, bitone and beef heart digest serve the same function as the casein and soy digests in TSA. The selective agents are the colistin sulfate and the naladixic acid. Colistin disrupts the cell membrane of Gram negative cells, while naladixic acid blocks DNA replication of many types of bacteria. Blood is often added to determine hemolysis, which is a characteristic associated with many disease causing bacteria.
MacConkey Agar: MacConkey agar is a selective and differential medium used for gram negative bacteria. It is usually supplemented with different specific carbohydrates to differentiate various bacteria.
MacConkey agar has the following formulation:
peptone 15 grams proteose peptone 3 grams lactose 10 grams Bile salts 1.5 grams sodium chloride 5 grams agar 13.5 grams neutral red 0.03 grams crystal violet 0.001 grams into 1 liter of waterNeutral red is a pH indicator. When a carbohydrate such as lactose is fermented, the pH usually drops. As the pH drops, the neutral red takes on an intensely red color. Bile salts and crystal violet are inhibitors of gram positive bacteria, and also inhibit some gram negative bacteria as well.MacConkey agar is frequently used in the analysis of medical, food or industrial samples. Many enteric bacteria (those that live in the intestinal tract of humans and animals) will ferment lactose. Typical bacteria isolated on MacConkey would be Klebsiella, Escherichia coli and Proteus mirabilis.
Hope this helps!
Try the links in the MadSci Library for more information on Microbiology.