MadSci Network: Microbiology

Re: What bacterial results can grow out of (TSA) (CNA)&MacConkey pet.dishes?

Date: Wed Feb 23 17:46:14 2000
Posted By: James S. Dickson, Faculty, Microbiology, Iowa State University
Area of science: Microbiology
ID: 950900743.Mi

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 water
The 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 water
Neutral 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!

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