por What is brilliant green agar?
He brilliant green agar It is a solid culture medium, with a high degree of selectivity. It is used exclusively for the isolation of strains of the genus Salmonella. However, there are some exceptions, such as the species typhi and paratyphi, that do not grow in this medium.
The search for gender Salmonella It is common in stool, water, or food samples. In this sense, this medium can be very useful. This agar was created in 1925 by Martin Kristensen, Vera Lester and Axel Jürgens, and later modified by Kauffmann.
It is composed of pluripeptones from peptic digest of animal tissue and pancreatic digest of casein, it also contains yeast extract, sodium chloride, lactose, sucrose, phenol red, brilliant green and agar-agar.
It is characterized by being a fairly inhospitable environment for most bacteria, favoring the growth of SalmonellaHowever, some coliforms are capable of subsisting in it, developing weakly.
It is important to note that in this environment the genre does not grow shigella and neither Salmonella typhimurium, neither Salmonella paratyphi. Therefore, if it is desired to isolate these microorganisms, other media must be used, such as XLD agar, among others.
Basis
brilliant green agar
Each of the components that make up the medium fulfills a specific function that determines the characteristics and properties of the agar.
Pluripeptones and yeast extract are the source of nutrients from which microorganisms take the nitrogen and minerals necessary for their development. Lactose and sucrose are energy sources for microorganisms capable of fermenting them.
Brilliant green is the inhibitory substance that prevents the growth of Gram-positive bacteria and a large number of Gram-negative microorganisms.
Sodium chloride provides osmotic stability to the medium. While phenol red is the pH indicator, it changes color by detecting acid production from carbohydrate fermentation.
Lactose and sucrose non-fermenting colonies grow on this medium pinkish white or transparent, on a red background. For example, bacteria of the genus Salmonella.
While lactose or sucrose fermenting bacteria capable of growing on this medium develop greenish-yellow or yellowish-green colonies on a greenish-yellow background. For example, Escherichia coli and Klebsiella pneumoniae.
Variants of Brilliant Green Agar (BGA)
There are other variants of Brilliant Green Agar: Novobiocin Brilliant Green Glucose (NBG) Agar and Novobiocin Brilliant Green Glycerol Lactose (NBGL) Agar.
Novobiocin Brilliant Green Glucose Agar (NBG)
Contains trypticase soy agar, ferric ammonium citrate, sodium thiosulfate pentahydrate, phenol red, glucose, brilliant green, novobiocin, and distilled water.
It is used to isolate colonies of Salmonella from stool samples.
In this case, brilliant green and novobiocin are the inhibitory substances that prevent the growth of gram-positive bacteria and some gram-negative microorganisms.
Sodium thiosulfate is the source of sulfide and ferric citrate is the source of iron, both of which are necessary to reveal the production of hydrogen sulfide through the formation of a black precipitate of ferric sulfide.
Glucose is the fermentable carbohydrate and phenol red is the pH indicator.
On this medium, the colonies of Salmonella they develop large with a black center, surrounded by a reddish halo and followed by a clear visible area. Some strains of Citrobacter freundii produce colonies identical to that of Salmonella.
Novobiocin Brilliant Green Glycerol Lactose Agar (NBGL)
This medium contains trypticase soy agar, ferric ammonium citrate, sodium thiosulfate, lactose, glycerol, brilliant green, novobiocin, and distilled water.
The difference between this medium and the previous one is that glucose is replaced by lactose and glycerol and phenol red is not used.
The medium is also used to isolate species of Salmonella, the colonies develop black, due to the production of hydrogen sulfide.
Only colonies that do not produce acid from glycerol or lactose achieve sufficient H2S production, because low pH interferes with H2S formation. This results in colorless colonies for most species of Proteus and Citrobacter.
Preparation
Weigh 58 grams of commercially obtained dehydrated medium. Add in a liter of bidistilled water. Mix, let stand for a few minutes and submit the preparation to a heat source until it is completely dissolved.
Autoclave at 121 °C for 15 minutes, without exceeding the sterilization time.
Let stand and serve still hot in sterile Petri dishes. The final pH should be 6.9 ± 0.2.
Allow to solidify and store in the refrigerator until use. Before inoculating the plates, room temperature must be taken.
The powdered medium is green, and when prepared it takes on an orange-brown or reddish-green color, depending on the pH and the commercial company. A very brown color is indicative that the agar has been overheated.
Once the agar has solidified, it is not recommended to recast it, as the medium deteriorates.
Uses/applications
This medium is used to search for strains of the genus Salmonella samples of feces and dairy foods, among others.
Because it is a rather inhospitable environment, it is advisable to sow an abundant inoculum if the direct sample is used. Otherwise, pre-enrichment and enrichment of the specimens should be done before sowing on this medium.
Like some strains of Salmonella are inhibited or grow with difficulty, it is advisable to accompany this medium with other selective agars to Salmonella. Any colony with typical characteristics of Salmonella must be subjected to biochemical tests for definitive identification.
QA
To test the proper functioning of the brilliant green agar medium, ATCC strains can be used to observe their development on it.
The most frequent strains used for quality control are: salmonella enteritidis ATCC 13076, Salmonella typhimurium ATCC 14028, proteus mirabilis ATCC 43071, Klebsiella pneumoniae ATCC 700603, Escherichia coli ATCC 25922, Shigella flexneri ATCC 12022, Staphylococcus aureus ATCC 6538.
The first 3 should give pinkish or transparent white colonies on a red background. Salmonella with good development, and Proteus with little or regular growth.
For Klebsiella and Escherichia greenish yellow colonies with a yellow background are expected, and in the case of shigella and staphylococcus they must be inhibited.
The dehydrated medium must be stored at room temperature, in a dry place, since the medium is very hygroscopic.
References
Brilliant Green Agar. Recovered from britanialab.com.
BD Brilliant Green Agar. Recovered from bd.com.