The BIGGY agar It is a solid, selective and differential culture medium. It was designed to isolate and aid in the identification of the main Candida species. Its name BIGGY comes from the acronym in English Bismuth Glucose Glycine Yeast.
The original formula was created by Nickerson, which is why this medium is also known as Nickerson agar. Currently it is composed of yeast extract, glycine, dextrose, ammonium citrate, bismuth citrate, sodium sulfite and agar..
BIGGY agar is formulated for the differentiation of species Complex C. albicans, C. tropicalis, C. krusei and C. kefyr, but other species also grow like Candida glabrata, C. parapsilosis, C. guilliermondii, Trichosporun sp, Geotrichum candidum and Saccharomyces cerevisiae.
It has good sensitivity and specificity, however, when compared with other media with similar function, it is in second place, being surpassed by CHROMagar. For this reason, many laboratories prefer the latter, although it is much more expensive..
It should be clear that the use of BIGGY agar, as well as other similar means, helps a quick presumptive identification, but never confirmatory. Therefore, additional identification methods must be available; e.g. Vitek 32 and API 20 identification systems.
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BiGGY agar is considered a partially selective medium for the isolation of yeasts, especially of the genus Candida, although other genera can grow.
It is also a differential medium because depending on the species involved, different characteristics will be observed in terms of appearance, color, shape and size. It is considered a chromogenic medium, due to the development of different colors in the colonies.
The color of the yeast colonies is due to the presence of bismuth sulfite in the agar. Nickerson realized that yeasts of the genus Candida carry out an extracellular reduction of bismuth sulfite, transforming it into bismuth sulfide (an insoluble black substance). This results in light brown to black colonies..
This agar contains yeast extract and dextrose, which provides the source of basic nutrients and energy for yeast development. Glycine is a yeast growth stimulant, while inhibiting the growth of certain bacteria.
In this same sense, ammonium and bismuth citrate, as well as sodium sulfite act as inhibitors of bacterial growth. Agar is the agent that gives the medium solid consistency.
Weigh 45 g of the dehydrated culture medium and dissolve in a liter of distilled water. Shake and heat the mixture, boiling for 1 minute until completely dissolved. This culture medium is not autoclavable.
Upon cooling to approximately 45 ° C in a water bath, pour 20 ml onto sterile Petri dishes..
The color of the dehydrated medium is yellowish gray and prepared is a yellowish white gel, in which a slight flocculating precipitate can be observed.
The pH should be 6.8 ± 0.2.
The prepared medium should be stored in a refrigerator (4 ° C) and protected from light. As it is a medium that cannot be autoclaved, it should be used as soon as possible. Its use is recommended within 3 days of its preparation.
The plates are incubated at room temperature (25 ° C) for 24, 48 and 72 hours, and the incubation time can be extended up to 5 days..
Each species of Candida develops certain characteristics that distinguish one from another. The manufacturers of the medium describe the following characteristics for each species:
-Candida albicans complex: develops on this agar as smooth, round, brown or black colonies, with a slight mycelial border. With the exception that the dark color does not diffuse to the middle.
-Candida tropicalis: the colonies are bright, small, dark brown, with a black center and a slight mycelial border. The dark color diffuses towards the middle, a characteristic that is typical of this species, after having been incubated for 72 hours..
-Candida krusei develops large, flat, rough-looking colonies with a brown to blackish border.
-Candida parakrusei: medium-sized colonies, most of the time rough, flat. The color can vary from bright dark reddish brown to light reddish brown. The border of the colonies has extensive, yellowish mycelium.
-Candida stellatoidea: Develops medium, flat, dark brown, almost black colonies. Mycelial development is very scarce.
A study carried out by Yücesoy and Marol in 2003, showed that CHROMagar medium has better sensitivity and specificity for the differentiation and identification of different Candida species than BIGGY agar..
For sterility control, uninoculated plates are incubated at 25 ° C, in the dark for 24 to 48 hours, it is expected that there is no growth of any kind, or color change..
The ATCC strains recommended for quality control of the medium are:
Candida albicans complex ATCC 60193, Candida glabrata ATCC 2001, Candida krusei ATCC 34135. In all of them, growth is expected as described above..
The following strains can also be used:
Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853 and Staphylococcus epidermidis ATCC 12228. In the first two, total inhibition is expected, but in case of partially growing colonies will be cream colored.
In the case of S. aureus it is expected to be totally inhibited, but if they grow partially their colonies are white.
Some bacteria are capable of developing in this medium and can even turn dark in color, simulating a Candida colony, but when making a smear of the colony and observing it under a microscope, it is easily discarded.
Table: Characteristics of the colonies of some yeasts on BIGGY agar according to the work of Yücesoy M and Marol S.
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