Brief Description:
The assimilation test, used for the taxonomy in yeast, is an agar that is made up and inoculated with a yeast and a carbohydrate. It is then incubates anywhere from 1-3 days, or ever up to 24 days to allow the identification of yeast that are slow metabolizers. It can be run in carbon and nitrogen based media made with distilled water (Ahearn et al, 1960).
The assimilation test checks the ability of a yeast to use a carbohydrate as its sole carbon source and can be run with nitrogen or carbon. If nitrate is used instead of carbon, the test tests the ability of yeast to use nitrate as sole nitrogen source. One observes the growth for 1 to 3 days. If there is an increase in growth then the yeast has a certain enzyme to assimilate that certain carbohydrate and the opposite is true too, if there is no growth then the yeast lacks a certain enzyme. In short, the pattern of how the yeast takes up the carbohydrate is observed and this characteristic pattern is called the yeast’s auxogram. There are many ways to apply the carbohydrates and inoculums onto the agar plate. They can be seeded into the agar, streaked onto the plate, applied via a filter, or applied drop-wise. When using the assimilation test, there are many options for techniques to use. There is the Wickerham Broth version in which a chemical broth is made in a tube and inoculated with a yeast colony that had been starved of nutrients. The yeast are allowed to grow for 48 hours and the growth is measured based on turbidity of the solution. Another technique would be to take the broth, add agar and allow a slant to form. These can then be stored for months before being used. These slants are usually used with Brom cresol blue indicator. When the slants pH changes (from the neutral 7) it changes color indicating a change in the pH and thus a change in the yeast (Lin et al, 1987)
Finally, there is the dye pour plate auxanographic technique (DPPA). This method has the ability to test a yeasts’ assimilation of 14 carbohydrates at a time (Lin et al, 1987).
Application in Wine Microbiology:
This test is one of many to aid in the identification of yeast. It identifies the yeast nased on whether or not it uses specific carbohydrates as carbon sources. It can also be used with nitrate to determine whether or not yeast can nitrate successfully as a nitrogen source. If the yeast can or cannot use the certain carbohydrate or nitrate, then it can be compared to the traits of certain yeasts. The agar can be infused with different carbohydrates or nitrogen sources to test for traits specific to the yeast testes for. This can be seen in the test by Pincus et al where potassium nitrate is used. This is an example of where a specific nitrogen source is used to test whether or not the yeast can use it and then can help to classify the organism.
References:
- Ahearn, D, Roth, F, Fell, J, Meyers, S. Use of Shaken Cultures in the Assimilation Test for Yeast Identification. 1960. J. Bacteriol, 79(3): 369–371
- Lin, C, Fung, D. Conventional and Rapid Methods fro Yeast Identification. 1987. Critical Reviews in Microbiology. Vol. 14, No. 4, Pages 273-289.
- Pincus, D, Salkin, I, Hurd, N, Levy, I, Kemna, M. Modification of potassium nitrate assimilation test for identification of clinically important yeasts. 1988. Journal of Clinical Microbiology, 26(2): 366–368.