Yeasts assimilation tests

L. J. Wickerham, J. Bacteriol., 52 293-301 (1946). A critical evaluation of the nitrogen assimilation tests commonly used in the classification of yeasts. 

Assimilation tests for carbon compounds use yeast nitrogen base (YNB) without carbon sources, and the assimilation test for nitrogen compounds use yeast carbon base (YCB) without assimilable nitrogen sources. For the tests described, YNB without amine acids and ammonium sulfate is used (Anonymous, 1984). Carbon sources normally examined include a number of pentoses, hexoses, disaccharides, trisaccharides, polysaccharides, alcohols, organic acids, and glycosides as specified by Yarrow (1998). Nitrogen sources commonly tested include nitrate, nitrite, ethyl-amine hydrochloride, cada-verine dihydrochloride, L-lysine, imidazole, glucosamine, creatine, and creatinine. When nitrite is used as a test compound, it is necessary to adjust the pH of the medium to 6.5 because toxic nitrous acid is formed at pH <6.0. 

Wickerham, L.J. and Bnrton, K.A. (1948) Carbon assimilation tests for the classification of yeasts. Journal of Bacteriology, 56 363-371. 

Assimilable nitrogen sources readily utilized by all the tested strains were peptone, hydrolysed casein, yeast extract, glutamic acid. 

In non-detoxified fully supplemented BSG hemicellulosie hydrolyzate, D. hansenii assimilates all sugars and consumes most of the inhibitors, presenting both superior kinetic and stoichiometric performance compared to the other tested yeast species. This performance is maintained for minimal (optimized) supplemented BSG medium, and it is expected that this performance can be further improved for more controlled oxygen and pH growth conditions. 

Several methods have been developed for the differentiation of yeasts. Traditional platting techniques may be adapted using selective and/or differential platting media. Such media have been developed for Zygosaccharomyces bailii, the most important of all food spoilage yeasts. However, yeast differentiation by differential media is poorly developed when compared with similar works applied to bacteria. Other phenotyping methods include conventional yeast identification by means of assimilation or fermentation tests and the use of morphological characteristics. Conventional methods are not suited to industrial laboratories even when these procedures are automated and computerized. 

---