Oxytetra Glucose Yeast Agar Base, Granulated (OGYE Agar Base, Granulated)

Principle And Interpretation

Acidic media are not completely suitable for counting yeasts and moulds in foods since yeast cells, stressed by heat do not tolerate the acidic conditions necessary to inhibit bacterial contamination. Yeast and mould growth is often limited by the presence of acid-tolerant bacterial flora. Therefore it is evident that more active media and different selective agents are needed in order to deal with various kinds of foodstuffs, incubation conditions and types of microorganisms to be studied. Under certain conditions and when testing certain foods like milk and milk products, the use of oxytetracycline alone was not sufficient to obtain reliable yeast and mould counts. In particular, Mossel et al (1) observed that with proteinaceous foods heavily contaminated with gram-negative rods, it is necessary to use both oxytetracycline and gentamicin (FD131) in order to obtain complete inhibition of the contaminants.

OGYE Media were formulated by Mossel et al for the selective isolation and enumeration of yeast and moulds from foods (1,2). They found that addition of Oxytetra selective supplement to a neutral pH medium increased the recovery / count of yeast and moulds as compared to acidified medium. Psychotrophic yeasts can also be isolated when gentamicin is also incorporated into the medium (3). ISO Committee (4) has recommended OGYE Media with pH 6.6 ± 0.2 for isolation and enumeration of yeasts and moulds from milk and milk products.

Yeast extract provides essential growth nutrients. Dextrose acts as carbon and energy source. Low pH helps to reduce the bacterial flora. Oxytetracycline makes the medium more selective by inhibiting the growth of Lactobacilli encountered in milk and milk-products at low pH. The choice of a suitable media for enumeration of yeasts and moulds greatly depends on the nature of foodstuffs to be tested and the organisms that grow on them. These media remain bacteriostatic when inoculated with not greater than 1 ml of a 10-1 food dilution and incubation at 22°C. The number of yeasts or moulds is calculated per one gram or 1 ml of sample under investigation by multiplying the number of colonies with the dilution factor. Lactic acid bacteria are inhibited on this medium.