Urease testing and yeast taxonomy.

Area of Science:

• Microbiology
• Yeast Taxonomy
• Enzymology

Background:

• Urease activity in yeasts is a key characteristic for identification and understanding metabolic pathways.
• Previous studies indicated certain yeast groups, like the Cryptococcus vishniacii complex, were urease-negative.
• Conventional urease testing methods can be prone to inaccuracies.

Purpose of the Study:

• To re-evaluate urease production in various yeast species using [14C]urea hydrolysis.
• To investigate the causes of anomalous results in conventional urease testing, specifically Roberts' rapid urea hydrolysis (RUH) test.
• To assess the impact of ethylenediaminetetraacetic acid (EDTA) on urease production and its potential for differentiating yeast varieties.

Main Methods:

• Assaying urease production via [14C]urea hydrolysis to measure 14CO2 production.
• Utilizing Roberts' rapid urea hydrolysis (RUH) test and comparing results with [14C]urea hydrolysis.
• Culturing yeasts in media with and without ethylenediaminetetraacetic acid (EDTA) to observe effects on urease production.
• Supplementing EDTA-containing media with Ni+2 to investigate restoration of urease activity.

Main Results:

• All tested basidiomycetous yeasts, including the Cryptococcus vishniacii complex, demonstrated significant urease activity.
• The Schizosaccharomycetaceae were the sole urease-positive ascomycetous yeasts identified; Yarrowia lipolytica was urease-negative.
• Anomalous RUH test results were attributed to medium pH changes from endogenous metabolism and autolysis, and also to prior growth in the presence of EDTA.
• Ethylenediaminetetraacetic acid (EDTA) inhibited urease production in most yeasts, except Filobasidiella (Cr.) neoformans var. neoformans.
• The RUH test differentiated between Filobasidiella neoformans varieties based on EDTA resistance, with var. neoformans being more resistant than var. bacillispora.
• Urease production in Fil. neoformans var. bacillispora was restored by Ni+2 in the presence of EDTA.

Conclusions:

• Basidiomycetous yeasts are generally urease-positive, necessitating a revision of previous classifications.
• Conventional urease tests require careful interpretation due to potential interference from metabolic byproducts and chelating agents like EDTA.
• Ethylenediaminetetraacetic acid (EDTA) sensitivity and Ni+2 supplementation can serve as valuable tools for differentiating yeast varieties within the Filobasidiella genus.