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The biochemical basis for the distinction between the two Cryptococcus neoformans varieties with CGB medium
Zentralblatt Fur Bakteriologie, Mikrobiologie, Und Hygiene. Series A, Medical Microbiology, Infectious Diseases, Virology, Parasitology
|July 1, 1986
Summary
Cryptococcus neoformans var. gattii utilizes glycine and resists canavanine, unlike most var. neoformans strains. This difference is due to glycine metabolism and ammonia production, explaining CGB agar reactions.
Area of Science:
- Mycology
- Biochemistry
- Microbial Physiology
Background:
- Cryptococcus neoformans is a fungus with distinct varieties (gattii and neoformans) that exhibit different biochemical characteristics.
- Canavanine-glycine-bromthymol blue (CGB) agar is a differential medium used to identify Cryptococcus species based on their metabolic reactions.
- Understanding the biochemical basis of CGB agar reactions is crucial for accurate fungal identification and epidemiological studies.
Purpose of the Study:
- To investigate the biochemical mechanisms underlying the differential CGB agar reactions observed in Cryptococcus neoformans var. gattii and C. neoformans var. neoformans.
- To determine the role of glycine utilization and L-canavanine resistance in the growth and differentiation of these fungal varieties.
Main Methods:
- Comparative analysis of glycine assimilation and L-canavanine resistance in isolates of C. neoformans var. gattii and C. neoformans (serotypes A and D).
- Growth studies on various media, including CGB agar and media containing glycine and/or canavanine.
- Enzymatic assays using cell-free extracts to detect glycine-cleaving enzyme activity and analyze metabolic byproducts.
Main Results:
- All C. neoformans var. gattii isolates utilized glycine and were resistant to L-canavanine, growing on media with both compounds.
- Most C. neoformans var. neoformans isolates showed limited glycine utilization or sensitivity to canavanine; some glycine-utilizing strains failed to grow with canavanine.
- Glycine-utilizing isolates possessed a discernible glycine-cleaving enzyme, producing ammonia, CO2, and serine, with enhanced activity in cell-free extracts.
Conclusions:
- The ability to utilize glycine as a sole carbon and nitrogen source and resist L-canavanine is a key differentiator for C. neoformans var. gattii.
- Glycine metabolism via glycine decarboxylase, leading to ammonia production, is responsible for the positive CGB agar reaction in glycine-utilizing Cryptococcus isolates.
- These biochemical differences provide a basis for distinguishing between C. neoformans varieties and have implications for diagnostic microbiology.