The polysaccharide capsule of Cryptococcus neoformans is the primary virulence factor and one of the most commonly studied aspects of this pathogenic yeast. Capsule size varies widely between strains, has the ability to grow rapidly when introduced to stressful or low-nutrient conditions, and has been positively correlated with strain virulence. For these reasons, the size of the capsule is of great interest to C. neoformans researchers. Inducing the growth of the C. neoformans capsule is used during phenotypic testing to help understand the effects of different treatments on the yeast or size differences between strains. Here, we describe one of the standard methods of capsule induction and detail two accepted methods of staining: (i) India ink, a negative stain, used in conjunction with conventional light microscopy and (ii) co-staining with fluorescent dyes of both the cell wall and capsule followed by confocal microscopy. Finally, we outline how to measure capsule diameter manually and offer a protocol for automated diameter measurement of India ink-stained samples using computational image analysis.
Keywords: Capsule; Cryptococcus neoformans; Fluorescent stain; Image analysis; India ink; Virulence; Yeast.
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