The Capsule of the Fungal Pathogen Paradoxically Inhibits Invasive Growth

ABSTRACT

Invasive growth in agar is an extended feature among many fungal species, and is correlated with morphological changes. In this paper we describe this phenomenon in the fungal pathogen Cryptococcus neoformans. This pathogen has a polysaccharide capsule around the cell body which is required for virulence. We found that acapsular mutants obtained by different methods invaded the agar in rich media, a phenomenon not observed in encapsulated strains. Agar invasion required the absence of capsule, since invasive growth was not present in cap59 mutants reconstituted with the wild type gene. We compared multiple C. neoformans strains, including both C. neoformans and C. gattii, and confirmed that none of the encapsulated strains invaded the agar. In contrast, for other fungi (such as Candida albicans, C. parapsilosis and C. krusei), invasive growth in C. neoformans was not correlated with hyphae or pseudohyphae formation, and only yeast forms were observed in the agar. Melanisation of the cells did not affect the pattern of invasive growth. Our results demonstrate that the C. neoformans polysaccharide capsule inhibits invasive growth, a phenomenon that could have consequences for the dissemination of the microorganism during in vivo infection.

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