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Infection and Immunity, November 2003, p. 6155-6164, Vol. 71, No. 11
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.11.6155-6164.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Induction of Capsule Growth in Cryptococcus neoformans by Mammalian Serum and CO2

Oscar Zaragoza,1 Bettina C. Fries,2 and Arturo Casadevall1,2*

Departments of Microbiology and Immunology,1 Medicine, Albert Einstein College of Medicine, Bronx, New York 104612

Received 28 May 2003/ Returned for modification 25 June 2003/ Accepted 25 July 2003

The pathogenic fungus Cryptococcus neoformans has a polysaccharide capsule that is essential for virulence in vivo. Capsule size is known to increase during animal infection, and this phenomenon was recently associated with virulence. Although various conditions have been implicated in promoting capsule growth, including CO2 concentration, osmolarity, and phenotypic switching, it is difficult to reproduce the capsule enlargement effect in the laboratory. In this study, we report that serum can induce capsule growth, and we describe the conditions that induce this effect, not only by serum but also by CO2. Capsule enlargement was dependent on the medium used, and this determined whether the strain responded to serum or CO2 efficiently. Serum was most effective in inducing capsule growth under nutrient-limited conditions. There was considerable variability between strains in their response to either serum or CO2, with some strains requiring both stimuli. Sera from several animal sources were each highly efficient in inducing capsule growth. The cyclic AMP (cAMP) pathway and Ras1 were both necessary for serum-induced capsule growth. The lack of induction in the ras1 mutant was not complemented by exogenous cAMP, indicating that these pathways act in parallel. However, both cAMP and Ras1 were dispensable for inducing a partial capsule growth by CO2, suggesting that multiple pathways participate in this process. The ability of serum to induce capsule growth suggests a mechanism for the capsular enlargement observed during animal infection.

* Corresponding author. Mailing address: Departments of Medicine and Microbiology and Immunology, Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, NY 10461. Phone: (718) 430-3665. Fax: (718) 430-8701. E-mail: casadeva{at}aecom.yu.edu.

Editor: T. R. Kozel

Infection and Immunity, November 2003, p. 6155-6164, Vol. 71, No. 11
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.11.6155-6164.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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