Cordero, R. J. | bioRxiv Science

bioRxiv2018

The buoyant cell density of Cryptococcus neoformans is affected by capsule size

Cryptococcus neoformans is an environmental pathogenic fungus with a worldwide geographical distribution that is responsible for hundreds of thousands human cryptococcosis cases each year. During infection, the yeast undergoes a morphological transformation involving capsular enlargement that increases microbial volume. To understand the factors that play a role in environmental dispersal of C. neoformans and C. gatii we evaluated the buoyant cell density of Cryptococcus using Percoll isopycnic gradients. We found differences in the buoyant cell density of strains belonging to C. neoformans and C. gatti species complexes. The buoyant cell density of C. neoformans strains varied depending on growth medium conditions. In minimal medium, the cryptococcal capsule made a major contribution to the buoyant cell density such that cells with larger capsules had lower density than those with smaller capsules. Removing the capsule, both by chemical or mechanical methods, decreased the C. neoformans cell density. Melanization of the C. neoformans cell wall, which also contributes to virulence, produced a small but consistent increase in cell density. C. neoformans sedimented much slower in seawater as its density approached the density of water. Our results suggest a new function for the capsule whereby it can function as a flotation device to facilitate transport and dispersion in aqueous fluids.

microbiology↗

bioRxiv2018

The capsule of Cryptococcus neoformans modulates phagosomal pH through its acid-base properties

Phagosomal acidification is a critical cellular mechanism for the inhibition and killing of ingested microbes by phagocytic cells. The acidic environment activates microbicidal proteins and creates an unfavorable environment for the growth of many microbes. Consequently, numerous pathogenic microbes have developed strategies for countering phagosomal acidification through various mechanisms that include interference with phagosome maturation. The human pathogenic fungus Cryptococcus neoformans resides in acidic phagosome after macrophage ingestion that actually provides a favorable environment for replication since the fungus replicates faster at acidic pH. We hypothesized that the glucuronic acid residues in the capsular polysaccharide had the capacity to affect phagosome acidity through their acid-base properties. A ratiometric fluorescence comparison of imaged phagosomes containing C. neoformans to those containing beads showed that the latter were significantly more acidic. Similarly, phagosomes containing non-encapsulated C. neoformans cells were more acidic than those containing encapsulated cells. Acid-base titrations of isolated C. neoformans polysaccharide revealed that it behaves as a weak acid with maximal buffering capacity around pH 4-5. We interpret these results as indicating that the glucuronic acid residues in the C. neoformans capsular polysaccharide can buffer phagosomal acidification. Interference with phagosomal acidification represents a new function for the cryptococcal capsule in virulence and suggests the importance of considering the acid-base properties of microbial capsules in the host-microbe interaction for other microbes with charged residues in their capsules.\n\nImportanceCryptococcus neoformans is the causative agent of cryptococcosis, a devastating fungal disease that affects thousands of individuals worldwide. This fungus has the capacity to survive inside phagocytic cells, which contributes to persistence of infection and dissemination. One of the major mechanisms of host phagocytes is to acidify the phagosomal compartment after ingestion of microbes. This study shows that the capsule of C. neoformans can interfere with full phagosomal acidification by serving as a buffer.

microbiology↗