Dormancy in Cryptococcus neoformans: 60 years of accumulating evidence

Abstract

Cryptococcus neoformans is an opportunistic yeast that is present worldwide and interacts with various organisms. In humans, it is responsible for cryptococcosis, a deadly invasive fungal infection that represents around 220,000 cases per year worldwide. Starting from the natural history of the disease in humans, there is accumulating evidence on the capacity of this organism to enter dormancy. In response to the harsh host environment, the yeast is able to adapt dramatically and escape the vigilance of the host's immune cells to survive. Indeed, the yeast exposed to the host takes on pleiotropic phenotypes, enabling the generation of populations in heterogeneous states, including dormancy, to eventually survive at low metabolic cost and revive in favorable conditions. The concept of dormancy has been validated in C. neoformans from both epidemiological and genotyping data, and more recently from the biological point of view with the characterization of dormancy through the description of viable but nonculturable cells.

Figure 1. Schematic representation of the evolution of C. neoformans phenotypes and morphology upon incubation under progressive nutrient deprivation and anaerobiosis.

The reference strain H99 was used in all conditions. STAT: stationary phase; yeast peptone dextrose (YPD), 22 hours with agitation at 150 rpm. VBNC: after incubation for 8 days in anaerobiosis and nutrient deprivation. DEAD: morphology of dead cells called Drop Cn, including one or two large vacuole-like structures. Each panel shows optical microscopy (left) and electron microscopy (right) images. Yeast cells under agitation and in glucose-rich medium (YPD) are actively multiplying in logarithmic phase (LOG). Quiescent yeasts are culturable (STAT) and do not need specific stimuli to grow in normal glucose-rich medium (YPD). Dormant yeasts (VBNC) are not spontaneously culturable and need a trigger stimulus for reactivation (addition of pantothenic acid). Dead yeasts (DEAD) are irreversibly unable to grow again (refs. 23, 95).