A Pathogen's Puzzling Premier
In 2009, doctors at the Tokyo Metropolitan Geriatric Hospital isolated a previously unknown yeast from the ear canal of a 70-year-old woman. They named it Candida auris, with "auris" being the Latin word for ear. This discovery was initially a microbiological curiosity. The situation became a global health puzzle when, just a few years later, genetically distinct versions of C. auris began causing severe infections almost simultaneously on three different continents.
Between 2012 and 2015, distinct clades of the fungus were identified in patients in India and Pakistan (South Asia clade), South Africa (African clade), and Venezuela (South American clade). Whole-genome sequencing revealed these strains were not closely related; they had emerged independently rather than spreading from a single source. This unprecedented, simultaneous emergence of a new pathogen in geographically separate locations baffled scientists and suggested a common global environmental pressure.
Breaching the Thermal Barrier
The leading hypothesis for the sudden appearance of Candida auris points to climate change. Mammals have a powerful, built-in defense against most fungal species: our high body temperature. Most fungi in the environment cannot replicate at the 37°C (98.6°F) of the human body. This "thermal barrier" has historically protected mammals from the planet's vast fungal kingdom.
The theory suggests that as global temperatures rise, fungi are adapting to survive in a warmer world. This process selects for strains with higher heat tolerance. C. auris may be the first example of a fungus that, after adapting to this new environmental heat, found it was pre-adapted to breach the human thermal barrier. Laboratory studies support this idea, showing C. auris can grow at higher temperatures—up to 42°C (107.6°F)—than most of its close fungal relatives. The discovery of C. auris in remote environmental locations, such as a sandy beach and a tidal swamp in the Andaman Islands, further establishes it as an environmental organism, a necessary condition for the global warming hypothesis.
The Superbug in the Hospital
Once inside a human host, C. auris is a formidable threat, particularly in healthcare settings. It causes invasive infections of the bloodstream, central nervous system, and internal organs. For patients with these invasive infections, the crude mortality rate is estimated to be between 30% and 60%.
What makes C. auris a "superbug" is its resistance to multiple classes of antifungal drugs. Many strains are resistant to azoles, the most common class of antifungals. Resistance to polyenes (like amphotericin B) and echinocandins is also increasing. Some isolates in the U.S. have showed resistance to all three major classes of available antifungal drugs, making infections exceptionally difficult to treat. The fungus is also notoriously persistent on surfaces, able to survive on plastic common in hospitals for at least two weeks. This tenacity, combined with its drug resistance, makes it a serious cause of hospital-acquired outbreaks worldwide.