Candida auris’s unknown origins

Candida auris has been discovered in a wide range of environments, including oceans and apples.

A search team was looking for a killer on a vast palm-fringed beach bordered by a sapphire-blue sea. The year was 2021, and the operation was taking place in the Andaman Islands, a remote archipelago in the north-eastern Indian Ocean, at Corbyn Cove, an impressive swath of pale golden sand and terracotta beach huts.

So far, the suspect has been identified in at least 33 countries across three continents, resulting in hundreds of deaths. However, this was no ordinary manhunt. To begin with, there were no search warrants, only swabs. Second, the attacker was a fungus.

Candida auris was discovered in 2009, when a 70-year-old woman presented to a hospital in Tokyo, Japan, complaining of ear pain. Hundreds of new cases emerged within a few years – It didn’t take long for the yeast to take its first victims. Today, C. auris has infected tens of thousands of people, including at least 7,413 in the United States alone. The Centers for Disease Control and Prevention (CDC) announced earlier this month that antibiotic resistance is a “urgent” threat – in some areas, the vast majority of cases are resistant to at least one antifungal medication, and cases in the country are expected to double by 2021.

Despite the growing concern, this new pathogen remains a mystery. Nobody knows where it came from or why it is spreading so quickly.

A rapid conquest

C. auris is a “budding yeast,” a microscopic oval-shaped fungus 2.5-5 micrometres long – about the width of a spider silk strand. The pathogen has specialised in infecting people who are already vulnerable, such as those with weakened immune systems or illnesses. It has swept through hospitals all over the world, lingering in the air and hopping from patient to patient.

The pathogen is particularly difficult to eradicate; once it has established itself on bedding and other surfaces, it can often withstand even the most thorough cleaning. It was discovered that it remained infectious after drying on plastic for at least 28 days. “Worse, there are a few Candida auris strains that are nearly untreatable because they are resistant to the three major classes of drugs that we have to treat fungal infections,” says Neil Gow, a professor of microbiology at the University of Exeter. “There are a few really, really concerning strains circulating.”

C. auris has a fatality rate of between 30 and 72% in infected people, which is significantly higher than that associated with other antibiotic-resistant microorganisms, which one study estimated at 19%.   

An elusive origin

The most surprising aspect of C. auris’ story thus far, however, was discovered by a team of researchers in 2017. When they examined the genomes of fungi found in hospitals in Pakistan, India, South Africa, Venezuela, and Japan, they discovered that while those found in the same geographic region were closely related, there were three distinct “strains.” Each continent had a distinct strain of the pathogen.

This implies that C. auris has made the transition from the environment to humans at least three times, all within the last 14 years. But why is this so? Is there a change that has made humans more hospitable to the fungus than they were previously?

C.’s nearest known relative. C. haemulonii, also known as auris, has been discovered in a variety of environments, including the guts of a fish found in the western Atlantic Ocean, the skin of dolphins, and the seas off the coast of Portugal.

C. auris, like its cousin, is not thought to be limited to animal hosts, which greatly expands the possibilities for its wild reservoir. However, there are some hints. To begin with, C. auris is extremely salt-tolerant, able to withstand concentrations far greater than those that would inhibit the growth of most bacteria. Second, it can grow well in warm conditions, even thriving at 42 degrees Celsius.

Armed with this knowledge and that of its closest relative, an international team of scientists decided to survey several environments in the Andaman Islands, where they believed the fungal flora would have been less affected by human activity than elsewhere on the planet. They collected samples from rocky shores, sandy beaches, marshes, and mangroves throughout the archipelago and tested them for C. auris.

They discovered fungus evidence at two locations: Corbyn Cove Beach and a nearby salt marsh. Could this new pathogen have originated in the sea?

Unfortunately, just as it appeared that the mystery was solved, another research team made an unexpected discovery. In India, C. auris was discovered on the surfaces of apples. and the strains found on the fruit were eerily similar to those found in the sea.

A human cause

The true origins of C. auris are unknown, but both findings support the leading theories about why the pathogen has only recently begun infecting humans. The first is global warming.

Fungi, in general, do not do well in hot environments. They infect animals with lower body temperatures, such as insects and amphibians, much more effectively. They can completely take over the former’s bodies, resulting in a zombie-like state that inspired the video game and television series The Last of Us. They have also wiped out more than 90 amphibian species in just 50 years.

This Achilles heel is thought to be one of the reasons mammals and birds evolved warmer bodies. giving us a significant advantage in our ongoing battle with these organisms – some experts believe it is partly responsible for the rise of mammals after dinosaurs died out. The factor is so potent that simply heating frogs can cure them of deadly fungal infections (do not try this at home). These dynamics, however, are changing as the climate warms.

“Of course, it has the chance to grow on a human body as they become more adapted to being able to live and thrive at something that approaches body temperature,” Gow says. If climate change has been heating up environments like tropical beaches, it’s possible that C. auris has been able to tolerate the warm temperatures in our bodies.

The other theory is that C. auris arose as a result of the widespread use of antifungals. These drugs and pesticides are now widely used to prevent microbes from growing on crops, but it is possible that this has allowed them to develop resistance. As a result, they can spread much more quickly among humans than they could previously, particularly in hospitals.

Despite these findings, Gow notes that there have been very few reports of C. auris in the wild, “so that’s a little bit of an unknown or underexplored area.” After all, with such a diverse range of habitats on the planet, What are the chances that the few surveys conducted so far have discovered the exact locations from which it emerged?