Candida auris, a recently emerged fungal pathogen, represents a significant global health concern. This yeast is known for its ability to cause severe illness, particularly in hospitalized patients and individuals requiring ventilator support. Its hardiness and resistance to commonly used antifungal treatments make it a difficult organism to manage within healthcare environments. The Centers for Disease Control and Prevention (CDC) has classified C. auris as an urgent threat due to its potential to cause outbreaks, resist disinfectants, and persist on human skin and surfaces.
The Cellular Growth Cycle
Candida auris primarily reproduces through an asexual process called budding. This involves a parent cell forming a small outgrowth, or bud, which then enlarges and eventually separates to become a new, independent daughter cell. These individual cells are ovoid or ellipsoid in shape, measuring approximately 2.5 to 5.0 micrometers in size.
While C. auris cells are predominantly single, they can also appear in pairs or groups, particularly if daughter cells fail to fully detach after budding. Although a sexual cycle has been identified in laboratory settings, asexual budding remains the main method by which C. auris proliferates in nature and during infections within a host. This efficient reproductive strategy allows for rapid increases in fungal populations.
Biofilm Formation and Environmental Persistence
A key aspect of C. auris’s life cycle involves its ability to form biofilms, which are communities of microorganisms encased in a self-produced protective, slimy matrix. This matrix allows the yeast to adhere strongly to various surfaces, including medical equipment, bedrails, and human skin. Biofilm formation is a significant factor in its persistence outside of a host.
The biofilm structure provides substantial protection for C. auris cells against environmental stressors, such as disinfectants and dehydration. Studies show that C. auris can survive on plastics and metals for up to 14 days, even in dry conditions, far longer than some other Candida species. This resilience makes C. auris particularly challenging to eradicate from healthcare environments, contributing to its widespread transmission.
Specific molecules within C. auris biofilms, such as adhesin proteins, play a role in its ability to cling to both living and non-living surfaces. These adhesins, along with beta-glucans in the cell wall, contribute to the robust attachment and persistence of the yeast. The protective extracellular matrix, rich in polysaccharides, also offers resistance against many antifungal agents and common hospital disinfectants.
Colonization and Invasion in a Host
Candida auris can colonize a patient, meaning it lives on the skin or in other body sites without causing active disease. This colonization can persist for months, or even indefinitely, providing continuous opportunities for spread. Colonized individuals may not show symptoms, yet they can serve as reservoirs for transmission within healthcare facilities.
Invasive infection occurs when C. auris breaches the body’s natural defenses and enters the bloodstream or deep tissues, leading to severe conditions like candidemia (bloodstream infection). This transition from colonization to invasion is more common in patients who are critically ill, have compromised immune systems, or possess indwelling medical devices. Catheters, breathing tubes, and feeding tubes provide easy access points for the fungus to enter the bloodstream.
Bloodstream infections are the most frequently reported invasive infections caused by C. auris. Once in the bloodstream, C. auris can spread to various distant anatomical sites, potentially causing infections in the urinary tract, wounds, ears, or internal organs.
Mechanisms of Survival and Resistance
Candida auris is known for its intrinsic multidrug resistance, meaning it is naturally resistant to at least one, and often multiple, classes of antifungal drugs commonly used for other Candida infections. This resistance complicates treatment, as some strains have shown resistance to all three main classes of antifungal medications. Efflux pumps, encoded by specific genes, contribute to this resistance by actively removing antifungal agents from the fungal cells.
Another characteristic that sets C. auris apart is its unusual tolerance for higher temperatures and salt concentrations compared to many other yeasts. It can grow at higher temperatures and survive in high salt concentrations. This thermotolerance and halotolerance contribute to its ability to thrive on human skin, which can be warm and salty, and to persist on various hospital surfaces, aiding its environmental survival and dissemination.