What Are Candida Glabrata and Candida Krusei Infections?

Candida glabrata and Candida krusei are two species of yeast that cause opportunistic infections in humans, and they matter clinically because both are far harder to treat with standard antifungal drugs than the more familiar Candida albicans. C. glabrata ranks as the second or third most common cause of both superficial and systemic Candida infections, while C. krusei is less common but carries intrinsic resistance to fluconazole, the most widely prescribed antifungal in the world.¹ Together, these two species sit at the center of a growing problem in hospitals: the rise of drug-resistant fungal infections that do not respond to first-line therapy.

The Shift Away From Candida Albicans

For decades, Candida albicans dominated the landscape of fungal infections. It was the species clinicians expected to find when a patient developed thrush, a bloodstream infection, or a urinary tract infection caused by yeast. That picture has changed. The incidence of candidemia has risen sharply, and this increase has come with a corresponding shift away from C. albicans toward non-albicans species that are often harder to treat.² C. glabrata, C. parapsilosis, and more recently C. auris have all become more prominent.³

A growing body of evidence points to the widespread use of azole antifungals, particularly fluconazole, as a driving force behind this shift. Fluconazole prophylaxis became routine in many intensive care units and transplant wards starting in the 1990s. That selective pressure appears to have favored species with natural or easily acquired resistance to azoles, especially C. glabrata.² The irony is hard to miss: a drug meant to prevent fungal infections has helped breed a population of fungi that the same drug cannot kill.

How C. Glabrata Survives Inside You

One reason C. glabrata is such a successful pathogen is its ability to stick to human tissue. The organism carries a large family of surface proteins called epithelial adhesins (Epa proteins) that function like tiny grappling hooks. These adhesins recognize sugar molecules on the surface of your cells and latch on, allowing the yeast to colonize mucosal surfaces in the mouth, gut, and urinary tract.⁴ The adhesin family is diverse enough that C. glabrata can recognize a wide variety of host cell sugars, making it a flexible colonizer across different body sites.⁵

What makes C. glabrata especially tricky is what happens after your immune system tries to eat it. When white blood cells called macrophages engulf the yeast, most pathogens die. C. glabrata does not. It survives and even replicates inside these immune cells while provoking surprisingly little alarm, causing low damage to the macrophage and triggering a weak inflammatory signal.⁶ The yeast accomplishes this partly by manipulating the internal environment of the compartment it gets trapped in, preventing the macrophage from acidifying it properly.⁷ The result is a pathogen that can hide inside the very cells sent to destroy it, buying time to spread.

Your immune system does have tools to recognize C. glabrata. Receptors called Dectin-1 on the surface of immune cells detect the yeast and trigger both an immediate response and a longer-term adaptive immune response. Mice lacking Dectin-1 are substantially more susceptible to C. glabrata infection, which helps explain why people with weakened immunity face the highest risk.⁸

Why Standard Antifungals Often Fail

The drug resistance story is different for each species, which matters for treatment decisions. C. krusei is born resistant to fluconazole. The enzyme that fluconazole targets in most Candida species works differently in C. krusei; fluconazole simply does not bind to it well. In lab experiments, this enzyme in C. krusei required concentrations of fluconazole roughly 16 to 46 times higher than those needed to inhibit the same enzyme in C. albicans.⁹ Additional work has confirmed that this low binding affinity, combined with reduced drug accumulation inside C. krusei cells, accounts for the species’ resistant nature.¹⁰ This is not resistance that the organism acquires over time. It is baked in from the start, which means fluconazole should never be used against C. krusei regardless of the dose.

C. glabrata takes a different path. It is not innately resistant to fluconazole, but it readily becomes resistant through use. The dominant mechanism involves pumping the drug back out of the cell before it can work. Resistant C. glabrata isolates massively overexpress efflux pump genes, sometimes cranking production up 10 to 85 times above normal.¹¹ This overexpression is often driven by mutations in a regulatory gene called PDR1. These gain-of-function mutations essentially remove the brakes on efflux pump production, and they also appear to boost the organism’s ability to adhere to tissue, potentially making it both more resistant and more virulent at the same time.¹²

The Multidrug Resistance Problem

When azoles stop working against C. glabrata, clinicians turn to echinocandins, a different class of antifungal that attacks the fungal cell wall. Echinocandins are now the first-line treatment for serious Candida infections. But resistance to echinocandins is rising too. In one large surveillance study, about 8% of C. glabrata isolates carried mutations in the FKS genes that confer echinocandin resistance, and the strongest predictor of having one of these mutations was prior echinocandin therapy.¹³

The most alarming finding is what happens when azole resistance and echinocandin resistance overlap. Among U.S. C. glabrata isolates resistant to at least one echinocandin, roughly a third were also resistant to fluconazole.¹⁴ A Candida infection that does not respond to azoles or echinocandins leaves clinicians with very few options, usually amphotericin B, a powerful but toxic older drug. This kind of multidrug resistance is still uncommon in absolute terms, but the trend is moving in the wrong direction.

Where These Infections Show Up

Both species cause a range of infections, from superficial to life-threatening. C. glabrata is a common cause of oral thrush, esophageal candidiasis, vaginal yeast infections, and urinary tract infections. Its prevalence has increased particularly among people with weakened immune systems, including those with HIV and those on immunosuppressive drugs after organ transplants.¹ When either species enters the bloodstream, the resulting candidemia is a serious, often fatal hospital-acquired infection.

A study comparing C. glabrata and C. albicans bloodstream infections found that patients who developed C. glabrata candidemia early in their hospital stay faced higher costs (roughly $56,000 versus $33,000) and longer stays (about 20 days versus 14.5 days) compared with those who developed C. albicans candidemia. Mortality was similar between the two groups, but the economic and resource burden of C. glabrata was substantially greater.¹⁵

C. krusei is less common overall but tends to appear in specific populations: patients with blood cancers, bone marrow transplant recipients, and others who have received prolonged fluconazole prophylaxis. Because fluconazole is often given preventively in these settings and C. krusei is inherently resistant, the drug inadvertently selects for C. krusei to fill the niche vacated by susceptible species.

Identifying the Right Species

Getting the species identification right is critical because the treatment differs so much. A C. krusei bloodstream infection treated with fluconazole will not improve, and a C. glabrata infection in a patient with prior azole exposure may need different drugs altogether. Traditional culture on chromogenic agar media can give a preliminary identification. Newer media such as CHROMagar Candida Plus correctly identify C. krusei and perform well as a screening tool for C. glabrata, though results should be confirmed with more precise methods like mass spectrometry.¹⁶

Speed matters when someone has a bloodstream infection. A newer approach using mass spectrometry on nucleic acids can detect and distinguish five Candida species directly from urine samples, with the entire process completed in about five hours and a diagnostic sensitivity of 100% when compared against culture.¹⁷ Faster identification means clinicians can switch from a broad-spectrum empiric antifungal to a targeted one sooner, reducing both toxicity and the selective pressure that breeds more resistance.

Current Treatment Recommendations

The Infectious Diseases Society of America (IDSA) guidelines recommend starting with an echinocandin for serious Candida infections, regardless of species, until cultures come back. Echinocandin options include caspofungin, micafungin, and anidulafungin. Once the species is identified, the path diverges:

• C. glabrata: Transition to an oral azole like fluconazole or voriconazole only if susceptibility testing confirms the isolate is sensitive. If not, echinocandins remain the mainstay.
• C. krusei: Fluconazole is never appropriate. Recommended options include echinocandins, a lipid formulation of amphotericin B, or voriconazole, which retains activity against this species even though fluconazole does not.

Voriconazole is specifically recommended as oral step-down therapy for C. krusei bloodstream infections once the patient is stable enough to leave intravenous therapy behind.¹⁸

New Drugs on the Horizon

The antifungal pipeline has been thin for years, but several new agents have emerged that could change treatment for resistant Candida infections. Ibrexafungerp is an oral drug that inhibits the same target as echinocandins (glucan synthase) but through a different binding site. It shows high activity against azole-resistant C. glabrata and C. krusei isolates, and most echinocandin-resistant C. glabrata strains remain susceptible to it, making it a potential bridge for multidrug-resistant infections.¹⁹ It is currently approved for vulvovaginal candidiasis and is being investigated as oral step-down therapy after initial intravenous treatment for invasive disease.²⁰

Rezafungin is a next-generation echinocandin with an unusually long half-life that allows once-weekly dosing instead of daily infusions. This is a practical advantage for patients who otherwise need a central intravenous line for daily drug administration. It shows strong activity against most wild-type and azole-resistant Candida species.¹⁹ Fosmanogepix works through an entirely new mechanism and shows potent activity against several Candida strains resistant to other antifungals, though it lacks activity against C. krusei specifically.²⁰ That gap highlights an ongoing challenge: no single new drug covers everything, and C. krusei in particular tends to fall through the cracks of new therapeutic development.

A Naming Change Worth Knowing About

If you encounter unfamiliar names in recent medical literature, you are not confused. Both species have been reclassified. C. glabrata is now formally called Nakaseomyces glabratus, and C. krusei has been renamed Pichia kudriavzevii. The old names remain in widespread clinical use, and most clinicians and laboratories still use them, but published research increasingly uses the new nomenclature. You may see studies on “N. glabratus” biofilms or “P. kudriavzevii” resistance and not immediately realize these are the same organisms.²¹

The reclassification reflects the fact that neither species is closely related to C. albicans. C. glabrata is actually more closely related to baker’s yeast (Saccharomyces cerevisiae) than to C. albicans, which is why it behaves so differently: it does not form the filamentous structures (hyphae) that C. albicans uses to invade tissue, and its genome is organized differently. Lumping all these organisms under “Candida” was always a convenience rather than a reflection of their biology.

C. Krusei’s Other Life in Food and Industry

Here is a fact that surprises most people learning about C. krusei for the first time: the same organism that causes drug-resistant infections in immunocompromised patients plays a valued role in food production around the world. Under its newer name Pichia kudriavzevii, it turns up in the spontaneous fermentation of traditional foods and beverages, where it contributes to flavor development, acid degradation, and enzyme release. Its tolerance to extreme pH, high temperatures, and high sugar concentrations makes it useful in industrial biotechnology as well.²²

This dual identity creates an unusual tension. The same stress-tolerance traits that make P. kudriavzevii attractive as an industrial yeast (thriving in harsh environments, resisting inhibitors) are part of what makes it a resilient pathogen. Genotyping studies have revealed that clinical isolates and environmental isolates are not always genetically distinct. One large study found clusters of closely related genotypes spanning geographically diverse countries, with evidence suggesting silent nosocomial transmission chains that may go undetected.²³ The clinical strains appear to belong to a long-standing clade with high transmission potential, particularly in neonatal intensive care units. Whether food and environmental reservoirs feed into clinical settings remains an open question, but the genetic overlap between the two worlds is closer than many researchers expected.

Biofilms and Device-Related Infections

Both species can form biofilms, the slimy communal structures that yeast and bacteria build on surfaces like catheters, prosthetic heart valves, and other medical devices. Biofilms are clinically important because organisms embedded in them are dramatically more resistant to antifungal drugs than free-floating cells, often requiring device removal rather than drug therapy alone. Interestingly, C. glabrata (now N. glabratus) produces less biofilm than C. albicans in laboratory comparisons, but it still forms enough to cause device-related infections that are difficult to clear.²¹ The newer echinocandin rezafungin has been tested against biofilm forms of C. glabrata from vascular infections and shows promise, though the clinical data are still in early stages.

Catheter-related candidemia is one of the most common scenarios where these species cause trouble. Current guidelines recommend removing the catheter whenever feasible, because antifungals alone often cannot sterilize a colonized device. For patients who cannot have their catheters removed, such as some dialysis patients, prolonged antifungal courses and close monitoring become necessary, and the odds of relapse are higher.