Kluyveromyces marxianus

Kluyveromyces marxianus

1. Identity

Taxonomic Classification and Nomenclature

Kluyveromyces marxianus is a thermotolerant, ascomycetous yeast species in the family Saccharomycetaceae, characterized by its rapid growth rate—the fastest among eukaryotic microbes with a generation time of approximately 70 minutes—and ability to assimilate diverse carbon sources such as lactose, xylose, and inulin. This homothallic, hemiascomycetous yeast reproduces via multilateral budding, forming oval to elongate cells and pseudohyphae under certain conditions, and is phylogenetically closely related to Saccharomyces cerevisiae as a sister species to Kluyveromyces lactis.

Its Crabtree-negative metabolism favors respiration over fermentation, enabling efficient growth across a wide pH range and temperatures up to 52°C, with optimal fermentation at 45°C. In contrast to Saccharomyces cerevisiae, most K. marxianus strains are apparently Crabtree-negative or have aerobic-respiring characteristics, which is a desirable phenotype for large-scale biosynthesis of products associated with biomass formation, as ethanol as an undesirable by-product can be avoided under aerobic conditions.

Synonyms and Anamorph

This species is the sexual stage (teleomorph) of Atelosaccharomyces pseudotropicalis, also known as Candida kefyr. The anamorph name of K. marxianus is Candida kefyr. Synonyms of this species include Dekkeromyces marxianus, Guilliermondella marxiana, Zygofabospora marxiana, Zygorenospora marxiana, and Zygosaccharomyces marxianus.

Taxonomic History

This species was first described in the genus Saccharomyces as S. marxianus by the Danish mycologist Emil Christian Hansen from beer wort. He named the species after the zymologist Louis Marx of Marseille, who first isolated it from grape. The species was transferred to the genus Kluyveromyces by van der Walt in 1956. This reclassification was part of broader efforts to reorganize yeast taxonomy based on ascospore formation, pseudohyphal development, and fermentation profiles, distinguishing Kluyveromyces from other endomycetaceous yeasts. The genus itself was first proposed by van der Walt in 1956 to accommodate species producing multi-spored asci, with subsequent expansions incorporating lactose-fermenting taxa like K. marxianus.

Natural Habitats and Sources

K. marxianus is one of the most auspicious nonconventional yeasts, generally isolated from wide-ranging natural habitats such as fermented traditional dairy products, kefir grain, sewage from sugar industries, sisal leaves, and plants. The yeasts included in the genus Kluyveromyces have been isolated from such diverse environments as soil, plants, fruits, beer, wine, insects, seawater, food, animals, and humans. K. marxianus has been isolated in dairy products, sisal leaves, and sewage from sugar manufacturing factories. It is also a naturally occurring colonist of plants, including corn.

Common Forms and Preparations

K. marxianus is a non-conventional yeast species used in nutritional products, particularly as a source of proteins, vitamins, and bioactive compounds. Historically, it has been utilized in the dairy industry for fermentation processes due to its rapid growth and ability to metabolize lactose. Its application in nutritional supplements and functional foods is based on its high content of essential amino acids, B-vitamins, and antioxidant metabolites.

In supplemental contexts, K. marxianus is typically supplied as:

• Dried yeast biomass / single-cell protein (SCP) powder — the heat-treated whole cell or cell-extract form used as a nutritional ingredient or protein supplement.
• Live or lyophilized (freeze-dried) cells — used in probiotic preparations for delivery of viable organisms. Freeze-drying has been applied to K. marxianus as a starter culture for kefir fermentations.
• Fermented dairy product component — present as an active constituent in kefir and certain fermented cheeses consumed as whole foods.
• Cell-wall extracts — including polysaccharide fractions such as β-glucans and mannans, used in research and emerging nutraceutical preparations.

2. Traditional and Historical Use

Kefir and Fermented Dairy Traditions

The origins of kefir predate written records and it has been consumed for centuries as part of the daily diet, valued for its perceived health-promoting properties. Owing to its long history of consumption, kefir has attracted increasing scientific interest as a functional food.

K. marxianus accounts for more than 90% of the yeast population of kefir, and recently, its probiotic potential has been actively explored with a focus on its health benefits and safety. Kefir is traditionally produced by fermenting milk with kefir grains, which consist of a complex microbial community of lactic acid bacteria and yeasts including Kluyveromyces marxianus. The Codex Alimentarius Commission's standard for fermented milks (CXS 243-2003) specifically names Kluyveromyces marxianus as one of the lactose-fermenting yeasts constitutive of authentic kefir grains, requiring that products sold as "kefir" be made with a culture originating from kefir grains containing that defined consortium of lactic acid bacteria and yeasts in symbiosis.

K. marxianus and related Kluyveromyces species possess weak proteolytic and lipolytic activities. The ability of Kluyveromyces spp. to metabolize milk constituents (lactose, proteins, and fat) makes them very important in cheese ripening and fermented milk products such as kefir, as they contribute to maturation and aroma formation. Ethanol and CO₂ production due to lactose fermentation by Kluyveromyces spp. gives kefir its particular alcoholic aroma.

Milk-producing animals (cow, sheep, and goat) were all domesticated between 8,000 and 10,000 years ago, and paleoproteomic analysis of dental calculus has shown that humans were consuming milk, most likely as cheese or other fermented products, by 5,500 years ago. Because the estimate for the age of lactose-metabolism gene introgression in Kluyveromyces extends into the period of milk animal domestication, it is plausible that selection occurred as a result of human activity during production of fermented milk products such as cheese or kefir.

Traditional Fermented Beverages

K. marxianus isolates have been characterized from the elaboration process of traditional Mexican alcoholic beverages derived from agave, including pulque and henequen (Agave fourcroydes) mezcal. K. marxianus contributes to spontaneous fermentations in dairy products such as cheese and kefir, where it ferments lactose to produce volatile aroma compounds like ethanol, acetaldehyde, and esters that enhance flavor profiles. In these natural processes, the yeast aids preservation by lowering pH and inhibiting spoilage organisms through metabolite production.

Single-Cell Protein as a Nutritional Resource

K. marxianus has historically been used to produce added-value compounds including bioethanol, aroma and flavor compounds, polysaccharides such as inulin, and single-cell protein (SCP). Since SCP from fungal species presents a rich nutritional profile containing valuable amino acids, it raises interest as an alternative protein source. This application, especially growth of K. marxianus on cheese whey to produce edible biomass, has been explored at least since the second half of the twentieth century as a practical means of valorizing dairy industry waste while producing a nutritional ingredient.

3. Key Constituents and Active Compounds

Macronutrient Composition: Protein and Amino Acids

The amino acid profile of the single-cell protein from Kluyveromyces marxianus var. marxianus (ATCC 8554) grown in whey as substrate was studied. The bioprocesses were carried out at pH 5.0, 30°C, and 200 rpm during 12 h in a bioreactor; humidity, lipids, and crude protein content of the biomass obtained were measured. Results indicated a crude protein content in K. marxianus of 42.19%. The content of lysine (5.47 ± 3.0%) and threonine (15.21 ± 2.6%) in this protein are higher than in wheat flour protein. The amino acid found in the highest amount is glutamic acid, characteristic of unicellular organisms, whereas methionine is the amino acid obtained in the smallest amount. The amino acid profile shows a reasonable distribution compared to FAO international reference patterns, which suggests the potential use of this protein as a protein source.

Non-essential amino acids including aspartic acid, glutamine, glutamic acid, glycine, proline, and serine have been detected in SCP. The most abundant essential amino acid in SCP was valine (9.5 mg/g), followed by leucine (4.0 mg/g) and isoleucine (3.9 mg/g). The production of fungal protein has many advantages, including the ability to regulate the amino acid composition, high protein content in dry matter, the possibility of production in a continuous process, independence from climatic factors, and the possibility of using waste substrates as media ingredients.

Enzymes

K. marxianus is known as a fast-growing organism applied in numerous studies for the production of endogenous enzymes including inulinase, β-galactosidase, β-glucosidase, and β-xylosidase. It can metabolize a broad range of carbon sources, including less conventional sugars like lactose, xylose, arabinose, and inulin.

β-Galactosidase (Lactase): K. marxianus strains are positive for lactose assimilation. This ability is attributed to two genes, lac4 and lac12, which encode a β-galactosidase and lactose permease, respectively. These lactase enzymes can hydrolyze lactose in fermented foods and potentially in the gastrointestinal tract.

Inulinase: K. marxianus produces both exo-inulinases and endo-inulinases. Exo-inulinases release fructose from the fructosyl terminal, while endo-inulinases act on the internal glycosidic linkages. The enzyme is optimally active at pH 4.0 and 50°C. TLC analysis revealed that inulinase hydrolyzed inulin exclusively into fructose.

Cell-Wall Polysaccharides

β-Glucans: The immune-cell stimulating properties of β-glucan-containing yeast cell wall extracts from K. marxianus have been investigated, with researchers exploring the mechanisms by blocking relevant dendritic cell surface receptors.

α-d-Mannan: α-d-mannan isolated from the K. marxianus cell wall produced antinociceptive effects in models of inflammatory pain (formalin and complete Freund's adjuvant tests). Furthermore, α-d-mannan reduced paw edema and interleukin-6 (IL-6) production after carrageenan-induced inflammation. The polysaccharide was characterized by gas chromatography–mass spectrometry, methylation analysis, and spectroscopic techniques.

Short-Chain Fatty Acid Production and Metabolic Activity

In model studies, the amounts of the short-chain fatty acids acetate and propionate increased following yeast supplementation with K. marxianus B0399. K. marxianus B0399 was also found to induce a decrease in the cytotoxic potential of culture supernatant from the first stage of a colonic model system.

Aroma and Flavor Metabolites

K. marxianus produces a wide diversity of flavor compounds such as 2-phenylethanol, ethyl decanoate, benzaldehyde, and 2,3-butanediol. Other high-value aroma and flavor substances such as citronellol, linalool, and geraniol may also be produced by K. marxianus.

4. Regulatory Status

Since K. marxianus has been isolated mainly from dairy products for years, it has GRAS (Generally Recognized as Safe) and QPS (Qualified Presumption of Safety) status, making it suitable for a range of applications. The long history of safe use in kefir and related products makes K. marxianus a generally recognized as safe (GRAS) yeast strain. Rare opportunistic infections have been caused by Saccharomyces cerevisiae and Kluyveromyces marxianus, both QPS yeasts. No information in successive EFSA QPS opinions has indicated a need to change their QPS status, which has been maintained.

The strain Kluyveromyces marxianus fragilis B0399 is described as the first non-Saccharomyces yeast approved as a probiotic for human consumption. While K. marxianus shows probiotic-like properties, it is not classified as a probiotic under all current regulatory frameworks.

5. Scientific Evidence by Area of Use

5.1 Gastrointestinal Health and Probiotic Properties

Gut Adhesion and Colonization

K. marxianus has demonstrated probiotic-like properties, such as resistance to gastrointestinal conditions and adhesion to intestinal mucosa, suggesting a possible role in maintaining healthy gut microbiota. In a study comparing K. marxianus strains from Korean kefir with the clinically established probiotic yeast Saccharomyces boulardii MYA-796 in a simulated gastrointestinal tract ranging from pH 2.0 to 7.5, K. marxianus A4 exhibited the best survivability among all tested strains, including S. boulardii MYA-796 (p = 0.014). K. marxianus strains showed a greater hydrophobicity of cell surface, abilities to biofilm formation and auto-aggregation, and phenol tolerance than S. boulardii MYA-796, suggesting greater potential for survival in the host gastrointestinal tract.

All strains in one study showed fully restored growth after 2 hours' incubation at pH 2, indicating that they could survive when passing through the gastric passage into the intestine.

In Vitro and Ex Vivo Colonic Model Evidence

The aim of a key study was to broadly investigate the beneficial properties of the lactic yeast Kluyveromyces marxianus (formerly Kluyveromyces fragilis) B0399. Several potential probiotic traits were investigated using in vitro assays, including adhesion and immune modulation, and the effect of the administration of 10⁷ CFU/day of K. marxianus B0399 on the composition and metabolic activity of the human intestinal microbiota was investigated in a 3-stage continuous-culture system simulating the human colon. K. marxianus B0399 impacted the colonic microbiota, increasing the bifidobacterial concentration in the stages of the colonic model system simulating the proximal and transverse colon. The amounts of the short-chain fatty acids acetate and propionate also increased following yeast supplementation. K. marxianus B0399 was found to induce a decrease of the cytotoxic potential of the culture supernatant from the first stage of the colonic model system. The effects of K. marxianus B0399 on adhesion, immune function, and colonic microbiota demonstrate that this strain possesses a number of beneficial and strain-specific properties desirable for a microorganism considered for application as a probiotic.

Evidence strength: This was an in vitro/ex vivo model study, not a randomized clinical trial. Results are hypothesis-generating and cannot be directly extrapolated to in vivo human outcomes.

Irritable Bowel Syndrome (IBS)

Strain B0399 has been reported to be effective in the recovery of digestive symptoms, with a potential in some other diseases such as intestinal bowel diseases, halitosis, lactose intolerance, and antibiotic side effects. In patients with IBS, administration of a fermented milk containing Kluyveromyces marxianus fragilis B0399 and other probiotic species improved symptoms. The specific IBS trial cited in this context has not been published as a standalone randomized controlled trial in publicly indexed databases, and should be considered preliminary.

Halitosis

In subjects with halitosis, the administration of Kluyveromyces marxianus B0399 capsules for two weeks allowed the elimination of halitosis in 91% of patients. The mechanism of action was reported as restoration of the gut microbiota, rather than acting at the oral level. This finding has been cited in one open-label clinical paper and requires confirmation through placebo-controlled study designs.

5.2 Immune Modulation

Cytokine and Dendritic Cell Responses (In Vitro)

The aim of a PLOS ONE study was to characterize the immune modulating properties of the food-related yeast Kluyveromyces marxianus in terms of adaptive immune responses indicating inflammation versus tolerance. Benchmarking against a Saccharomyces boulardii strain with probiotic effects documented in clinical trials, researchers evaluated the ability of K. marxianus to modulate human dendritic cell (DC) function in vitro. The study also assessed yeast-induced DC modulation of naive T cells toward effector responses dominated by secretion of IFNγ and IL-17 versus induction of a T-reg response characterized by robust IL-10 secretion.

K. marxianus B0399 was demonstrated to be highly adhesive to human enterocyte-like Caco-2 cells and modulated the immune response, inducing proinflammatory cytokines in peripheral blood mononuclear cells (PBMCs). Interactions between members of the intestinal microbiota and the mucosal immune system can significantly impact human health, and fungi and food-related yeasts are known to influence intestinal inflammation through direct interactions with specialized immune cells in vivo.

Evidence strength: All immune modulation evidence is in vitro or from animal models. No published randomized controlled trials in humans have specifically evaluated immune endpoints for K. marxianus alone.

Animal Study: Broiler Immune Responses

To investigate the effects of Kluyveromyces marxianus on immune responses, intestinal structure, and microbiota in broilers, 840 one-day-old broiler chicks were randomly divided into seven groups and fed basal diets without or with 0.25, 0.50, 1.0, 1.5, 2.0, and 2.5 g/kg of K. marxianus (2.0×10¹⁰ CFU/g). Serum and intestine samples were collected at 21 days of age. The results showed that increasing K. marxianus addition linearly reduced feed conversion ratio but linearly elevated relative thymus weight, as well as quadratically increased serum lysozyme and IgG levels, with the medium dose (1.0 g/kg) being the most effective.

Evidence strength: Animal (avian) study. Results cannot be directly applied to human supplementation.

5.3 Anti-Inflammatory Activity

Inflammatory Bowel Disease Models (Animal and In Vitro)

The anti-inflammatory capacity of Kluyveromyces marxianus CIDCA 8154 was tested in multiple experimental models. In vitro, pretreatment of epithelial cells with the yeast reduced the levels of intracellular reactive oxygen species. Furthermore, in a murine model of trinitrobenzene sulfonic acid (TNBS)-induced colitis, yeast-treated animals showed a reduced histopathological score (P<0.05) and lower levels of circulating interleukin-6 (P<0.05). The capacity to modulate oxidative stress in vivo was also assessed using a Caenorhabditis elegans model.

An investigation on the anti-inflammatory effects of K. marxianus CIDCA 8154 in IBD concluded that pretreatment of cells with K. marxianus might decrease the levels of intracellular reactive oxygen species and IL-6.

Mannan-Mediated Anti-Inflammatory Effects

α-d-mannan isolated from the K. marxianus cell wall produced antinociceptive effects in models of inflammatory pain (formalin and complete Freund's adjuvant tests). α-d-mannan also reduced paw edema and interleukin-6 (IL-6) production after carrageenan-induced inflammation. These findings were from rodent models; no human trials have evaluated this cell-wall fraction for inflammatory pain endpoints.

Antioxidant Activity

Extracts from K. marxianus S-02-5 showed antioxidative effects, which were particularly important after anaerobic incubation. K. marxianus S-02-5 displayed a high level of activity against reactive oxygen species. The antioxidative potential evidenced for K. marxianus S-02-5 is considered another advantage that could justify the utilization of this strain as a probiotic for countering intestinal inflammatory processes.

Evidence strength: All IBD-related and antioxidant evidence is from animal models, invertebrate models (C. elegans), and in vitro cell culture. No published randomized clinical trials have specifically investigated K. marxianus mono-supplementation for inflammatory bowel disease in humans.

5.4 Alcoholic Liver Disease (Preclinical)

The intervention effect of the potential probiotic Kluyveromyces marxianus YG-4, isolated from Tibetan kefir grains, was evaluated in alcoholic liver disease (ALD). Eight-week-old male C57BL/6J mice were fed a Lieber-DeCarli diet containing ethanol with a progressively increasing concentration from 1% to 4% (vol/vol) to establish an ALD mouse model. The results suggested that K. marxianus treatment improved ALD, as demonstrated by the reduction of serum ALT and AST levels and the suppression of TLR4/NF-κB-mediated inflammatory response in the liver. K. marxianus administration significantly elevated antioxidant activities of SOD, CAT, and GSH-Px, and reduced the MDA level in mice. K. marxianus supplementation repaired the gut barrier by increasing tight junction proteins and the number of goblet cells in the colon of ALD mice.

Evidence strength: Preclinical mouse study only. No human clinical data exist for this indication.

5.5 Gut Microbiota Modulation

In broiler studies, the increase in Firmicutes and the reduction of Cyanobacteria in the ileum was related to the improved feed efficiency and intestinal structure after K. marxianus addition. At the order level, increases in the abundance of Clostridiales and Lactobacillales (Lactobacillus genus in particular) were observed after K. marxianus addition.

In the human colonic model system, K. marxianus B0399 impacted the colonic microbiota, increasing the bifidobacterial concentration in the stages of the colonic model system simulating the proximal and transverse colon. These findings are from in vitro simulation and animal studies; direct human microbiome modulation by K. marxianus as an isolated supplement has not been demonstrated in randomized controlled human trials.

5.6 COVID-19 Digestive Symptoms (Human Clinical Trial)

A prospective, open-label, case-control intervention study evaluated the impact of the oral intake of probiotic yeast Kluyveromyces marxianus B0399 together with Lactobacillus rhamnosus CECT 30579, administered for 30 days, on the evolution of COVID-19 patients. Analysis of digestive symptoms at the end of the follow-up shows a benefit in the number of patients without pyrosis (100% vs 33.3%; p 0.05) and without abdominal pain (100% vs 62.5%; p 0.04). Results also show a better evolution when evaluating the difference in the overall number of patients without non-digestive symptoms at the end of the follow-up (41.7% vs 13%; p 0.06).

Evidence strength: This was an open-label, non-blinded study using a combination product (K. marxianus B0399 plus L. rhamnosus CECT 30579), not K. marxianus alone. The open-label design and absence of blinding limit interpretation. Effects attributable specifically to K. marxianus cannot be separated from those of the co-administered bacterial strain.

5.7 Inflammatory Bowel Disease (Ongoing Clinical Research)

K. marxianus is described in clinical trial literature as a probiotic yeast commonly found in kefir whose probiotic potential has been investigated in preclinical models. Studies have shown that K. marxianus survives gastrointestinal transit efficiently, interacts with the host immune system, reduces pro-inflammatory signaling, and supports a more balanced intestinal microbiota.

A clinical study (registered at ClinicalTrials.gov) was designed to investigate whether the combination of butyrate and probiotic yeast may represent a natural adjunctive strategy to standard pharmacological therapy for ulcerative colitis. The study population includes 40 patients with ulcerative colitis in clinical remission on stable background therapy. The study duration is 8 weeks, with two sachets of the combined supplement (sodium butyrate plus K. marxianus) taken daily before meals. This study was in the enrollment stage as of 2025–2026; published results are not yet available.

5.8 Lactose Metabolism

K. marxianus and K. lactis are the only lactose-fermenting species regularly found in milk and dairy products. Strain B0399 has been reported to have potential benefits in lactose intolerance. The mechanistic basis lies in the constitutive production of β-galactosidase (lactase), which can hydrolyze lactose. No standalone randomized controlled trial specifically evaluating K. marxianus supplementation for lactose intolerance has been identified in indexed databases.

5.9 Antihypertensive Effects

In vitro studies have identified antihypertensive effects for K. marxianus strain Z17. Specifically, fermented milk products involving K. marxianus have been investigated for angiotensin-converting enzyme (ACE) inhibitory activities, but human clinical evidence in this area is lacking.

5.10 Kefir Clinical Trial Evidence (Contextual)

Because K. marxianus is the dominant yeast constituent of kefir, human clinical trials on kefir consumption are contextually relevant, though they evaluate the whole fermented product rather than K. marxianus in isolation. The studies included diverse populations, ranging from healthy individuals to patients with metabolic syndrome, non-alcoholic fatty liver disease (NAFLD), inflammatory bowel disease (IBD), osteoporosis, chronic functional constipation, and athletes, and focused on the influence of kefir on gastrointestinal health, metabolic health, cardiovascular health, oral health, and lactose intolerance. Overall, kefir consumption was associated with various beneficial effects; however, the observed outcomes were dose-dependent, duration-dependent, and the population profile was critical in determining clinical efficacy. These results cannot be attributed specifically to K. marxianus alone.

6. Body Systems and Health Areas

• Gastrointestinal system: Gut microbiota modulation, intestinal barrier integrity, short-chain fatty acid production, lactose hydrolysis, IBS symptom relief, adhesion to intestinal mucosa.
• Immune system: Modulation of dendritic cell responses, cytokine balance (IL-6, IL-10, IFNγ, IL-17), T-regulatory cell induction, immunoglobulin (IgG) stimulation.
• Hepatic system: Preclinical evidence for reduction of alcohol-induced liver injury via antioxidant and anti-inflammatory pathways.
• Antioxidant/oxidative stress: Reduction of reactive oxygen species; elevation of SOD, CAT, and GSH-Px antioxidant enzyme activities in animal models.
• Cardiovascular system: In vitro ACE-inhibitory activity in fermented milk preparations (preliminary).
• Nutritional status: As a single-cell protein source providing essential amino acids, B-vitamins, and minerals.

7. Dosage Forms and Reported Dosages

The following dosages are reported in the cited scientific literature and pertain to specific strains tested in those studies. They do not constitute recommendations.

• In vitro colonic model (human gut simulation): 10⁷ CFU/day of K. marxianus B0399 administered in a 3-stage continuous-culture system simulating the human colon.
• Broiler (animal) feed supplementation: 0.25, 0.50, 1.0, 1.5, 2.0, and 2.5 g/kg of K. marxianus (at a concentration of 2.0×10¹⁰ CFU/g) in basal diet.
• Human clinical (COVID-19 symptom study): Oral intake of Kluyveromyces marxianus B0399 combined with Lactobacillus rhamnosus CECT 30579 administered for 30 days. The exact CFU dose per administration was not separately specified in the available abstract.
• Registered clinical trial (ulcerative colitis): Two sachets daily of a combination supplement (sodium butyrate plus K. marxianus) taken before meals for 8 weeks.
• Broiler dose summary: K. marxianus supplementation at high dose (2.5 g/kg) was more effective for feed efficiency and intestinal health of broilers, while innate immunity was optimized at a medium dose (1.0 g/kg).

No standardized human supplemental dose has been established, as robust dose-finding clinical trials in humans are lacking.

8. Safety Considerations

General Safety Profile and Regulatory Status

The long history of safe use of kefir and related products makes K. marxianus a generally recognized as safe (GRAS) yeast strain. K. marxianus has GRAS and QPS status due to its long history of isolation from dairy products.

Opportunistic Infection Risk in Immunocompromised Individuals

K. marxianus is not usually an agent of human disease, although infection in humans can occur in immunocompromised individuals. This species has been associated with candidemia and has been recovered from catheters. It has also been found in biofilms on indwelling devices such as pacemakers and prosthetic heart valves. Between 1–3% of cases involving K. marxianus have been reported in oncology patients, surgical wards, female genital infections, and upper respiratory infections.

Five retrospective studies of clinical isolate collections, often from specific hospitals, reported opportunistic infections in immunocompromised patients, or nosocomial infections in hospitalized patients with underlying disease. The numbers of isolates confirmed as K. marxianus were low, between one and three, corresponding to between <1% and 8% of the total number of fungal isolates obtained.

The literature update mentioned only the isolation of K. marxianus from patients who are immunocompromised and/or have underlying disease. Methodological problems concerning identification and source attribution were also noted.

New studies confirm that, in rare cases, K. marxianus can cause opportunistic or superficial infections. The anamorph name of K. marxianus is Candida kefyr. Several studies reported opportunistic infections with K. marxianus in humans with various predisposing conditions, but could not be appropriately evaluated due to uncertainties regarding methodology for species identification.

Antifungal Susceptibility

Treatment with amphotericin B has been effective against K. marxianus in case reports. All isolates of K. marxianus from a clinical study were susceptible to all three tested antimycotics: amphotericin B, fluconazole, and voriconazole.

EFSA QPS Status Maintained

Successive EFSA QPS review articles have found that reviewed articles did not identify any information that would change the QPS status of K. marxianus. Papers submitted in successive review cycles did not identify any information that would change the QPS status of K. marxianus.

Strain-Specificity and Identification Caveats

Multiple EFSA review cycles have noted that identification of clinical isolates attributed to K. marxianus (or its anamorph Candida kefyr) is sometimes uncertain due to methodological limitations in routine clinical mycology, and that some reported safety signals may reflect misidentification. Three publications in one review cycle contributed information related to human safety concerns, and all three presented identification problems. This underscores the need for molecular confirmation when evaluating safety data attributed to this species.

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