Candida albicans is without question the most significant fungal pathogen encountered by humans.1 From birth, C. albicans is a normal commensal microbe colonizing the skin and mucosal surfaces of the mouth, genitals, and intestines in 30% to 70% of healthy people at any given time.2 In all likelihood, everyone is colonized with C. albicans at some point in life. Most of the time, Candida colonization is contained and tolerated; no disease ensues. Critical factors known to modulate Candida colonization are host immune system responses and the competitive functions of the normal microbiota inhabiting the skin, mucosal, and intestinal surfaces. Disruption of normal immune responses and/or composition of the microbiota may lead to C. albicans fungal disease that can be localized, invasive, or systemic. Familiar examples of localized Candida infections are oral thrush and vulvovaginal candidiasis, while candidal esophagitis and disseminated Candida fungemia represent invasive and systemic manifestations. Less appreciated is the ability of C. albicans to cause immune-mediated disease.3 Candida has long been associated with allergies, dermatitis, and asthma; however current evidence suggests that immune responses to Candida antigens may trigger celiac and Crohn's disease in susceptible individuals.4,5 A more controversial association is that between C. albicans and polysomatic symptomatology sometimes termed Candida hypersensitivity syndrome; chronic candidiasis sensitivity; chronic Candida; or, popularly the "yeast connection."6
Chronic Candidiasis Sensitivity
Primary care practitioners are very familiar with patients who complain of polysomatic symptoms for which no etiology can be firmly established.7,8 These symptoms usually include fatigue; lethargy; cognitive and memory impairment; weakness; muscle, joint, and abdominal pain; diarrhea; constipation; gas; skin rashes; allergies; vaginitis; dysuria; and urinary frequency. There is considerable overlap of these symptoms among patients suffering from chronic fatigue syndrome, fibromyalgia, irritable bowel syndrome, environmental exposure syndrome, and sensory hypersensitivity and central sensitization syndromes. In 1977, Truss, a Birmingham internist, presented his experiences over 16 years of treating patients with chronic polysomatic complaints.10 He reported good responses, sometimes dramatic, to nystatin and desensitization with Candida extract. He hypothesized that the cause of polysomatic symptoms in many people may be related to an inappropriate immunological response to Candida albicans antigens.
In the early 1980s, Crook, a Jackson, Tennessee, pediatrician with an interest in allergies, contacted Truss after a mutual patient responded to a low–sugar/nystatin regimen.11 Crook began treating patients with polysomatic complaints with diet and antifungals and saw success. In 1983, he published the Yeast Connection, thereby popularizing the putative link between Candida and chronic, polysomatic symptomatic disease. Academic medicine was quick to deny any association between Candida or yeast colonization in general and chronic polysomatic symptoms.12 However, critics uniformly failed to adequately explain the observations that dietary restrictions, antifungals, and Candida extract desensitization were of benefit in a subgroup of people with chronic symptoms involving multiple organ systems. Critics also ignored historical evidence in the medical literature that showed dietary, antifungal, and other interventions could improve polysomatic symptoms in similar populations.
Gut Fermentation Syndrome
In 1931, Hurst and Knott proposed the term intestinal carbohydrate dyspepsia to describe patients suffering from gastrointestinal and systemic symptoms with no established etiology.13 They recommended carbohydrate restriction, betaine HCL, pancreatic enzymes, Lactobacillus acidophilus, and vitamin supplements. What would be now diagnosed as small intestine bacterial overgrowth was proposed by some as the mechanism.14 This condition was accepted for decades in the gastroenterology literature, but by 1976 such patients were called "essentially unhappy individuals" and the disorders were considered, as they are by many to this day, psychosomatic.15,16 In 1990, Eaton, a British allergist, revived the gut fermentation concept.7 He emphasized a clinical syndrome recognizable by a favorable response to a diet low in fermentable and yeast- or mold-containing foods, with or without antifungal medications. He also proposed the utility of measuring an increase in blood ethanol levels following the administration of a glucose load as a diagnostic test for gut fermentation syndrome. In time, Eaton renamed this chronic polysomatic symptomatic condition fungal-type dysbiosis.17 He steadfastly denied a specific etiologic role for Candida albicans, noting that the syndrome was clinically indistinguishable from type B food allergy, Candida overgrowth is uncommonly documented, Candida skin test and serum antibody abnormalities have not been shown to be causal, and clinical responses to antifungals do not necessarily specify an etiological role for Candida albicans.18 Indeed, a major obstacle to the consideration of a candidiasis sensitivity syndrome or fungal-type dysbiosis has been that while patients and many practitioners alike refer to the syndrome as "yeast overgrowth," high numbers of intestinal Candida and other fungi are rarely documented.19 Nevertheless, Candida species are the most common human commensal fungal microorganisms, and any clinical syndrome that appears to respond to antifungals must necessarily be evaluated in the light of a possible etiologic role for Candida species in general and Candida albicans in particular.
Candida species are diploid, polymorphic yeasts belonging to the kingdom Fungi, distinguished from plants by the presence of chitin in their cell walls.20 Replication is usually asexual and occurs by budding. There are nearly 200 species of Candida, but most Candida species cannot replicate at 37 °C, so fewer than a dozen are important for humans. These include C. albicans, C. glabrata, C. krusei, C. tropicalis, C. parapsilosis, C. inconspicua, C. dubliniensis, and C. guilliermondii. Candida species alternate between budding yeast cells known as blastospores and filamentous hyphae and pseudohyphae. Most fungi grow as hyphae outside the human body in a saprophytic environment and as blastospores within the body. Candida albicans has no extracorporeal life cycle and grows as both blastospores and hypha within the body. The ability to transform from blastospore to hypha and back, known as morphogenesis, is closely linked with virulence.1 Transition from blastospore to hypha is important to both adhesion to cellular and abiotic surfaces as well as to tissue invasion. Filamentous morphogenesis is critical to Candida biofilm formation.21 It also facilitates evasion of host immune responses by modulating immune detection and response. It is becoming clear that Candida can produce subtly diverse hyphae in terms of genetic expression and cell wall composition depending on the microecological niche within the host, thereby eliciting variable immune responses.
Candida Cell Wall Structure and Immunogenicity
Between 80% and 90% of the C. albicans cell wall is composed of carbohydrates.19 There are outer fibrils made up of mannoproteins, underlying which is a dense layer of O- and N-linked mannans covalently bonded to proteins. This is followed by a thin layer of ß-1,6-glucan, under which is a thicker layer of chitin and ß-1,3-glucan. Mannoproteins cover and traverse the cell membrane. C. albicans is highly immunogenic and contains up to 178 antigens, with mannoproteins being the most reactive.22 Candida mannoproteins have been shown to stimulate histamine release and prostaglandin E2 release.23,24 Among the most interesting Candida antigens is hyphal wall protein 1 (Hwp1). This cell wall protein is an important adhesin molecule highly expressed in hyphae and pseudohyphae and plays a pivotal role in both C. albicans colonization and disease.25 Hwp1, specifically amino acids 40–197, is a substrate for transglutaminase,and Candida adherence to intestinal epithelial cells is dependent on transglutaminase. These same Hwp1 amino acid sequences are homologous, in instances identical, to known celiac disease-related epitopes in a- and g-gliadin.4 Patients with mucocutaneous candidiasis, a genetic disorder of chronic T cell dysfunction, have been shown to develop elevated antigliadin antibodies in response to Hwp1, and Candida colonization is hypothesized to be a trigger for celiac disease, which has inexplicably increased in prevalence over the past decades in tandem with increased antibiotic use and Candida infections.4,26 Other documented immunologic findings in chronic or recurrent candidiasis are a decrease in natural killer (NK) cell function and cell-mediated immunity, a decline in interleukin (IL)-1, -2, and interferon-g production, and an increase in IL-6, -8, -17, and IgE production.27,28
Candida Gastrointestinal and Systemic Effects
Normal fungal populations, primarily Candida species, range from 0 to 102 CFU/mL of intestinal contents in the stomach and jejunum to 102 to 103 and 102 to 106 CFU/mL in the ileum and colon respectively.19 Increased rates of Candida colonization have been observed in both patients suffering from inflammatory bowel disease (IBD) and their family members.28 Patients with Crohn's disease (CD) have a high prevalence of anti-Saccharomyces cerevisiae antibodies (ASCA), which are cross-reacting antibodies generated in response to C. albicans colonization.5 An altered immune sensing of Candida gastrointestinal colonization is hypothesized to be associated with the onset of some instances of CD. A high percentage of people with ulcerative colitis (UC) are colonized (105 CFU/g) with C. albicans (91%), C. glabrata (7%), or C. inconspicua (2%).29
The addition of fluconazole or probiotics to conventional mesalamine and azathioprine therapy has been shown to improve remission rates in patients with UC.29 Animal models show that Candida colonization facilitates the induction of experimental UC by 2,4,6-trinitrobenzenesulfonic acid (TNBS).29 The presence of C. albicans delays healing in TNBS-induced colitis, and this can be reversed by fluconazole. High rates of Candida colonization havealso been described in peptic ulcer disease (PUD) and appear to impede healing.29 Human studies have shown that probiotics promote clearance of Candida and reduce inflammation in the setting of both PUD and IBD.30 Animal research reveals that intestinal Candida colonization intensifies collagen-induced arthritis, a model for rheumatoid arthritis, suggesting a permissive and aggravating role for Candida in autoimmune diseases.31 Intestinal Candida colonization has also been shown in animal models to promote sensitization to food antigens by increasing gut mucosal permeability.32 In humans, intestinal Candida colonization has been linked to allergic asthma, eczema, and hives.4 Neuropsychiatric symptoms are often prominent in patients diagnosed with fungus-related diseases. One possible mechanism deserving of investigation is the cross-reactivity of Hwp1 and a- and g-gliadin. Neuropsychiatric symptoms are very common in celiac disease and may sometimes be present in the absence of gastrointestinal symptoms.33 These symptoms are thought to be immunologically mediated, and among the most common are depression, personality changes, and attention deficit. The evidence for plausible mechanisms of action whereby Candida may cause polysomatic symptomatic disease is compelling.
Studies of Diet and Antifungals for Candidiasis Syndromes
An expanding body of human and animal research clearly implicates gastrointestinal, oral, and genital Candida colonization with localized and systemic inflammation, allergies, and autoimmune processes. Nevertheless, the concept of candidiasis sensitivity of fungus-related disease continues to be criticized in part because of the alleged lack of human studies on the efficacy of diet and antifungal interventions.6 However, a body of clinical evidence suggests that dietary interventions and antifungal therapy may result in symptomatic improvement in patients with multisystem symptomatic complaints. Many clinicians have long noted that a subset of patients who meet the diagnostic criteria for chronic fatigue syndrome will respond to a diet that restricts sugars, starches, and yeast-related foods along with antifungals. In 1989, Jessup presented her experience treating 1100 such patients at the University of California San Francisco.34 Prominent symptoms included fatigue, myalgia, headache, depression, dizziness, arthralgias, night sweats, morning stiffness, and postexercise malaise. The neurological examination was abnormal in 30% of patients. Approximately 80% of patients had repeated antibiotic courses for a variety of indications. Of the 1100 patients, 685 were on disability. Jessup prescribed ketaconazole 200 mg a day along with a diet free of alcohol, added sugar, fruit, and fruit juice. The average length of treatment was 5 months (range: 3–12 months). In follow-up, 84% of patients recovered and only 12 of 685 patients remained on disability. Jessup concluded that Candida infection with production of an unknown systemic toxin was the cause of disease. Unfortunately, the study was retrospective and never published, and the interventions were unblinded and uncontrolled.
In 1990, Dismukes and coworkers published a study of nystatin for candidiasis hypersensitivity syndrome. The trial involved 42 women with a history of Candida vaginitis and unexplained polysomatic symptoms. Subjects were randomized to receive oral and vaginal nystatin, oral nystatin alone, vaginal nystatin alone, or placebo. Every 8 weeks, participants were crossed over to a different study group so that all subjects received each regimen. Follow-up included clinical assessments, completion of a questionaire assessing presence and severity of 18 different symptoms, and the SCL-90-R multidimensional symptom inventory, which focuses on psychiatric complaints. The investigators found that while, unsurprisingly, nystatin improved symptoms related to vaginitis, it was no better than placebo at relieving polysomatic complaints. At the time, the Dismukes study, published in the prestigious New England Journal of Medicine, was widely touted as disproving the concept of candidiasis sensitivity. However, the study had significant limitations. The number of subjects was extremely small, and dividing participants in 4 intervention groups served to render the trial so vastly underpowered as to make any conclusions meaningless. Further invalidating the findings, the study was poorly designed, as crossover in antimicrobial studies is widely regarded as inappropriate unless subjects can return to their baseline status.36
In 1998, Eaton and Howard published a small, retrospective study of 25 patients diagnosed with fungus-type gut dysbiosis on the basis of an increase in blood ethanol levels following an oral glucose load. All subjects were instructed in a diet that restricted starch and excluded yeast and mold-containing foods. Patients who had not improved after 1 month received either oral nystatin or amphotericin B. Interventions were continued for a minimum of 3 months, but usually 6 months. The investigators found that 21 of 25 patients improved on diet alone and the remainder experienced resolution of symptomatic with antifungal agents. Small numbers, lack of a control group, the absence of blinding, and lack of objective assessment at baseline and in follow-up compromise the findings.
To date, the best study of diet and antifungal interventions for candidiasis sensitivity or fungus-related disease comes from Santelmann and coworkers in Norway.8 These investigators designed a prospective, double-blind, randomized, placebo-controlled trial. The investigators developed a 7-item questionnaire termed the Fungus-Related Disease Questionnaire-7 (FRDQ-7) derived from a statistical discrimination analysis of the Candida-Related Complex (CRC) scheme found in Crook's The Yeast Connection and the Woman. A high score (≥10) was used to qualify subjects for entry into the study. Patients were randomized to oral nystatin or placebo for 4 weeks. They were also allowed to adhere to their usual diet or to avoid foods containing sugars, yeasts, or molds. Participants were assessed with a 70-item CRC questionnaire assessing the presence and severity of polysomatic symptoms. Nystatin was found to be significantly better (P = 0.003) than placebo in improving symptoms. Diet plus nystatin was significantly better than nystatin alone, and diet alone significantly improved polysomatic symptoms better than placebo. Remarkably, this well-conducted study has received scant attention, and over the past decade no additional clinical trials have been conducted to confirm or refute its findings.
A consistent observation by both patients and practitioners is that while select individuals with polysomatic symptoms may respond to diet and antifungal interventions, the response is usually limited in time and relapse is the rule. One possible explanation is that C. albicans, other Candida species, and fungi reside within biofilm.21,38 A biofilm is a gathering of sessile microorganisms encased by a self-generated hydrated exopolysaccharide matrix, firmly attached to a surface.39,40 Biofilm formation is an integral part of Candida colonization on mucosal surfaces and skin. Transition from blastospores to hyphae is an integral part of biofilm formation. As Candida enters into biofilm mode, genes coding for antifungal resistance and drug efflux pumps are expressed. Additional mechanisms of antifungal drug resistance manifested by Candida organisms residing within biofilm are reduced diffusion of antifungals into the biofilm, binding of antifungal drugs by biofilm matrix carbohydrates such as b-glucan, reduced metabolic rates, and the appearance of highly drug-resistant persister cells.38,41,42 The biofilm properties of Candida make it highly resistant to eradication. Within biofilm, Candida remains highly immunogenic, stimulating host defensive immune responses as well as generating allergic and autoimmune reactions. Immunogenic, colonizing Candida biofilm communities offer an explanation for why fungal overgrowth is rarely documented in polysomatic symptomatic patients and for high relapse rates following treatment. Therapeutic modalities aimed at disrupting Candida morphogenesis and biofilm formation hold great promise for improving treatment responses for polysymptomatic people with candidiasis sensitivity and fungus-related disease.
Novel Approaches to Candida Biofilm
A nonpharmaceutical approach to Candida biofilm eradication involves the use of enzymes, chelating agents, and probiotics available as nutritional supplements together with naturally occurring antimicrobial agents.
Enzymes that disrupt the polysaccharide components of biofilm are effective antibiofilm agents. Cellulose is a major component of most biofilms.43 Cellulase is a group of glycoside hydrolase enzymes that hydrolyze the b-(1®4) glycosidic bonds in cellulose. Cellulase has been shown to significantly reduce biofilm formation.44 b-(1®3)- and b-(1®6)-glucans compose up to 60% of the cell wall of fungi such as C. albicans and are the major component of candidal biofilms. Chitin is also a major component of Candida cell walls. Combining beta-gluconase and chitinase with cellulase makes an enzyme formulation with powerful anticandidal biofilm activity.45
Metal Chelating Agents
Metals such as calcium, magnesium, and iron are critical to biofilm formation and maintenance.46 The disodium salt of ethylene diamine tetraacetic acid (EDTA) is a powerful chelator of bi- and trivalent cations.47 Disodium EDTA has well-established antibiofilm activity mediated by complexing with metals required for cross-linking biofilm matrices. Disodium EDTA also causes structural damage to bacterial cell walls, making them more permeable to antimicrobial agents.48 EDTA has been shown to inhibit filamentation and biofilm formation by C. albicans. The combination of disodium EDTA with enzymes synergistically disrupts fungal biofilms, potentially facilitating their eradication.
S. boulardii is a probiotic yeast related to S. cervesiae.49 It has been used worldwide for 60 years for a variety of indications such as IBD, prevention of antibiotic-associated diarrhea, treatment of Clostridium difficile-associated disease, and treatment of Blastocystis hominis and other unicellular parasite infections. S. boulardii has been widely used as an anticandidal agent, but its mechanisms of action have been unclear. Recently, S. boulardii has been found to significantly inhibit Candida adhesion, hyphae growth, and biofilm formation, an effect mediated by capric acid secretion.50 S. boulardii's ability to disrupt critical aspects necessary for Candida colonization and virulence makes it an important addition to the therapeutic tool kit for treating people with fungus-related diseases.
Lactobacillus probiotics have been shown in human studies to promote clearance of Candida associated with gastrointestinal inflammatory lesions.30 In combination with standard therapies, they have been shown to reduce inflammation and accelerate healing in the setting of PUD and IBD.
Optimizing Treatment of Candidiasis Sensitivity Syndrome
While candidiasis sensitivity syndrome, or for those wishing a broader term, fungus-related disease, is likely to remain controversial, there can be no question for patients and practitioners involved in their care that a significant number of people with polysomatic symptoms with no other apparent etiology will respond to a diet that restricts starch and yeast-derived food and beverages and the administration of antifungal agents. Unfortunately, definite diagnostic tests are unavailable, and stool cultures rarely reveal fungal overgrowth. A high score on the FRDQ-7 remains the best predictor of who will respond to these interventions. A common error is a short duration of treatment. Experienced clinicians recommend a minimum of 3 months. An immune response in susceptible individuals to colonizing biofilm communities of Candida represents a likely mechanism of disease. A therapeutic approach that incorporates enzymes, chelating agents, and probiotics to disrupt fungal biofilm is likely to make antifungal eradication more effective. For optimal results, antibiofilm agents should be consumed together with antifungals. The use of S. boulardii as an anticandidal agent is to be encouraged and it should be consumed away from antifungals. Probiotics are valuable both to promote Candida clearance and to support a healthy, balanced gut microbiota. Prebiotics are underappreciated as a valuable means to promote healthful gastrointestinal microbial populations.
C. albicans is the most important human fungal commensal and pathogen. The relation of Candida to polysomatic symptomatic syndromes has been long hypothesized, but remains highly controversial. An emerging body of research involving animal studies and human trials is revealing an association between Candida species and gastrointestinal and systemic diseases. Candida species are highly immunogenic microorganisms and are associated with allergic and autoimmune responses. Colonization and not overgrowth is all that is required for a dysfunctional immune response in susceptible individuals. Candida colonization occurs within mucosal and skin biofilms, where it is highly resistant to eradication. Enzymes, chelating agents, and probiotics including S. boulardii represent novel therapeutic modalities that may render dietary and antifungal treatment of people with candidiasis sensitivity more successful.
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Stephen Olmstead, MD, is chief science officer at ProThera Inc., where he directs clinical trials of ProThera and Klaire Labs nutraceutical products. His current research focus is the use of enzymes and chelating agents to disrupt pathogenic GI biofilm. Dr. Olmstead graduated from the University of New Mexico with distinction in biology and chemistry. He attended the University of New Mexico School of Medicine, and trained at Harvard Medical School, Massachusetts General Hospital. He is board certified in both internal medicine and cardiovascular diseases.