Several species of the yeast genus Candida are capable of causing candidiasis. They are members of the normal flora of the skin, mucous membranes, and gastrointestinal tract. Candida species colonize the mucosal surfaces of all humans soon after birth, and the risk of endogenous infection is ever-present. Candidiasis is the most prevalent systemic mycosis, and the most common agents are Candida albicans, Candida parapsilosis, Candida glabrata, Candida tropicalis, Candida guilliermondii, and Candida dubliniensis. The widespread use of fluconazole has precipitated the emergence of more azole-resistant species, such as C glabrata, Candida krusei, and Candida lusitaniae. As indicated in Table 45-1, species of Candida cause both cutaneous and systemic infections, and these clinical manifestations have different mechanisms of pathogenesis. In addition, there are several other clinical forms of candidiasis.
Morphology and Identification
In culture or tissue, Candida species grow as oval, budding yeast cells (3–6 μm in size). They also form pseudohyphae when the buds continue to grow but fail to detach, producing chains of elongated cells that are pinched or constricted at the septations between cells. Unlike other species of Candida, C albicans is dimorphic; in addition to yeasts and pseudohyphae, it can also produce true hyphae (Figure 45-22). On agar media or within 24 hours at 37°C or room temperature, Candida species produce soft, cream-colored colonies with a yeasty odor. Pseudohyphae are apparent as submerged growth below the agar surface. Two simple morphologic tests distinguish C albicans, the most common pathogen, from other species of Candida: After incubation in serum for about 90 minutes at 37°C, yeast cells of C albicans will begin to form true hyphae or germ tubes (Figure 45-23), and on nutritionally deficient media C albicans produces large, spherical chlamydospores. Sugar fermentation and assimilation tests can be used to confirm the identification and speciate the more common Candida isolates, such as C tropicalis, C parapsilosis, C guilliermondii, Candida kefyr, C krusei, and C lusitaniae. C glabrata is unique among these pathogens because on routine culture media it produces only yeast cells and no pseudohyphae.
Candida albicans. Budding yeast cells (blastoconidia), hyphae, and pseudohyphae. 400×.
Germ tube. Unlike other species of Candida, Candida albicans produces true hyphae as well as budding yeast cells and pseudohyphae. After incubation in serum at 37°C for 60–90 minutes in the laboratory, clinical isolates of C albicans are stimulated to form hyphae, and this process is initiated by the production of germ tubes, which are thinner and more uniform than pseudohyphae (see Figure 45-22). 1000×.
The use of adsorbed antisera have defined two serotypes of C albicans: A (which includes C tropicalis) and B. During infection, cell wall components—such as mannans, glucans, other polysaccharides and glycoproteins, as well as enzymes—are released. These macromolecules typically elicit innate host defenses and Th1, Th17, and Th2 immune responses. For example, sera from patients with systemic candidiasis often contain detectable antibodies to candidal enolase, secretory proteases, and heat shock proteins.
Pathogenesis and Pathology
Cutaneous or mucosal candidiasis is established by an increase in the local census of Candida and damage to the skin or epithelium that permits local invasion by the yeasts and pseudohyphae. The histology of cutaneous or mucocutaneous lesions is characterized by inflammatory reactions varying from pyogenic abscesses to chronic granulomas. The lesions contain abundant budding yeast cells and pseudohyphae. The administration of broad-spectrum antibacterial antibiotics often promotes large increases in the endogenous population of Candida in the gastrointestinal tract as well as the oral and vaginal mucosa. Systemic candidiasis occurs when Candida enters the bloodstream and the innate phagocytic host defenses are inadequate to contain the growth and dissemination of the yeasts. The yeasts can enter the circulation by crossing the intestinal mucosa. Many nosocomial cases are caused by contamination of indwelling intravenous catheters with Candida. Once in the circulation, Candida can infect the kidneys, attach to prosthetic heart valves, or produce candidal infections almost anywhere (eg, arthritis, meningitis, endophthalmitis). The critical host defense against systemic candidiasis is an adequate number of functional neutrophils capable of ingesting and killing the yeast cells.
As noted above, Candida cells elaborate polysaccharides, proteins, and glycoproteins that not only stimulate host defenses but facilitate the attachment and invasion of host cells. C albicans and other Candida species produce a family of agglutinin-like sequence (ALS) surface glycoproteins, some of which are adhesins that bind host cell membrane receptors and mediate attachment to epithelial or endothelial cells. The innate host defense mechanisms include pattern recognition receptors (eg, lectins, Toll-like receptors, macrophage mannose receptor) that bind to pathogen-associated molecular patterns. A key example is the host cell lectin, dectin-1, which binds to the β-1,3-glucan of C albicans and other fungi to stimulate a robust inflammatory response. This response is characterized by the production of cytokines, especially tumor necrosis factor-α, interferon-g, and granulocyte colony-stimulating factor, which activate anti-fungal effector cells, neutrophils, and monocytes. In addition, the binding of β-glucan to dectin 1 on dendritic cells induces the Th17 lymphocytes, which secrete interleukin-17. Th17 lymphocytes differ from T and B cells. They are activated by innate, usually mucosal defense mechanisms as well as adaptive immune responses.
In addition to the family of eight ALS adhesion genes, many other virulence factors have been identified in C albicans and other species of Candida. They include 10 secreted aspartyl proteinases (SAP) that are able to degrade host cell membranes and destroy immunoglobulins. Another virulence factor is phospholipase (PLB1), which is secreted by yeasts and pseudohyphae. In addition, on a variety of biological and prosthetic surfaces, the accumulation of yeasts and pseudohyphae readily form biofilms. The fungal biofilm is protected by extracellular matrix material that resists penetration by host immune responses and antifungal drugs.
A. Cutaneous and Mucosal Candidiasis
The risk factors associated with superficial candidiasis include AIDS, pregnancy, diabetes, young or old age, birth control pills, and trauma (burns, maceration of the skin). Thrush can occur on the tongue, lips, gums, or palate. It is a patchy to confluent, whitish pseudomembranous lesion composed of epithelial cells, yeasts, and pseudohyphae, which can lead to the formation of an intractable biofilm. Thrush develops in most patients with AIDS. Other risk factors include treatment with corticosteroids or antibiotics, high levels of glucose, and cellular immunodeficiency. Yeast invasion of the vaginal mucosa leads to vulvovaginitis, characterized by irritation, pruritus, and vaginal discharge. This condition is often predisposed by conditions such as diabetes, pregnancy, or antibacterial drugs that alter the microbiota, local acidity, or secretions. Other forms of cutaneous candidiasis include invasion of the skin, which occurs when the skin is weakened by trauma, burns, or maceration. Intertriginous infection occurs in moist, warm parts of the body such as the axillae, groin, and intergluteal or inframammary folds; it is most common in obese and diabetic individuals. Before newborns establish a balanced microbiome, they are susceptible to extensive diaper rash and skin infection caused by Candida. The infected areas become red and moist and may develop vesicles. Interdigital involvement between the fingers follows repeated prolonged immersion in water; it is most common in homemakers, bartenders, cooks, and vegetable and fish handlers. Candidal invasion of the nails and around the nail plate causes onychomycosis, a painful, erythematous swelling of the nail fold resembling a pyogenic paronychia, which may eventually destroy the nail.
Candidemia can be caused by indwelling catheters, surgery, intravenous drug abuse, aspiration, or damage to the skin or gastrointestinal tract. In most patients with normal innate immune responses and circulating neutrophils, the yeasts are eliminated and candidemia is transient. However, patients with compromised innate phagocytic defenses may develop occult lesions anywhere, especially the kidney, skin (maculonodular lesions), eye, heart, and meninges. Systemic candidiasis is most often associated with chronic administration of corticosteroids or other immunosuppressive agents; with hematologic diseases such as leukemia, lymphoma, and aplastic anemia; or with chronic granulomatous disease. Candidal endocarditis is frequently preceded by the deposition and growth of the yeasts and pseudohyphae on prosthetic heart valves or vegetations and the formation of recalcitrant biofilms. Kidney infections are usually a systemic manifestation, whereas urinary tract infections are often associated with Foley catheters, diabetes, pregnancy, and antibacterial antibiotics.
C. Chronic Mucocutaneous Candidiasis
Chronic mucocutaneous candidiasis (CMC) is a rare but distinctive clinical manifestation characterized by the formation of granulomatous candidal lesions on any or all cutaneous and/or mucosal surfaces. There are several classifications of CMC based on the age of onset, endocrinopathy, genetic predisposition, and immune status. The most common forms present in early childhood and are associated with autoimmunity and hypoparathyroidism. The patients may develop chronic, raised, and crusty highly disfiguring keratitic lesions on the skin, oral mucosa, and scalp. Many patients with chronic mucocutaneous candidiasis are unable to mount an effective Th17 response to Candida.
Diagnostic Laboratory Tests
A. Specimens and Microscopic Examination
Specimens include swabs and scrapings from superficial lesions, blood, spinal fluid, tissue biopsies, urine, exudates, and material from removed intravenous catheters.
Tissue biopsies, centrifuged spinal fluid, and other specimens may be examined in Gram-stained smears or histopathologic slides for pseudohyphae and budding cells (Figure 45-24). As with dermatophytosis, skin or nail scrapings are first placed in a drop of KOH and calcofluor white.
Candidiasis. Yeasts and pseudohyphae in tissue, stained with periodic acid-Schiff. 1000×.
All specimens are cultured on fungal or bacteriologic media at room temperature or at 37°C. Yeast colonies are examined for the presence of pseudohyphae (see Figure 45-22). C albicans is identified by the production of germ tubes (see Figure 45-23) or chlamydospores. Other yeast isolates are speciated phenotypically by using any of several commercial kits to test for the metabolic assimilation of a battery of organic substrates. CHROMagar(®) is a useful commercial medium for the rapid identification of several Candida species based on fungal enzymatic action on chromogenic substrates in the medium. After incubation for 1–4 days on CHROMagar, colonies of C albicans are green, C tropicalis is blue, C glabrata is dark purple, and C parapsilosis, C lusitaniae, C guilliermondii, and C krusei acquire a pinkish hue.
The interpretation of positive cultures varies with the specimen. Any positive culture from normally sterile body sites is significant. The diagnostic value of a quantitative urine culture depends on the integrity of the specimen and the yeast census. Contaminated Foley catheters may lead to “false-positive” urine cultures. Positive blood cultures may reflect systemic candidiasis or transient candidemia due to a contaminated intravenous line. Sputum cultures have no value because Candida species are part of the oral microbiota. Cultures of skin lesions are confirmatory and distinguish cutaneous candidiasis from dermatophytosis or another infection.
In many clinical laboratories, blood cultures for Candida are augmented by real-time PCR with species-specific primers, which are commonly designed from sequences of the multicopy ribosomal DNA genes. The specificity of DNA tests for candidemia is excellent, but the sensitivity can be compromised by a low census of yeast cells in the blood sample. A crucial problem is the method used to extract the DNA from the yeast cells, as well as eliminating inhibition of the PCR by human DNA and hemoglobin. The ideal molecular test would detect candidemia early in the course of infection before the yeasts have developed chronic infection in the kidneys and other organs, when blood cultures are usually negative.
Yeast cultures, especially non–C albicans species often take several days for definitive identification. With its ease of sample preparation and automation, matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS), has become a popular rapid method of identifying species of Candida, as well as other pathogenic fungi and bacteria.
Serum antibodies and cell-mediated immunity are demonstrable in most people as a result of lifelong exposure to Candida. In systemic candidiasis, antibody titers to various candidal antigens may be elevated, but there are no clear criteria for establishing a diagnosis serologically. The detection of circulating Candida cell wall mannan, using a latex agglutination test or an enzyme immunoassay, is much more specific, but the test lacks high sensitivity because many patients are only transiently positive or because they do not develop significant and detectable antigen titers until late in the disease. Nevertheless, a positive test can be most helpful (see Table 45-6). The biochemical test for circulating β-(1,3)-D-glucan, described earlier in this chapter, has become widely used to screen for fungemia in patients at risk who often have negative blood cultures. Although the test is not specific for Candida, most patients with an invasive fungal infection have serum β-glucan levels above 80 pg/mL. Normal levels are 10–40 pg/mL.
The basis of resistance to candidiasis is complex and incompletely understood. Innate immune responses, especially circulating neutrophils, are crucial for resistance to systemic candidiasis. Several Candida polysaccharide antigens are recognized by host pattern recognition receptors (PRR), such as dectin-1, which binds β-(1,3)-glucan, and β-(1,2)-mannan, which binds Toll-like receptor (TLR)-4. As noted above, cell-mediated immune responses are important for controlling mucosal candidiasis. Stimulation of specific Th17 lymphocytes triggers a cascade of cytokines that activate macrophages, inflammation, and enhance phagocytic activity.
Thrush and other mucocutaneous forms of candidiasis are usually treated with topical nystatin or oral ketoconazole or fluconazole. The clearing of cutaneous lesions is accelerated by eliminating contributing factors such as excessive moisture or antibacterial drugs. Systemic candidiasis is treated with amphotericin B, sometimes in conjunction with oral flucytosine, fluconazole, or caspofungin. Chronic mucocutaneous candidiasis responds well to oral ketoconazole and other azoles, but patients have a genetic cellular immune defect and often require lifelong treatment.
It is often difficult to establish an early diagnosis of systemic candidiasis—the clinical signs are not definitive, and blood cultures are often negative. Furthermore, there is no established prophylactic regimen for patients at risk, though the administration of an azole or a short course of low-dose amphotericin B is often indicated for febrile or debilitated patients who are immunocompromised and do not respond to antibacterial therapy (see below).
The most important preventive measure is to avoid disturbing the normal balance of microbiota and intact host defenses. Candidiasis is not communicable, since virtually all persons normally harbor the organism. However, molecular epidemiological studies have documented outbreaks caused by the nosocomial transmission of particular strains to susceptible patients (eg, leukemics, translplants, neonates, ICU patients). Candida species are the fourth most common blood culture isolate and the attributable mortality ranges from 30% to 40%.