Candida species grow as 4 to 6 μm, budding, round or oval yeast-like cells (Figure 46–1) under most conditions and at most temperatures. Under certain growth conditions, including those encountered during infection, certain pathogenic Candida species can also form hyphae. Of the over 150 Candida species, fewer than 10 cause human infections. Particular attention is given to the differentiation of C albicans from other species, because it is by far the most common cause of disease. For serious infections, identifying Candida isolates to the species level is important for prognostic and treatment decisions (Figure 46–1).
Candida albicans. This scanning electron micrograph demonstrates dimorphism with both yeast-like blastoconidia and hyphae. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)
Formation of hyphae and chlamydoconidia are distinguishing features
Carbohydrate assimilation and fermentation help clinical labs to distinguish between Candida species
Most Candida species grow rapidly on Sabouraud’s agar and on enriched bacteriologic media such as blood agar. Smooth, white, 2 to 4 mm colonies resembling those of staphylococci are produced on blood agar after overnight incubation. Aeration of cultures favors their isolation. The primary identification procedure involves presumptive differentiation of C albicans from the other Candida species with the germ tube test. Germ tube–negative strains may be further identified biochemically or reported as “yeast not C albicans,” depending on their apparent clinical significance.
Rapidly produce colonies resembling bacteria
✺ C albicans produces germ tubes
Candida albicans is a budding yeast that grows readily in culture. Candidiasis occurs in localized and disseminated forms. Localized disease is seen as erythema and white plaques in moist skinfolds (diaper rash) or on mucosal surfaces (oral thrush). It may also cause the itching and thick white discharge of vulvovaginitis. Candida bloodstream and urinary tract infections are especially common among hospitalized patients with intravenous and urinary catheters. Deep tissue and disseminated infections are limited almost exclusively to the immunocompromised.
Candida albicans grows in multiple morphologic forms, most often as a budding yeast. Candida albicans is also able to form hyphae triggered by changes in conditions such as temperature, pH, and available nutrients. When observed in their initial stages of germination from the yeast cell, these nascent hyphae resemble sprouts and are called “germ tubes” (Figure 46–2A). Other elongated forms with restrictions at regular intervals are called pseudohyphae because they lack the parallel walls and septation of true hyphae. There is evidence that these three forms have distinct stimuli and genetic regulation, making C albicans a polymorphic fungus. Unless otherwise specified, the term hyphae is used here to encompass both the true hyphal and pseudohyphal forms. Being able to recognize these different fungal morphologies can help clinicians and laboratory personnel to rapidly distinguish C albicans from other, similar yeast species in clinical specimens.
Candida albicans. A. When incubated at 37°C, C albicans rapidly forms elongated hyphae called germ tubes. B. On specialized media, C albicans forms thick-walled chlamydoconidia, which differentiate it from other Candida species. (Reprinted with permission from Dr. E. S. Beneke and the Upjohn Company: Scope Publications, Human Mycoses.)
✺ C albicans grows with varied morphologies: yeast, hyphae, and pseudohyphae
The C albicans cell wall is made up of a mixture of the polysaccharides mannan, glucan, and chitin alone or in complexes with protein. A fibrillar outer layer extending to the surface contains several glycoproteins and complexes of mannan with protein called mannoproteins. The exact composition of the cell wall and surface components varies under different growth conditions.
Cell wall includes surface mannoproteins
Candida albicans is present in the microbiota of 30% to 50% of healthy persons, especially common in the oropharyngeal, gastrointestinal, and female genital tracts. Infections are endogenous, with most symptoms arising from infections with one’s own resident species. However, transmission and new acquisition of Candida colonization can occur by direct mucosal contact with others (eg, through sexual intercourse). Similarly, in nosocomial C albicans infections, the strains involved are usually derived from the patient’s own flora rather than from cross-infection. Invasive procedures and indwelling devices may provide the portal of entry, and the number of Candida present on the skin and mucosal surfaces may be enhanced by the use of antibacterial agents.
✺ Candida albicans infections are often from endogenous flora
Because C albicans is regularly present on mucosal surfaces; disease implies a change in the organism, the host, or both. The ability of this microorganism to change between the yeast and hyphal forms is strongly associated with its pathogenic potential, and these different morphological forms are likely required for different phases of candidiasis. In histologic preparations, hyphae are seen during Candida invasion, either superficially into the mucosa or within deep tissues. However, dissemination in the bloodstream is likely enhanced during the yeast growth phase. Therefore, it is the plasticity between the two forms, rather than one morphology or the other, that results in the ability of C albicans to so effectively colonize and infect the host. The yeast-hyphal switch can be controlled in vitro by the manipulation of a wide variety of environmental conditions (serum, pH, temperature, amino acids). Various sensors and signaling pathways for morphogenesis have been described including one in which C albicans induces its own morphological change by directly altering the local pH.
Shift from yeast to hyphae is associated with invasion
Switch is triggered by environmental conditions
Candida albicans hyphae have the capacity to form strong attachments to human epithelial cells. One mediator of this binding is a surface hyphal wall protein (Hwp1), which is found only on the surface of germ tubes and hyphae. Other surface mannoproteins that have similarities to vertebrate integrins may also mediate binding to components of the extracellular matrix (ECM), such as fibronectin, collagen, and laminin. Hyphae also secrete proteinases and phospholipases that are able to digest epithelial cells and probably facilitate invasion (Figures 46–3 and 46–4A and B). One family of hyphal enzymes, the secreted aspartic proteinases (Saps), is able to digest keratin and collagen, which would facilitate deep tissue invasion. The pattern of Sap production may be tissue-specific with individual isolates producing distinct sets of Saps depending whether they are invading gastrointestinal or vaginal epithelium.
Pathogenesis of Candida albicans infections. Proposed mechanisms of C albicans attachment and invasion are shown. Surface glucomannan receptor(s) on the yeast may bind to fibronectin covering the epithelial cell or to elements of the extracellular matrix (ECM) when the epithelial surface is lost or when the Candida cells have invaded beyond it. Invasion is associated with formation of hyphae and production of proteinases, which may digest tissue elements.
Invasiveness of Candida albicans. Two features of invasiveness are seen in these scanning electron micrographs taken from experiments with murine corneocytes. A. Both yeast-like blastoconidia and mycelial elements are present. The mycelial elements spread over the surface and invade the cell cuticle. B. A C albicans strain that produces a protease is seen producing cavity-like depressions in the cell surface. This action could play a role in invasion of the cell. (Reprinted with permission of Thomas L. Ray and Candia D. Payne. Infect Immunol. 1988;56:1945-1947, Figures 4, 6B. Copyright American Society for Microbiology.)
One of the most important pathogenic features of C albicans is its ability to form biofilms. These complex structures include yeast and hyphal forms of the fungus along with host-derived proteins. Once formed, the biofilm strongly adheres to components of the ECM as well as to plastics. Neither host immune cells nor antifungal agents are able to penetrate Candida biofilms well, making this structure an important source of microbial persistence during infection. In a very practical sense, fungal biofilms that develop on prosthetic surfaces (eg, intravenous catheters, prosthetic joints, prosthetic heart valves) are almost impossible to sterilize without device removal.
Hwp1 binds to epithelial cells
Mannoproteins bind to ECM
Hyphae produce Saps, other enzymes
Candida albicans has various mechanisms which facilitate evasion of innate immune mechanisms. These include the masking of surface structures from Toll-like receptors (TLRs) and the accelerated degradation of surface complement C3b. The latter can be accomplished by binding of serum factor H or by secretion of its own protease. Hyphae also have surface proteins that resemble the complement receptors (CR2, CR3) on phagocytes. This seems likely to confuse the phagocyte’s ability to recognize C3b bound to the candidal surface. Enhanced production of these receptors under various conditions, such as elevated glucose concentration, is associated with resistance to phagocytosis by neutrophils. If phagocytosed, hyphal growth interferes with lysosomal fusion and leads to the death of macrophages.
Surface proteins resemble complement receptors
Many factors predispose to both local and invasive Candida infections. Antibacterial therapy reduces microbial competition on mucosal surfaces and increases the relative abundance of C albicans within the microbiota. Alterations in innate immunity (eg, leukopenia or corticosteroid therapy) or adaptive immunity (eg, AIDS) are important contributing factors to systemic and mucosal candidiasis. Additionally, anatomic disruptions of the skin and mucosa may enhance the invasion process by exposing Candida binding sites in the ECM and by allowing direct access to deeper tissues. Biofilm formation on the plastics used in medical devices also contributes to fungal persistence in this host. Diabetes mellitus predisposes to C albicans infection, possibly due to greater production of surface mannoproteins in the presence of high glucose concentrations.
Antimicrobials and immunosuppression increase risk
Mechanical disruptions may provide access to ECM
Both humoral immunity and cell-mediated immunity (CMI) are involved in defense against Candida infections. Neutrophils are the primary first-line defense, and defects in neutrophil number or function are among the most common immune correlates of serious C albicans infections. Yeast forms of C albicans are readily phagocytosed and killed by these innate immune cells, especially when opsonized by antibody and complement. In the absence of specific antibody, the process is less efficient, but a naturally occurring antimannan IgG is able to activate the classical complement pathway and facilitate the alternate pathway. Hyphal forms may be too large to be ingested by polymorphonuclear neutrophils (PMNs), but these immune cells can still kill the fungi by attaching to the hyphae and discharging metabolites generated by the oxidative burst.
Opsonized yeast forms are killed by PMNs
Antimannan IgG activates complement
Many immunodeficiency syndromes involving T-lymphocyte dysfunction result in severe mucocutaneous candidiasis, emphasizing the importance of this arm of the immune system in defense against Candida infections. For example, patients with AIDS develop frequent episodes of oral and esophageal candidiasis, suggesting that protection against even superficial infections involves CD4-mediated immune responses. As with other fungi, cytokine activation of macrophages and other immune effector cells enhances their ability to kill C albicans. Therefore, a favorable outcome of infection appears to require the proper balance between TH1- and TH2-mediated cytokine responses, and this balance is lost in the absence of intact CD4 cell function. Typically, the cytokines associated with TH1-mediated immunity (interleukin-2 [IL-2], IL-12, interferon-γ, tumor necrosis factor-α) are correlated with enhanced resistance against Candida infection. In contrast, TH2 responses (IL-4, IL-6, and IL-10) are associated with chronic disease. In animal studies, the fungus appears to play an active role in regulating this host response. For example, Candida cell wall mannan has been shown to play an immunoregulatory function by downregulating protective CMI responses.
Compromised CMI is associated with progressive infection
Candida mannan may downregulate CMI responses
Balance between TH1 and TH2 cytokines is necessary
CANDIDIASIS: CLINICAL ASPECTS
Superficial invasion of the mucous membranes by C albicans produces a white, cheesy plaque that is loosely adherent to the mucosal surface. Oral lesions, called thrush, occur on the tongue, palate, and other mucosal surfaces as ragged white patches (Figure 46–5). Scraping the fungal plaque with a tongue blade will reveal varying degrees of underlying mucosal invasion and inflammation, helping to differentiate this infectious process from other causes of superficial oral films. A similar infection in the vagina, vaginal candidiasis, produces a thick, curd-like discharge and itching of the vulva. Although many women have at least one episode of vaginal candidiasis in a lifetime, a small proportion suffers chronic, recurrent infections. No general or specific immune defect has yet been linked to this syndrome.
Thrush. The white plaques on this AIDS patient’s tongue are caused by Candida albicans. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)
✺ White mucosal plaque is called thrush
✺ Vaginitis may be recurrent
Superficial C albicans infections also occur in skinfolds and other areas in which wet, macerated skin surfaces are opposed. For example, one type of diaper rash is caused by C albicans (Figure 46–6A). Other infections of the skinfolds and appendages occur in association with recurrent immersion in water (eg, dishwashers). The initial lesions are erythematous papules or confluent areas of erythema, tenderness, and skin fissures. Infection usually remains confined to the chronically irritated area with adjacent “satellite” lesions.
Candida albicans skin infection. A. This rash is preceded by chronically damp skin in the diaper area. B. This Gram stain demonstrates yeast cells and pseudohyphae. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)
✺ Macerated skin is a common site of Candida skin infections
Rarely, chronic and relapsing Candida infections occur in patients with a specific defect in TH1 immune defenses. This condition, known as chronic mucocutaneous candidiasis (CMC), manifests with recurrent severe skin and mucosal lesions. With time, patients with CMC experience considerable skin disfigurement. Although lesions may become extensive, they usually do not result in fungal dissemination.
In contrast, most people live their entire lives in constant contact with C albicans but without developing symptomatic infections. This observation largely reflects the ability of normal hosts to effectively control the growth of this fungus. However, Candida colonization without associated symptomatic inflammation may also represent a clinical example of immunologic tolerance—to be able to be exposed to a microorganism without developing an excessive immune response. Therefore, it is possible that a subset of patients with frequent episodes of symptomatic candidiasis (eg, recurrent vulvovaginal candidiasis) actually has difficulty suppressing an overexuberant immune reaction to resident fungi, as opposed to trouble controlling microbial growth.
Chronic mucocutaneous candidiasis is associated with specific T-cell defects
Inflammatory patches similar to those in thrush may also develop in the esophagus and upper GI tract. These lesions occur most frequently in immunocompromised patients and are characterized by painful swallowing or substernal chest pain. Extensive ulcerations, deformity, and occasionally perforation of the esophagus may ensue.
✺ Esophagitis and intestinal candidiasis are similar to thrush
In addition to infection of mucosal surfaces, Candida infection often involves the urinary tract. Ascending infections may produce cystitis, pyelonephritis, renal abscesses, or expanding fungus ball lesions in the renal pelvis. Patients with urinary catheters, kidney transplants, or other types of chronic urinary devices are at particular risk for these infections.
Disseminated infections are particularly serious forms of candidiasis. The fungus often gains access to the bloodstream through skin lesions (eg, burns, intravascular catheters), disruption of the GI tract (eg, intestinal perforations, abdominal surgery), or prosthetic devices colonized with Candida biofilms. Once in the bloodstream, Candida species can infect many organs, including the kidneys, brain, and heart valves; however, symptoms are generally not sufficiently characteristic to suggest C albicans over the bacterial pathogens. Importantly, disseminated candidiasis frequently involves the eye. Candida endophthalmitis has the characteristic funduscopic appearance of a white cotton ball expanding on the retina or floating free in the vitreous humor. Endophthalmitis and infections of other eye structures can lead to blindness, and ocular complications must be considered in every case of disseminated candidiasis.
Urinary tract infections are ascending or hematogenous
Endophthalmitis appears as white cotton-like retinal lesions
What specific interventions might limit the incidence of mucosal candidiasis (oral thrush) and vaginal candidiasis in immunologically normal hosts?
What specific interventions might limit the incidence of candidemia in hospitalized patients?
Exudate or epithelial scrapings examined by KOH preparations (Figure 46–6B) demonstrate abundant budding yeast cells; if associated hyphae are present, the infection is almost certainly caused by C albicans. C albicans is readily isolated in culture from clinical specimens including blood. Cultures from specimens, such as sputum, run the risk of contamination from yeasts present in the normal flora.
KOH and Gram smears of superficial lesions show yeast and hyphae
✺ Candida species are readily grown in routine cultures of clinical samples
Deep organ involvement is difficult to prove without a direct aspirate or biopsy. However, Candida species often grow in routine blood cultures, and every episode of candidemia must be carefully evaluated for evidence of dissemination of involvement of prosthetic devices.
Think ➪ Apply 46-1. Many cases of oral thrush could be prevented by limiting unnecessary antibacterial use, avoiding the associated disruption of the mucosal microbiota that predisposes to yeast overgrowth. Candidemia can be limited by minimizing unnecessary intravenous and urinary catheter use in hospitalized patients. In health-care settings, adherence to good hand-hygiene limits spread from person to person.
Candida albicans is usually susceptible to amphotericin B, nystatin, flucytosine, the echinocandins, and the azoles. Superficial infections are generally treated with topical nystatin or azole preparations. Measures to decrease moisture and chronic trauma are important adjuncts in treating Candida skin infections. All C albicans infections may also require addressing predisposing conditions. For example, removal of an infected catheter, control of diabetes, or an increase in peripheral leukocyte counts can be important aspects of the complete treatment of infection. Systemic therapy with amphotericin B, echinocandins, or azoles is required for disseminated or deep tissue infections. The choice of treatment is often guided by speciation and antifungal susceptibility testing. Fluconazole has been effective treatment for chronic mucocutaneous candidiasis and recurrent mucosal infections, although antifungal resistance can develop with the prolonged use of this agent.
✺ Topical nystatin or azoles for superficial lesions
✺ Amphotericin B, fluconazole, and echinocandins for invasive disease
Candida species other than C albicans can produce very similar infections to those described earlier, especially disseminated and urinary tract infections. However, these nonalbicans Candida species are isolated almost exclusively from patients with nosocomial infections. Antibiotic use, wounds, and prosthetic devices also predispose hospitalized patients to infections with diverse Candida species. Some species, such as C glabrata and C krusei, display increased levels of resistance to the azole antifungals. Therefore, selection for colonization by these species may occur in patients previously treated with azoles. Other species, such as C tropicalis and C parapsilosis, are also isolated mostly from hospitalized patients. Importantly, nosocomial transmission of Candida species may occur with poor adherence to proper handwashing and other infection control practices. Little is known of the pathogenesis of these species with the exception of Candida tropicalis. Both experimental and clinical evidence indicate that C tropicalis has virulence at least equal to that of C albicans. Candida tropicalis produces extracellular proteinases similar to those of C albicans, which may enhance its invasiveness.
✺ Candida glabrata and Candida krusei are often resistant to azoles