Fungi rarely cause disease in healthy immunocompetent hosts. Disease results when fungi accidentally penetrate host barriers or when immunologic defects or other debilitating conditions exist that favor fungal entry and growth.
Adaptation and Propagation
Fungi often develop both virulence mechanisms (e.g., capsule and ability to grow at 37oC) and morphologic forms (e.g., yeasts, hyphae, spherules, and sclerotic bodies) that facilitate their multiplication within the host.
Dissemination of fungi in the body indicates a breach or deficiency of host defenses (e.g., endocrinopathies and immune disorders).
Healthy, immunologically-competent individuals have a high degree of innate resistance to fungi. Resistance to fungi is based primarily upon cutaneous and mucosal physical barriers. Severity of disease depends on factors such as inoculum, magnitude of tissue destruction, ability of fungus to multiply in the tissue, and the immune status of the host.
Enzymes such as keratinase, the presence of capsule in Cryptococcus neoformans, the ability to grow at 37°C, dimorphism, and other as yet undefined factors contribute to fungal pathogenesis which involves a complex interplay of many fungal and host factors.
Fungi are ubiquitous in nature and exist as free-living saprobes that derive no obvious benefits from parasitizing humans or animals. Since they are widespread in nature and are often cultured from diseased body surfaces, it may be difficult to assess whether a fungus found during disease is a pathogen or a transient environmental contaminant. Before a specific fungus can be confirmed as the cause of a disease, the same fungus must be isolated from serial specimens and fungal elements morphologically consistent with the isolate must be observed in tissues taken from the lesion. In general, fungal infections and the diseases they cause are accidental. A few fungi have developed a commensal relationship with humans and are part of the indigenous microbial flora (e.g., various species of Candida, especially Candida albicans, and Malassezia furfur). Although a great deal of information is available concerning the molecular basis of bacterial pathogenesis, little is known about mechanisms of fungal pathogenesis. Infection is defined as entry into body tissues followed by multiplication of the organism. The infection may be clinically inapparent or may result in disease due to a cellular injury from competitive metabolism, elaboration of toxic metabolites, replication of the fungus, or an immune response. Immune responses may be transient or prolonged and may be cell-mediated, humoral (with production of specific antibody to components of the infecting organism), or both. Successful infection may result in disease, defined as a deviation from or interruption of the normal structure or function of body parts, organs, or systems (or combinations thereof) that is marked by a characteristic set of symptoms and signs and whose etiology, pathology, and prognosis are known or unknown.
Fungi infect the body through several portals of entry (Table 74-1). The first exposure to fungi that most humans experience occurs during birth, when they encounter the yeast C. albicans while passing through the vaginal canal. During this process the fungus colonizes the buccal cavity and portions of the upper and lower gastrointestinal tract of the newborn, where it maintains a life-long residence as a commensal.
Another fungus, Malassezia furfur, is common in areas of skin rich in sebaceous glands. How it colonizes the skin is not known, but both M furfur and C albicans are the only fungi that exist as commensals of humans and are considered part of the indigenous flora. Only under certain unusual circumstances have they caused disease. Other fungi that have been implicated in human diseases come from exogenous sources, where they exist as saprobes on decaying vegetation or as plant parasites. Fungi rarely cause disease in healthy, immuno-competent hosts, even though we are constantly exposed to infectious propagules. It is only when fungi accidentally penetrate barriers such as intact skin and mucous membrane linings, or when immunologic defects or other debilitating conditions exist in the host, that conditions favorable for fungal colonization and growth occur. When C albicans, for example, is implicated in disease processes, it may indicate that the patient has a coexisting immune, endocrine, or other debilitating disorder. In most cases, the underlying disorder must be corrected to effectively manage the fungal disease.
Adaptation and Propagation
Although most fungal diseases are the result of accidental encounters with the agent, many fungi have developed mechanisms that facilitate their multiplication within the host. For example, the dermatophytes that colonize skin, hairs, and nails elaborate enzymes that digest keratin. Candida albicans as a commensal organism exists in a unicellular yeastlike morphology, but when it invades tissues it becomes filamentous; conversely, the systemic fungi Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidoides brasiliensis exist as molds in nature and change to a unicellular morphology when they cause disease. Other properties, such as capsule production by C neoformans and the adherence properties of Candida species to host tissues, also contribute to their pathogenicity. In general, the fungi that cause systemic disease must be able to grow and multiply at 37°C.
Disseminated fungal diseases usually indicate a breach in host defenses. Such a breach may be caused by endocrinopathies or immune disorders, or it may be induced iatrogenically. Effective management of the fungal infection requires a concerted effort to uncover and correct the underlying defects.
The high degree of innate resistance of humans to fungal invasion is based primarily on the various protective mechanisms that prevent fungi from entering host tissues. Fungal growth is discouraged by the intact skin and factors such as naturally occurring long-chain unsaturated fatty acids, pH competition with the normal bacterial flora, epithelial turnover rate, and the desiccated nature of the stratum corneum. Other body surfaces, such as the respiratory tree, gastrointestinal tract, and vaginal vault, are lined with mucous membranes (epithelium) bathed in fluids that contain antimicrobial substances, and some of these membranes are lined with ciliated cells that actively remove foreign materials. Only when these protective barriers are breached can fungi gain access to, colonize, and multiply in host tissues. Fungi gain access to host tissues by traumatic implantation or inhalation. The severity of disease caused by these organisms depends upon the size of the inoculum, magnitude of tissue destruction, the ability of the fungi to multiply in tissues, and the immunologic status of the host.
Most of the fungi that infect humans and cause disease are classified by tissue or organ levels that are primary sites of colonization. These are discussed below.
Superficial Fungal Infections
Superficial fungal infections involve only the outermost layers of the stratum corneum of the skin ( Phaeoannellomyces werneckii [syn. Exophiala werneckii] and M furfur) or the cuticle of the hair shaft (Trichosporon beigelii and Piedraia hortae). These infections usually constitute cosmetic problems and rarely elicit an immune response from the host (except occasionally M furfur infections). Recently T beigelii and M furfur were implicated as opportunistic agents of disease, particularly in immunosuppressed or otherwise debilitated patients. Patients are accidentally infected with these common organisms via indwelling catheters or intravenous lines. Virtually nothing is known concerning the pathogenic mechanisms of these fungi.
The dermatophytes are fungi that colonize skin, hair, and nails on the living host. These fungi possess greater invasive properties than those causing superficial infections, but they are limited to the keratinized tissues. They cause a wide spectrum of diseases that range from a mild scaling disorder to one that is generalized and highly inflammatory. Studies have shown that the disease-producing potential of these agents depends on various parasite and host factors, such as the species of organism, immunologic status of the host, type of clothing worn, and type of footwear used. Trauma plays an important role in infection. These organisms gain entry and establish themselves in the cornified layers of traumatized or macerated skin and its integument and multiply by producing keratinase to metabolize the insoluble, tough fibrous protein. The reason why these agents spread no deeper is not known, but it has been speculated that factors such as cell-mediated immunity and the presence of transferrin in serum inhibit fungal propagation to the deeper tissue layers and systemic disease does not occur. Some dermatophytes have evolved a commensal relationship with the host and are isolated from skin in the absence of disease. Little is known about specific pathogenic mechanisms of the dermatophytes, but they do not cause systemic disease.
The fungi that have been implicated in the subcutaneous mycoses are abundant in the environment and have a low degree of infectivity. These organisms gain access to the subcutaneous tissues through traumatic implantation. Again, little is known about mechanisms of pathogenesis. Histopathologic evidence indicates that these organisms survive in the subcutaneous tissue layers by producing proteolytic enzymes and maintaining a facultative microaerophilic existence because of the lowered redox potential of the damaged tissue. In eumycotic mycetoma there is extensive tissue damage and production of purulent fluid, which exudes through numerous intercommunicating sinus tracts. Microabscesses are common in chromoblastomycosis, but the clinical manifestation of disease indicates a vigorous host response to the organism, as seen by the intense tissue reaction that characterizes the disease (pseudoepitheliomatous hyperplasia).
Although most of the fungi implicated in this category of disease exist in a hyphal morphology, the agents of chromoblastomycosis and sporotrichosis are exceptions. Chromoblastomycosis is caused by a group of fungi that have several features in common. They are all darkly pigmented (dematiaceous) and exhibit a pleomorphism consisting of two distinct morphologies: the organism may exist in a mycelial state or as a thick-walled spherical cell that divides by cleavage. The latter cell morphology, called a muriform cell, sclerotic cell or Medlar body, is the pathologic morphology seen in tissue sections. However, transition to the sclerotic morphology may not be a crucial requirement for pathogenesis. Several dematiaceous fungi cause a disease called phaeohyphomycosis, which clinically consists of a broad group of diseases characterized by the presence of various darkly pigmented yeastlike to hyphal elements, but not sclerotic cells, in pathologic specimens. Alternatively, the immune reaction of the host may dictate the morphology that the organism assumes. Again, there is no information about mechanisms or the role of morphogenesis in the pathogenesis of this group of fungi.
Sporotrichosis is caused by Sporothrix schenckii, which grows as a mold in nature or when cultured at 25°C, but as yeastlike cells when found in tissues. The clinical manifestations of disease caused by S schenckii vary, depending on the immune status of the patient. The classic condition, subcutaneous lymphanigitic sporotrichosis, is characterized by numerous nodules, abscesses, and ulcerative lesions that develop along the lymphatics that drain the primary site of inoculation. The disease does not extend beyond the regional lymph nodes that drain the site of the original infection. Alternatively, infection may result in solitary lesions or pulmonary disease. Clinical manifestations of pulmonary infections vary depending on the immune status of the patient. The immunocompetent individual has a high degree of innate resistance to disease, and when infection occurs the organism is often a secondary colonizer of old infarcted or healed cavities of the lungs. If the patient is immunocompromised, dissemination can occur. There is no information about mechanisms of pathogenesis of this dimorphic fungus.
Of all the fungi that have been implicated in human disease, only the six agents that cause the systemic mycoses have the innate ability to cause infection and disease in humans and other animals. The primary site of infection is the respiratory tract. Conidia and other infectious particles are inhaled and lodge on the mucous membrane of the respiratory tree or in the alveoli, where they encounter macrophages and are phagocytosed. To successfully colonize the host these organisms must be able to survive at the elevated temperature of the body and either elude phagocytosis, neutralize the hostility they encounter, or adapt in a manner that will allow them to multiply.
Several factors contribute to infection and pathogenesis of these organisms. Of the six systemic agents, five, Histoplasma capsulatum, Blastomyces dermatitidis, Paracoccidioides brasiliensis, Coccidioides immitis, and Penicillium marneffei are dimorphic, changing from a mycelial to a unicellular morphology when they invade tissues, except C immitis that forms spherules. The change from mycelial to yeast morphology in H. capsulatum appears critical for pathogenicity. Several physiologic changes occur in the fungus during the transition, which is induced by the temperature shift to 37°C. The triggering event is a heat-related insult: the temperature rise causes a partial uncoupling of oxidative phosphorylation and a consequent decline in the cellular ATP level, respiration rate, and concentrations of electron transport components. The cells enter a period of dormancy, during which spontaneous respiration is maintained at a decreased level. Then there is a shift into a recovery phase, during which transformation to yeast morphology is completed. Mycelial cells of H capsulatum that are unable to undergo this morphologic transition are avirulent. Similar observations have been made when mycelia of B dermatitidis and P brasiliensis are shifted from 25°C to 37°C, and it has been implied that transformation to the yeast morphology is critical for infection.
Coccidioides immitis is also dimorphic, but its parasitic phase is a spherule. Little is known about the role of morphologic transformation in infection and disease of this organism. Dimorphism does not appear to play a role in C neoformans pathogenesis since the organism is an encapsulated yeast both at 25°C and in host tissues. The sexual phase of C neoformans, Filobasidiella neoformans, is known, and the organism assumes a filamentous morphology, producing small basidiospores. It has been suggested that these propagules are relevant in infection.
In addition to adjustment to the elevated temperature of the host, the infectious propagules must deal with the hostile cellular environment of the lungs. Studies with mutants of C neoformans have shown that the acidic mucopolysaccharide capsule is important in pathogenesis. Acapsular variants of the yeast are either avirulent or markedly deficient in pathogenicity. Since these mutants were obtained by mutagenesis, it is difficult to rule out the contribution of other genetic defects to their decreased pathogenicity. However, at the cellular level, the capsular polysaccharide inhibits phagocytosis of the yeast. Encapsulated C neoformans cells are highly resistant to phagocytosis by human neutrophils, whereas acapsular variants are effectively phagocytosed. The active component of the capsular polysaccharide has been identified as glucoronoxylomannan. In addition, the capsular polysaccharide is poorly immunogenic in humans and laboratory animals, and the glucoronoxylomannan component persists for extended periods in the host.
In addition to the capsular polysaccharide, elaboration of phenyl oxidase (an enzyme that catalyzes the oxidation of various phenols to dopachrome) by C neoformans appears to be a determinant of virulence, although the role of this enzyme in virulence is unknown. The infectious propagules of H capsulatum, B dermatitidis, P brasiliensis, and C immitis are readily phagocytosed by alveolar macrophages. To survive phagocytosis and to multiply, these fungi must neutralize the effects of the phagocytes. The production of reactive oxygen metabolites by phagocytic cells is an important host defense against microorganisms. Studies have shown that the yeast phase of H capsulatum fails to trigger release of reaction oxygen metabolites in unprimed murine macrophages despite extensive phagocytosis. How they avoid destruction by the fungicidal mechanisms within lysosomes is unclear. Arthroconidia of C immitis inhibit phagosome-lysosome fusion and survive within normal murine peritoneal macrophages. Phagosome-lysosome fusion takes place after H capsulatum infection, but the yeast cells survive in the phagolysosome. It has been speculated that the fungus neutralizes the fungicidal components of the lysosome by a mechanism not yet elucidated.
There is very little information about mechanisms of fungal pathogenicity, in contrast to what is known about molecular mechanisms of bacterial pathogenesis. Fungal pathogenesis is complex and involves the interplay of many factors. Studies to elucidate these mechanisms are needed because of the increasing incidence of opportunistic infections.
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