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. 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.

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.

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Microbiology: Current Research
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