Historically, the fungi were regarded as relatively in-^gnificant causes of infection. However, these microorganisms began to be recognized as important causes of ¡¡disease in the early to mid-twentieth century, particularly because of changes in patient profiles, which continues today. In recognition of the endemic systemic jnycoses, which may cause disease in healthy hosts, a pumber of fungal species that are normally found in the Environment have been recognized as important causes |f human disease, particularly in the immunocompromised host.108',4S The modern clinical laboratory, therefore, needs to provide methods for isolating and Identifying the common causes of mycologic disease. Susceptibility testing of these isolates is also often l^cessary. Some clinical microbiology laboratories have (Jiept pace with changing times and have offered somewhat complete mycologic services. However, because of (Continued economic restraints in the health care environment, some laboratories are unable to offer these Services. In such instances, diagnostic clinical mycology is performed by reference laboratories that have varying degrees of experience. The lack of experience in clinical faycology has been influenced by a shortage of trained Individuals, lack of quality educational programs, and the inability of clinical laboratories to support the cost of ■sending personnel to training courses. Commonly, Individuals with experience who retire or leave their .position are replaced by someone with considerably less Experience. Braining and continuing education pro-tpams are needed to assist in the development of such jpdividuals, if quality laboratory services are to be offered. A real concern is that the changing health care environment and implementation of cost containment ¿measures without continuing education will prevent future generations from being well trained in diagnostic finical mycology.
This chapter is designed to assist technologists or ffticrobiologists with the basics of diagnostic clinical fnycology with the hope that this information will allow some laboratories to offer clinical mycology services.
overview of clinical mycology
More than 50,000 valid species of fungi exist, but only 100 to 150 species are generally recognized as causes of pitman disease and approximately 25 species cause the majority of human disease. Most of these organisms normally live a saprophytic (living on dead or decayed organic matter) existence in nature, enriched by nitrogenous matter, but they are capable of maintaining a separate existence as an opportunistic pathogen in humans or animals. Fungal infections are generally not communicable in the usual sense from person-to-person transmission. Animal-to-person transfer is also rare, with most such transmissions involving certain dermatophytes (a particular group of fungi that infect the skin and grow primarily in the keratin layer). Humans become an accidental host for fungi by inhaling spores or by the introduction of fungal elements into tissue by trauma. Except for disease caused by the dimorphic fungi, humans are relatively resistant to infections caused by the fungi. The major factors responsible for the increase in the number of fungal infections have been alterations in the host, particularly in the immune system. Whether caused by immunosuppressive agents or serious underlying diseases, these alterations may lead to infection by organisms that are normally nonpathogenic or are part of the patient's normal microbiota (i.e., normal flora). These infections may occur in patients with debilitating diseases such as progressive infection with the human immunodeficiency virus (HIV) or diabetes mellitus, or in patients with unpaired immunologic function resulting from corticosteroid or antimetabolite chemotherapy,108 Other common predisposing factors also include long-term intravenous cannulation, complex surgical procedures, and antibacterial therapy. It is because of this ability of normally saprophytic fungi to cause disease in the immunocompromised patient that laboratories now must be able to identify and report a wide array of fungi. No fungus species can be considered completely innocuous. It is therefore difficult at times for the mycologist at the bench to determine the significance of an organism recovered from a clinical specimen. In some instances, clinical and histopathologic correlations are the only means by which usually saprophytic fungi can be proven to represent true pathogens.
general features of the fungi_
Fungi seen in the clinical laboratory can generally be separated into two groups based on the appearance of the colonies formed. The yeasts produce moist, creamy, opaque or pasty colonies on media, whereas the filamentous fungi or molds produce fluffy, cottony, woolly or powdery colonies. Several systemic fungal pathogens exhibit either a yeast (or yeastlike) phase and filamentous forms are referred to as being dimorphic. When dimorphism is temperature dependent, the fungi are designated as thermally dimorphic. In general, these fungi produce a mold form at 25° to 30° C and a yeast form at 35° to 37° C under certain circumstances. The medically important dimorphic fungi include Histo-plasma capsulation, Blastomyces dermatitidis, Cocddwides immitis, Paracoccidioides brasiliensis, Sporothrix schettckii, and Penidllium marneffei. Cocddioides immitis is not thermally dimorphic. Additionally, some of the medically important yeasts, particularly the Candida species, may produce yeasts forms, pseudohyphae, and/or true hyphae. The polymorphic features of this group of organisms are not temperature dependent.
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