Fig. 9.3 Bacterialmorphologies.Thebacteriaareindicatedbyarrows.(A)Gramstainpreparationofsputumfromapatientwithpneumonia.Gram-positive,elongatedcocciinpairsandshortchains(Streptococcus pneumoniae) andaneutrophilareevident.(B)Gramstainpreparationofabronchoalveolarlavagespecimenshowinggram-negativeintracellularrodstypicalofmembersofEnterobacteriaceae suchasKlebsiella pneumoniae or Escherichia coli. (C)SilverstainpreparationofbraintissuefromapatientwithLymediseasemeningoencephalitis.Twohelicalspirochetes(Borrelia burgdorferi) areshown(arrows). (A),(B),and(C)areatdifferentmagnifications.(Aand C,Courtesy of Dr. Kenneth Van Horn, Focus Diagnostics, Cypress, California. B,Courtesy of Dr. Karen Krisher, Clinical Microbiology Institute,Wilsonville, Oregon.) http://ebooksmedicine.net fluorescent stain that binds chitin, provides a useful way to identify fungi in patient specimens. Assays for betaglucans in blood are used to diagnose disseminated fungal infections. Fungi can grow either as rounded yeast cells or as slender, filamentous hyphae. An important distinguishing characteristic is whether hyphae are septate (with cell walls separating individual cells) or aseptate. Some of the most important pathogenic fungi exhibit thermal dimorphism; that is, they grow as hyphal forms at room temperature but as yeast forms at body temperature. Fungi may produce sexual spores or, more commonly, asexual spores called conidia. The latter are produced on specialized structures or fruiting bodies arising along the hyphal filament.
Fungi may cause superficial or deep infections.
Superficial infections involve the skin, hair, and nails. Fungal species that cause superficial infections are called dermatophytes. Infection of the skin is called tinea; thus, tinea pedis is “athlete’s foot” and tinea capitis is scalp ringworm. Certain fungi invade the subcutaneous tissue, causing abscesses or granulomas. Chronic infections, often in the foot, are called mycetomas.
Deep fungal infections can spread systemically and invade tissues, destroying vital organs in immunocompromised hosts, but usually resolve or remain latent in otherwise normal hosts.
Fungi are divided into endemic and opportunistic species.
Endemic fungi are invasive species that are usually limited to particular geographic regions (e.g., Coccidioides in the southwestern United States, Histoplasma in the Ohio River Valley).
Opportunistic fungi (e.g., Candida, Aspergillus, Mucor, Cryptococcus), by contrast, are ubiquitous organisms that either colonize individuals or are encountered from environmental sources but do not cause severe disease in healthy individuals. In immunodeficient individuals, opportunistic fungi give rise to life-threatening invasive infections characterized by vascular occlusion, hemorrhage, and tissue necrosis, with little or no inflammatory response (
Fig. 9.4 ). Patients with AIDS are very susceptible to infection with the opportunistic fungus Pneumocystis jiroveci (previously called Pneumocystis carinii).