1 Microbial Pathogens: an Overview

Abstract:

This chapter provides an introduction to the main human microbial pathogens, with a description of the clinical features of the disease and emphasis on the cell biology of the infectious process. There is increasing evidence for association of Chlamydia pneumoniae with atherosclerosis. Chlamydia psittaci is primarily an animal pathogen, and only in rare cases is it responsible for human respiratory tract infections, oropharyngitis, and atypical pneumonia. Chlamydia has a biphasic developmental cycle with two morphologically different forms: the elementary body (EB), which is the infectious form and is metabolically inactive, and the reticulate body (RB), which results from the differentiation of the EB in the parasitophorous vacuole. The genus Entamoeba includes two related species, the commensal Entamoeba dispar and the pathogenic Entamoeba histolytica, the latter being the agent of amebic dysentery and visceral amebiasis, the second leading cause of death due to parasitic disease. Continuous cell growth leads to ulceration of the intestinal mucosa, causing diarrhea and severe intestinal cramps. The diarrhea is then replaced by a condition referred to as dysentery, characterized by intestinal bloody and mucoid exudates. If the condition is not treated, trophozoites of E. histolytica can migrate to the liver, lungs, bones, and brain, where large abscesses may appear. Infections caused by the encapsulated basidiomycetous yeast Cryptococcus neoformans are initiated by inhalation of the yeast into the lungs and show a remarkable propensity to spread to the brain and meninges.