Porth's Essentials of Pathophysiology, 4e

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Mechanisms of Infectious Disease

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like the viruses, but produce a rigid peptidoglycan cell wall, reproduce asexually by cellular division, and con- tain RNA and DNA, similar to the bacteria. The Rickettsiaceae depend on the host cell for essential vitamins and nutrients, but the Chlamydiaceae appear to scavenge intermediates of energy metabolism such as adenosine triphosphate (ATP). The Rickettsiaceae infect but do not produce disease in the cells of certain arthro- pods such as fleas, ticks, and lice. The organisms are accidentally transmitted to humans through the bite of the arthropod (i.e., the vector) and produce a number of potentially lethal diseases, including Rocky Mountain spotted fever and epidemic typhus. The Chlamydiaceae are slightly smaller than the Rickettsiaceae but are structurally similar and are trans- mitted directly between susceptible vertebrates without an intermediate arthropod host. Transmission and rep- lication of Chlamydiaceae occur through a defined life cycle. The infectious form, called an elementary body , attaches to and enters the host cell, where it transforms into a larger reticulate body . This undergoes active replication into multiple elementary bodies, which are then shed into the extracellular environment to initi- ate another infectious cycle. Chlamydial diseases of humans include sexually transmitted genital infections ( Chlamydophila trachomatis; see Chapter 41); ocular infections and pneumonia of newborns ( C. trachoma- tis ); upper and lower respiratory tract infections in chil- dren, adolescents, and young adults ( Chlamydophila pneumoniae ); and respiratory disease acquired from infected birds ( Chlamydophila psittaci ). Organisms within the family Anaplasmataceae (including the reorganized genera Ehrlichia, Anaplasma, Neorickettsia , and Wolbachia ) are also obligate intra- cellular organisms that resemble the Rickettsiaceae in structure and produce a variety of veterinary and human diseases, some of which have a tick vector. These organ- isms target host mononuclear and polymorphonuclear white blood cells for infection and, similar to the Chlamydiaceae , multiply in the cytoplasm of infected leukocytes within vacuoles called morulae. Unlike the Chlamydiaceae , however, the Anaplasmataceae do not have a defined life cycle and are independent of the host cell for energy production. The most com- mon infections caused by Anaplasmataceae are human monocytic and granulocytic ehrlichiosis. Human monocytic ehrlichiosis is a disease caused by Ehrlichia chaffeensis that can easily be confused with Rocky Mountain spotted fever. Clinical disease severity ranges from mild to life-threatening. Manifestations include generalized malaise, anorexia and nausea, fever, and headache. Decreases in white blood cells (leukopenia) and platelets (thrombocytopenia) often occur. Severe sequelae include severe respiratory fail- ure, encephalopathy, and acute renal failure. The dis- ease is usually more severe in the elderly and persons with compromised immune function (e.g., those with HIV/AIDS). Human granulocytic ehrlichiosis is also transmitted by ticks, but is caused by Anaplasma phagocytophilum . The symptoms are similar to those seen with human monocytic ehrlichiosis.

Pathogenic Treponema species require no intermediates and are spread from person to person by direct contact. The most important member of the genus is Treponema pallidum , the causative agent of syphilis. Mycoplasmas. The mycoplasmas are unicellular pro- karyotes capable of independent replication. These organisms are less than one-third the size of bacteria and contain a small DNA genome approximately one- half the size of the bacterial chromosome. The cell is composed of cytoplasm surrounded by a membrane, but, unlike bacteria, the mycoplasmas do not produce a rigid peptidoglycan cell wall. As a consequence, the microscopic appearance of the cell is highly variable, ranging from coccoid forms to filaments, and the myco- plasmas are resistant to cell wall–inhibiting antibiotics such as penicillins and cephalosporins. The mycoplasmas affecting humans are divided into three genera: Mycoplasma, Ureaplasma , and Acholeplasma. The first two require cholesterol from the environment to produce the cell membrane; the acholeplasmas do not. In the human host, mycoplas- mas are commensals. However, a number of species are capable of producing serious diseases, including pneu- monia ( Mycoplasma pneumoniae ), genital infections ( Mycoplasma hominis and Ureaplasma urealyticum ), and maternally transmitted respiratory infections to infants with low birth weight ( U. urealyticum ). Rickettsiaceae, Anaplasmataceae, Chlamydiaceae, and Coxiella This interesting group of organisms combines the char- acteristics of viral and bacterial agents to produce dis- ease in humans. All are obligate intracellular pathogens, FIGURE 14-7. A photomicrograph of a Treponema pallidum bacterium that causes syphilis.This microscopic bacterium (spirochete) is a wormlike spiral-shaped microorganism that wiggles vigorously when viewed under the microscope. (From the Centers for Disease Control and Prevention Public Health Images Library. No. 2323.)