Porth's Essentials of Pathophysiology, 4e

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Mechanisms of Infectious Disease

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specimen from the host is inoculated into broth or onto the surface of an agar plate, and the culture is placed in a controlled environment such as an incubator until the growth of microorganisms becomes detect- able. In the case of a bacterial pathogen, identification is based on microscopic appearance and Gram stain reaction, shape, texture, and color (i.e., morphology) of the colonies, by a panel of biochemical reactions that fingerprint salient biochemical characteristics of the organism, and by the organism’s protein profile. Certain bacteria such as Mycobacterium leprae , the agent of leprosy, and T. pallidum , the syphilis spi- rochete, do not grow on artificial media and require additional methods of identification. Fungi and myco- plasmas are cultured in much the same way as bacteria, but with more reliance on microscopic and colonial morphology for identification. Chlamydiaceae, Rickettsiaceae , and all human viruses are obligate intracellular pathogens. As a result, the propagation of these agents in the laboratory requires the inoculation of eukaryotic cells grown in culture (cell cultures). Cell culture is becoming more infrequent in modern clinical laboratories due to the extended time required for growth of viruses and reported poor sen- sitivity of culture. Diagnosis of viruses, Chlamydiaceae , and Rickettsiaceae now relies primarily on serology and nucleic acid detection, which will be discussed later in this chapter. Although culture media have been developed for the growth of certain human-infecting protozoa and helminths in the laboratory, the diagnosis of parasitic infectious diseases has traditionally relied on micro- scopic or, in the case of worms, visible identification of organisms, cysts, or ova directly from infected patient specimens.

■■ Virulence factors, which are substances or products generated by infectious agents that enhance their ability to cause disease, include toxins (endotoxins and exotoxins), adhesion factors, evasive factors, and invasive factors. ■■ The natural history of an infectious disease includes its incubation period, as well as its prodromal, acute, convalescent, and resolution stages.

Diagnosis of Infectious Diseases

The diagnosis of an infectious disease requires two cri- teria: the recovery of a probable pathogen or evidence of its presence from the infected sites of a diseased host and accurate documentation of clinical signs and symp- toms (symptomatology) compatible with an infectious process. In the laboratory, the diagnosis of an infectious agent is accomplished using three basic techniques: culture, serology, or the detection of characteristic anti- gens, genomic sequences, or metabolites produced by the pathogen. Culture Culture refers to the propagation of a microorganism outside of the body, usually on or in artificial growth media such as agar plates or broth (Fig. 14-10). The

FIGURE 14-10. Use of agar culture for propagation and identification of microorganisms. (A) Photograph showing numerous Legionella species colonies that have been cultivated on an agar culture plate and illuminated using ultraviolet light. (B) Quantitative differences in hemolytic reactivity seen in trypticase soy agar culture plate containing 5% sheep’s blood growing group D Streptococci (left wedge), group B Streptococci (middle wedge), and group A Streptococci (right wedge) bacteria.The plate was grown under normal atmospheric conditions, at 35°C, for a period of 18 hours. It is important to note that the group B Streptococci (GBS) in the center wedge produced a hemolytic reaction that is less than the reaction produced by group A Streptococci (GAS) in the right wedge of bacterial growth. Hemolysis, which results from destruction of red blood cells, is one of the traits used to help identify the bacteria. (From the Centers for Disease Control and Prevention Public Health Images Library. Nos. 7925, 10861. A courtesy James Gathany; B courtesy Richard Facklam.) A B