CHAPTER 24 Diagnostic Microbiology and Immunology Growth-Dependent Diagnostic Methods Isolation of Pathogens from Clinical Specimens Proper sampling and culture of a suspected pathogen is the most reliable way to identify an organism that causes a disease ( Figure 24.1 ).
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Diagnostic Microbiology and Immunology
Growth-Dependent Diagnostic MethodsIsolation of Pathogens from Clinical Specimens
Most clinical samples are first grown on general-purpose media, media such as blood agar that support the growth of most aerobic and facultatively anaerobic organisms.
Differential media are specialized media that allow identification of organisms based on their growth and appearance on the media.
Bacteremia is the presence of bacteria in the blood.
Growth-Dependent Identification Methods
Traditional methods for identifying pathogens depend on observing metabolic changes induced as a result of growth. These growth-dependent methods provide rapid and accurate pathogen identification.
Antimicrobial Drug Susceptibility Testing
Antimicrobial drugs are widely used for the treatment of infectious diseases.
Agar media are inoculated by evenly spreading a defined density of a suspension of the pure culture on the agar surface. Filter paper disks containing a defined quantity of the antimicrobial agents are then placed on the inoculated agar.
Antibiograms are periodic reports that indicate the susceptibility of clinically isolated organisms to the antibiotics in current local use.
Safety in the Microbiology Laboratory
Safety in the clinical laboratory requires effective training, planning, and care to prevent the infection of laboratory workers with pathogens.
Immunology and Clinical Diagnostic Methods Immunoassays for Infectious Disease
Specific immune responses, particularly antibody titers and skin tests, can be monitored to provide information about past infections, current infections, and convalescence (Figure 24.11).
Common immunodiagnostic tests for pathogens are shown in Table 24.5.
Polyclonal and Monoclonal Antibodies
Polyclonal and monoclonal antibodies are used for research and clinical applications.
In Vitro Antigen-Antibody Reactions: Serology
The study of antigen-antibody reactions in vitro is called serology.
Neutralization (Figure 24.13) and precipitation (Figure 24.14) reactions are examples of antigen-binding tests that produce visible results involving antigen-antibody interactions.
Direct agglutination tests are widely used for determination of blood types (Figure 24.15).
A number of passive agglutination tests are available for identification of a variety of pathogens and pathogen-related products. Agglutination tests are rapid, relatively sensitive, highly specific, simple to perform, and inexpensive.
Fluorescent antibodies are used for quick, accurate identification of pathogens and other antigenic substances in tissue samples and other complex environments.
Enzyme-Linked Immunosorbent Assay and Radioimmunoassay
Both involve linking a detection system, either an enzyme or a radioactive molecule, to an antibody or antigen, significantly enhancing sensitivity.
Immunoblot (Western blot) procedures (Figure 24.27) are used to detect antibodies to specific antigens or to detect the presence of the antigens themselves.
The antigens are separated by electrophoresis, transferred (blotted) to a membrane, and exposed to antibody.
Molecular and Visual Diagnostic MethodsNucleic Acid Methods
To design a nucleic acid probe, a nucleic acid sequence specific for the microorganism of interest must be available.
Virus propagation in vitro can be accomplished only in tissue culture. Therefore, most diagnostic techniques for viral identification are not growth-dependent but routinely rely on immunoassays and nucleic acid–based techniques.