Principles of Antigen Detection and Nucleic Acid Assays for Detection Identification of Microorganisms

The laboratory diagnosis of most infectious diseases involves the isolation in culture and subsequent identification of a microbial agent from a clinical specimen. Diagnoses can also be made by detecting antibodies in a patient’s serum. Some of these traditional procedures are now being supplanted by new and more rapid methods that detect the presence of microorganisms or their products directly in patient specimens without the need for culture. In addition, these methods, which are often referred to as nonculture methods, can be used instead of biochemical tests to identify organisms that have already grown in culture. When used judiciously, these nonculture methods not only eliminate the need to perform culture, but also can be performed within minutes or hours. Thus, the time for reporting the result to the physician is shortened, and appropriate therapy can be administered to the patient sooner.

Clinical evaluations of nonculture technologies have shown that they are as reliable as, and in some cases, better than routine culture (i.e., more sensitive in detecting the microbe being sought). The result does not require growth of living, multiplying organisms but only detection of certain microbial cell structures or products. Another advantage is that these methods can detect infectious agents that, as yet, cannot be cultivated in the laboratory. An important example is the rotavirus, a common cause of infantile diarrhea that spreads rapidly in the hospital environment. Because this viral agent can now be detected directly in infant stool specimens by a rapid, nonculture method, its recognition helps prevent possible nursery-wide transmission.

Two types of nonculture methods are generally available. One type depends on detection of microbial antigens, a technology that has come into everyday use in clinical microbiology laboratories. Some examples are included in experiments you will perform in Section VIII. The second type of nonculture method uses probes to detect microbial nucleic acids, sometimes in combination with techniques that greatly expand (amplify) small amounts of microbial DNA or RNA present in a patient specimen.

This discussion should aid your understanding of the principles of these antigendetection and nucleic acid assays.