The time required to obtain bacterial identification depends heavily on the length of incubation needed before the test result is available. In turn, the duration of incubation depends on whether the test is measuring metabolic activity that requires bacterial growth or whether the assay is measuring the presence of a particular enzyme or cellular product that can be detected without the need for bacterial growth.
Because the generation time (i.e., the time required for a bacterial population to double) for most clinically relevant bacteria is 20 to 30 minutes, growth-based tests usually require hours of incubation before the presence of an end product can be measured. Many conventional identification schemes require 18 to 24 hours of incubation, or longer, before the tests can be accurately interpreted. Although the conventional approach has been the standard for most bacterial identification schemes, the desire to produce results and identifications in a more timely fashion has resulted in the development of rapid identification strategies.
In the context of diagnostic bacteriology, the term rapid is relative. In some instances a rapid method is one that provides a result the same day that the test was inoculated. Alternatively the definition may be more predse whereby "rapid" is only used to describe tests that provide results within 4 horns of inoculation.
TWo general approaches have been developed to obtain more rapid identification results. One has been to vary the conventional testing approach by decreasing the test substrate medium volume and increasing the concentration of bacteria in the inoculum. Several conventional methods, such as carbohydrate fermentation profiles, use this strategy for more rapid results.
The second approach uses unique or unconventional substrates. Particular substrates are chosen, based on their ability to detect enzymatic activity at all times. That is, detection of the enzyme does not depend on multiplication of the organism (i.e., not a growth-based test) so that delays caused by depending on bacterial growth are minimized. The catalase, oxidase, and PYR tests discussed previously are examples of such tests, but many others are available as part of commerdal testing batteries.
Still other rapid identification schemes are based on antigen-antibody reactions, such as latex agglutination tests, that are commonly used to quickly and easily identify certain beta-hemolytic strepto.cocd and S. aureus (for more information regarding these test formats, see Chapter 10).
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