The number and types of tests that are selected for inclusion in a battery depends on various factors, including the type of bacteria to be identified, the clinical significance of the bacterial isolate, and the availability of reliable testing methods.
Certain organisms have such unique features that relatively few tests are required to establish identity. For example, Staphylococcus aureus is essentially the only gram-positive coccus that appears microscopically in dusters, is catalase-positive, and produces coagulase. Therefore, identification of this common pathogen usually requires the use of only two tests coupled with colony and microscopic morphology. In contrast, identification of most clinically relevant gram-negative bacilli, such as those of the Enterobacteriaceae family, requires establishing metabolic profiles often involving 20 or more tests.
Although a relatively large number of tests may be required to identify a particular bacterial spedes, the number of tests actually inoculated may depend on the clinical significance of an isolate. For instance, if a gram-negative bacillus is mixed with five other bacterial spedes in a urine culture, it is likely to be a contaminant. In this setting, multiple tests to establish spedes identity are not warranted and should not routinely be performed. However, if this same organism is isolated in pure culture from cerebrospinal fluid the full battery of tests required for definitive identification should be performed.
With the complicated medical procedures that patients are subjected to and the increasing population of immunocompromised patients, uncommon or unusual bacteria are isolated. Because of the unusual nature that some of these bacteria exhibit, reliable testing methods and identification criteria may not be established in most clinical laboratories. In these instances, only the genus of the organism may be identified (e.g., Bacillus spp,), or identification may not go beyond a description of the organism's microscopic morphology (e.g., gram-positive, pleomoiphic bacilli, or gramvariable, branching organism). When such bacteria are encountered and are thought to be clinically significant, they should be sent to a reference laboratory whose personnel are experienced in identifying unusual organisms.
Although the number of tests induded in an identification battery may vary and different identification systems may require various inoculation techniques, the one common feature of all systems is the requirement for inoculation with a pure culture. Inoculation with a mixture of bacteria produces mixed and often uninterpretable results. lb expedite identification, cultivation strategies (described earlier in this chapter) should focus on obtaining pure cultures as soon as possible. Furthermore, controls should be run with most identification systems as a check for purity of the culture used to inoculate the system.
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