As we observed previously, bacterial colonies can differ greatly in their morphologies. These differences can help us in identifying different species of bacteria. Likewise, bacterial species differ in their cellular morphologies and staining properties. Again, these differences can be used to aid in identifying different species. Finally, as we observed previously, we can use selective and/or differential media to aid in identifying bacterial species. Generally, selective and differential media rely on some structural or metabolic property of the species that is preferentially selected.
We also have observed, however, that most of these tests are not extremely specific. Gram staining, for instance, can allow us to distinguish Gram positive from Gram negative organisms and rod-shaped organisms from coccus-shaped organisms, but does not allow us to make a more specific identification. Likewise, a selective and differential medium like MacConkey allows us to identify Gram negative, lactose fermenting organisms, but does not allow us to positively identify what specific Gram negative, lactose fermenting organism we are examining. To aid in the more definitive identification of bacteria, microbiologists have developed a series of biochemical tests that can be used to differentiate even closely related organisms. These various tests were designed to identify various metabolic properties of different bacterial species. More importantly, these tests, in conjunction with a dichotomous tree, can lead to the unambiguous identification of an organism. Today, we will examine the usefulness of several common biochemical tests.
Because clinical microbiologists often must identify bacteria quickly and accurately, a number of companies produce rapid identification systems. These systems allow one to quickly perform approximately 20 biochemical tests on a sample. These systems also contain an easy to read chart that allows one to quickly identify an unknown isolate based on the color changes that occur in the various tests.