3: Section Three Assessing Metabolic Characteristics of Bacteria (Biochemical Tests, Selective and Differential Media)

Microscopy and staining techniques allow the observer to examine cell shape, cell size, and cellular arrangement of microbial specimens. Differential staining techniques allow a microbiologist to gather information about the presence or absence of specific structural features. These techniques, however, do not allow laboratory personnel to determine the bacterial sample’s species.

Two different species within the same genus will look morphologically identical when stained and observed. For example, Staphylococcus aureus and Staphylococcus epidermidis would both present with the same Gram stain results and acid-fast stain results; both are cocci that grow in clusters and are approximately the same size. These two species cannot be differentiated using microscopy alone. In order to differentiate two species within the same genus, we must examine the physiological characteristics of the bacteria. Each species will be unique in its physiology, meaning that it will utilize a unique set of metabolic pathways. These metabolic pathways are catalyzed by protein molecules known as enzymes.

Since a bacterial species will have a unique set of enzymes, microbiologists can conduct what are known as biochemical tests to determine whether specific enzymes are present. After completing sets of biochemical tests, microbiologists interpret the data using a systematic key known as Bergey’s Manual.

Selective and differential media are also used to aid in the isolation and identification of bacteria. Selective media contain ingredients that inhibit the growth of some organisms while encouraging the growth of others. Differential media allow the observer to differentiate between organisms on the media.

Differentiation may be the result of a color change in the media, a color change in the bacterial colony, the production of a product, or another observable change. Some media are classified only as selective media, others are classified only as differential media, and some combine both selective and differential properties.

In this section, you will examine a variety of biochemical tests, selective media, and differential media.

 

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