Identification of bacteria may require a number of specialized tests designed to reveal a number of characteristics. One such characteristic is whether or not the bacterium causes a disease (and if yes, which one). The color of a bacterium colony as it grows on a nutrient medium is an important identifying characteristic, as is the kind of medium on which a bacterium will grow. Whether a bacterium is Gram positive or negative is also very useful in identifying the bacterium. (The Danish microbiologist Hans Christian Gram discovered that the cell walls of certain bacteria have a lipopolysaccharide that renders the bacterial cells unable to retain a dye called gentian violet. Such bacteria are labeled Gram negative. Conversely, bacterial cells lacking the lipopolysaccharide will retain the dye, and are, thus, labeled Gram positive.) Whether a bacterium forms spores and requires oxygen, and the temperature at which the bacterium best grows are also valuable in determining species. The presence of a gelatinous sheath outside the bacterial cell wall can also be useful in identification. While more than 1,600 species of bacteria have thus far been discovered and described, many more are as yet unknown.
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