Taxonomy Classification of Bacteria
Taxonomy, is literally the science of classification. Look at the picture above, and imagine that all those little divisions, like “firmicutes” are different phyla under the kingdom of bacteria. Then those phyla are further subdivided into different classes, then orders, then families, then genera, and then finally species! Take a look at how this works for one particular bacteria, called streptococcus mutans.
Wow, there’s a lot to classify, probably why it’s taxonomy: it’s such a taxing job.
Ahem, right, so. As you’ve probably noticed, Streptococcus mutans is named using its Genera and its Species name. Similarly, all organisms have a scientific name comprising of two parts: The genus, followed by the species. It is very important to classify organisms in this way because:
- It establishes criteria for identifying organisms.
- Allows arrangement of related organisms into groups.
- Provides important information on how organisms evolved.
Bacteria are classified, usually, according to their morphological, metabolic and biochemical differences, although genetic and immunologic factors are also now being considered.
(Picture courtesy Principles and Explorations in Microbiology, 8th ed. – J. Black).
One of the earliest, and most fundamental methods of classifying bacteria depended on the use of the Gram Stain.
Unlike large organisms like humans, parrots and dra-, erm, Komodo Dragons, which are easy to spot and have a distinct appearance to the eye, bacteria are colourless and invisible to light microscopy. Thus, gram staining had to be developed to give bacteria a colour, and visualize them. Since bacteria would either respond to the stain, or not, all bacteria were subsequently classified into gram-positive and gram-negative bacteria.
There are 4 steps to the Gram Stain Procedure.
- Pour crystal violet stain (a blue dye) and wait for 60 seconds.
- Wash off with water and flood with iodine solution. Wait for 60 seconds.
- Wash off with water and then “decolourize” with 95% alcohol solution.
- Counter-stain with safranin (a red dye). Wait 30 seconds and then wash off with water.
Basically, when viewed under the microscope, cells that absorb the crystal violet dye and hold on to it become blue: These are gram-positive. Alternatively, if the crystal violet is washed off by the 95% alcohol, the cells absorb the safranin and appear red. These are gram-negative.
Gram Positive = Blue
Imagine yourself sitting by the beach, opposite crystal blue waters, or kayaking across deep waters, or even river tubing across the bluest of rivers.. won’t you say yes to that? So, Blue = Positive. Note that Gram-Positive bacteria may also appear purple if the red safranin is not effectively washed off. This is because blue + red is purple.
Gram Negative = Red
Now you’re sitting in sweltering red heat, sweat pouring down your body, the sun red in the sky. You don’t want that, do you? Or for comic book fans, you can picture Superman’s face when he sees a Red Sun in the sky. NOPE, thinks Superman. So Red = Negative.