How Are Bacteria Classified?

Bacteria can be classified in several ways. Learning how they are classified can help us to better control or manipulate them. Here are nine ways in which bacteria are classified.

Based on Stain

Bacteria a classified as gram positive or gram negative. Gram positive stains purple while gram negative stains pink. The difference is due to the thickness of their peptidoglycan layer. The thick peptidoglycan layer in gram positive bacteria enables them to retain their purple stain during a gram-staining procedure.

Gram stain showing both gram positive (purple) and gram negative (pink) bacteria.

Based on Shape

Bacteria cells may assume several shapes. The most common ones you will come across are cocci (round), bacilli (rod-shaped), and spiral.

Common bacteria shapes. *Creative Commons.*

Based on Colony Morphology

Bacteria grow on solid media as a colony. They colony starts from a single cell and then multiply by binary fission until they occupy a visible space on the media. At this point they can be differentiated based on morphology such as form, elevation, margin, size, color, and surface characteristics (e.g., translucent, glistening, dull, wrinkled, smooth).

Form: The shape of the colony

Elevation: How much the colony rise from the agar when viewed from the side

Margin: The conformation of the edge of the colony

Bacteria colony morphology (form, elevation, and margin). *Creative Commons License.*

Based on Oxygen Tolerance

Bacteria are generally classified as aerobic if they need oxygen to grow, anaerobic if they survive and grow where oxygen is absent, and facultative anaerobes if they can survive in both the presence and absence of oxygen.

Based on Temperature Tolerance

Bacteria may be classified as psychrophiles if they are cold-loving, mesophiles if they prefer warm to lukewarm conditions, and thermophiles if they prefer hot conditions.

Based on Salt Tolerance

Bacteria vary based on their ability to tolerate salt. Those that are not able to survive in salt are called nonhalotolerant. Those that can tolerate a low concentration of salt (up to 1%) are called halotolerant. Halophiles can survive salt concentrations of up to 5%, while extreme halophiles can live and thrive in salt levels as high as 20-30%.

Based on Acid Tolerance

Most bacteria do not survive low pH conditions. The few that do are called acidophiles. Neutrophiles are those that prefer to grow at pH from 5.5 to just over the neutral pH. Alkaliphiles grow at pH above 7.5

Based on Spore Formation

Some bacteria can produce spores (also called endospores) while others cannot. Spores are a very hardy non-reproductive structure that are produced by bacteria, especially when they are under stress. Spores can remain dormant for years until the right nutrient conditions exist for them to grow. Example of bacteria that grow spores include Bacillus cereus, Bacillus thuringiensis, Clostridium botulinum, and Clostridium tetani.

Bacteria Spore. Creative Commons License.

Based on Toxin Formation

Bacteria may produce toxins which can cause intoxication. The ones that do not produce toxins instead cause direct injury to cells and disrupts tissue function.