How Many Types of Bacteria Are There?

How many types of bacteria are there? It depends on what method is used for classifying them. Domain, morphology, Gram staining, metabolism, oxygen needs, and phyla are methods that yield different numbers.

Domain

• Biologists classify all bacteria under the monera kingdom. The monera kingdom, in turn, divides bacteria into three different domains: archaebacteria, mycoplasmas, and eubacteria.

  Eubacteria are also called true bacteria. Most bacteria studied in science classrooms belong to this domain.

  Mycoplasmas are among the smallest living organisms to be discovered. They lack a cell wall and reproduce without a host cell.

  Archaebacteria are some of the oldest organisms on earth. Despite their similarities to other bacteria, they have some genes that more closely resemble the genes in animal cells, or eukaryotes.

Morphology

• Bacteria have three basic types of morphology, or shape. Some bacteria are shaped like spheres, and these are called cocci. Others are shaped like rods, or bacilli. Yet other look like spirals under a microscope, and these are called spirilla. A fourth type of bacteria, known as pleomorphic bacteria, can change shapes.

Gram Staining

• The Gram stain divides bacteria into two basic types, Gram positive and Gram negative. Here's the process:

  Scientists will collect a sample of bacteria and stain them with crystal violet, a dark purplish chemical. After adding iodine for stability and a counterstain with saffranin, a pink dye, they can then observe the results under a microscope. Gram positive bacteria will appear brown or purple. This is because a polymer called peptidoglycan, which strengthens their cells walls, absorbs the crystal violet.

  Gram negative bacteria will appear pink. Because they contain low amounts of peptidoglycan or have an external membrane that repels crystal violet, they take the color of saffranin.

  Two further categories exist: Gram variable bacteria may appear show positive or negative, and Gram indeterminate bacteria do not respond to the stain.

Metabolism

• Based on metabolism, or how bacteria nourish themselves, there are two basic types: autotrophs or heterotrophs. Autotrophs can nourish themselves through direct sources of energy, like sunlight and carbon dioxide gas. Heterotrophs, on the other hand, must acquire nutrients other living organisms.

Oxygen Needs

• Based on their oxygen needs, there are five types of bacteria:

  The first type are called strict anaerobes. Strict anaerobes cannot survive in the presence of oxygen.

  The second type are facultative anaerobes. Facultative anaerobes can survive in the presence of oxygen, but they develop best where oxygen is absent.

  The third type are microaerophilic bacteria. Microaerophilic bacteria require oxygen to survive, but only in small amounts.

  The fourth type are strict aerobes. Strict aerobes require an abundance of oxygen for their growth and survival.

  The fifth and final type are indifferent bacteria. As their name suggests, indifferent bacteria are not sensitive to oxygen or its absence.

Phyla

• Phyla are determined by the bacteria's genetics. Currently, scientists recognize fifteen phyla, and the first is Aquificae, which live in hot, airless places. The fifteenth is Verrucomicrobia, which live in soil and fresh water.