How to Identify Different Types of Bacteria

Bacteria can be identified by their shape and how they react to chemical tests. Among the most common tests are the Gram stain, slide agglutination and phage typing. However, due to advancements in microbiology, scientists can further identify bacteria by DNA analysis.


Bacteria come in three basic shapes: Cocci, bacilli and spirilla. Spirilla--like vibrios, spirilla and spirochaetes--are shaped like spirals. They are some of the largest bacteria and the easiest to identify.

Bacilli are rod-shaped. Depending on how they divide, they may be further classified into other groups. For instance, diplobacilli form groups of twos, streptobacilli form long chains and staphylobacilli form amorphous shapes.

Cocci are circle-shaped. Like bacilli, they are also classified by the way they divide. Diplococci form groups of twos, streptococci form chains and staphylococci are amorphous.

Some bacteria assume different shapes, even within the same petri dish. These are known as pleomorphic, as opposed to monomorphic, bacteria.

Gram Staining

Gram staining is a simple but highly useful process in identifying bacteria. First, a researcher will stain the bacteria with crystal violet and iodine. Next comes an alcohol rinse, then a counterstain with safranine, a reddish dye.

Under a microscope, Gram-positive bacteria should appear deep purple or brown. Because of the peptidoglycan in their cell walls, they hold the crystal violet stain well, even after the alcohol rinse.

Gram-negative bacteria, on the other hand, will appear pink or red. They contain relatively low amounts of peptidoglycan and thus have low affinity for crystal violet. Three-quarters of all known eubacteria (or "true bacteria") are Gram-negative.

While the most common, the Gram stain is not the only stain test in use. Other stain tests can determine whether bacteria have endospores, capsules or flagella.

Slide Agglutination

One widespread serological test is slide agglutination. In this test, researchers expose the unknown bacteria to a number of different antiserums. These antiserums contain antibodies that combat a specific type of bacteria, and a positive identification comes when the bacteria agglutinate, or clump together.

Phage Typing

Phage typing operates on a similar principle. In this test, researchers grow the unknown bacteria in a petri dish, then introduce bacteriophage, a special kind of virus. Observing which bacteriophage attacks the bacteria, researchers can then determine the bacteria's identity.

DNA Analysis

To identify bacteria through DNA analysis, researchers can measure the ratio of guanine-cytosine pairs. These pairs reveal how an unknown bacterium relates to those that are known. They can also reveal how bacteria are evolving.

Another valuable means of identifying bacteria is nuclear hybridisation. In nuclear hybridisation, researchers heat a bacterium's complementary DNA strands, breaking the bonds between them. After a strand is isolated, researchers introduce DNA probes, which are short strands of another bacterium's DNA, and observe to what extent the strands combine, or hybridise. The greater the hybridisation, the closer the relationship between the two bacteria.