Different kinds of bacteria are classified by physical characteristics

Bacteria are classified in a number of different ways. The traditional and most commonly used classification system is Gram stain, but bacteria are also classified based on shape and other physical characteristics, such as growth requirements. A basic method of differentiating bacteria are as benign (“good” or helpful) or pathogenic (i.e. disease-causing) organisms. Here are some of the various kinds of bacteria.

Gram-positive versus Gram-negative

After a series of staining reactions with crystal violet, iodine and safranan, Gram-positive bacteria appear blue-purple, whereas Gram-negative bacteria appear red. The difference is thought to be due to differences in the size of the peptidoglycan cell wall. Gram-negative bacteria also have lipopolysaccharide (LPS) in and an outer membrane that Gram-positive bacteria lack.

Aerobic versus anaerobic

Bacteria can be classified by their requirement for oxygen. Anaerobic bacteria grow only when no, or very little, oxygen is present in the environment. Aerobic bacteria are the opposite, they grow when oxygen is present. A more versatile type of bacterium is facultative anaerobic – these types of bacteria can grow in the presence of high or low concentrations of oxygen.

Chemoautotrophs versus heterotrophs

Chemoautotrophic bacteria oxidize reduced elements in their environment to gain energy. Classified as beta-proteobacteria in some classification systems, they fall into three groups: bacteria that use iron, bacteria that use nitrogen and bacteria that use sulfur. Most bacteria are heterotrophs, which require energy from another organic source, much like humans and food.

Phenotypic nomenclature: Cocci, bacilli and spirilla

Probably the most well known classification system for the general public is the one used to name bacterial species. “Coccus” (cocci, plural) is well known as part of the names of infamous bacteria (i.e. streptococcus, staphylococcus) – specifically it refers to spherically shaped cells. Some bacteria, which can be either Gram-positive or negative, aerobic or anaerobic, grow in colonies. Those that grow in grape-like clusters are “staphylo” and those that grow in chains are “strepto”. Growing as a couple of spherical bacterial cells are diplococci. Another type of bacteria is rod-shaped, called bacillus (bacilli, plural). If the bacterial cell has curved walls, it is a spirillum (spirilla, plural).


Mycoplasmas are extremely small bacteria, the smallest living organisms known on Earth. They are assumed to be descendents of Gram-positive bacteria but do not stain well and are obligate parasites similar to viruses.


Actinobacteria are similar to fungi. They grow in long filaments rather than as single cells, making up a large portion of microbial life in the soil. They are the source of many antibiotics. This type of bacterial classification includes the Gram-positive organisms Mycobacteria and Corynebacteria, which include species that cause leprosy, diphtheria and tuberculosis.


Cyanobacteria, also known as blue-green algae, are clustered photosynthetic organisms. They are considered to be the forerunners of plants.


Other bacterial names are based on the species they infect, where the strain was discovered, how they infect, how they survive, and what they infect. Often referred to as proteobacteria, these “other” species of bacteria are classified and named according to agreed upon nomenclature rules.

Classification systems

There are a number of ways to classify bacteria into “types”. The method depends on the information that is available about the species and the features that are most important. If detecting the strain is important, Gram stain classification may be most helpful. If identifying the strain visually is important, then classification by shape may be most helpful. Thus, each species of bacteria can be classified in different ways.