Alphaproteobacteria: Definition & Characteristics

Instructor: Catherine Paul

Catherine has taught high school science and has a master's degree in biology.

Learn about the group of bacteria named Alphaproteobacteria, a subgroup of the phylum Proteobacteria. See how plant life benefits from the existence of this uniquely shaped group of bacteria.


Microbiologists attempt to classify bacteria into groups in order to make sense of the relationships between such a plentiful and diverse form of life. Bacteria exist in all shapes and sizes; with a wide variety of characteristics. In the past, bacteria were categorized according to their physical characteristics, such as shape, size, or other structural growths. Now, we have the ability to organize bacteria according to their rRNA similarities. This gives us a more accurate picture of the evolutionary relationships between bacterial species.

When bacteria are organized according to their rRNA, the phylum Proteobacteria emerges. The name Proteobacteria is based on the Greek god Proteus, who could assume many shapes. As you may have guessed, this group has a diversity of shapes. All Proteobacteria are Gram-negative bacteria, meaning they have an outer membrane covering their cell wall. Proteobacteria are even further broken down into subgroups called alpha, beta, gamma, delta, and epsilon Proteobacteria.


Some Alphaproteobacteria have a distinct appearance because they display unique extensions called prosthecae. Prosthecae are a characteristic of Alphaproteobacteria, which use these outward buds (picture outstretched arms) to increase their surface-to-volume ratio. By doing this, the bacteria are able to live in environments with little nutrients.

The Alphaproteobacteria group can utilize nutrients in a variety of ways and can be nitrogen fixating, chemoheterotrophs, or chemoautotrophs. Chemoheterotrophs obtain the nutrients they need to live by utilizing organic molecules in their environment. They can oxidize these molecules, which converts them to a life-sustaining supply of carbon and energy. Chemoautotrophs receive their energy by converting inorganic materials and acquire carbon from carbon dioxide.

Azospirillum, Nitrobacter, and Nitrosomonas

One type of Alphaproteobacteria that is found in soil is called Azospirillum. It grows close to plant roots in order to absorb the nutrients they excrete as waste. In turn, it fixes nitrogen from the atmosphere and gives the plant an essential nitrogen nutrient supply. Azospirillum is commonly found with grass, sugar cane, and corn; it is considered beneficial to plant life. As the 'nitro' in the name suggests, Nitrobacter and Nitrosomonas are a pair of bacteria that work together to provide crucial nitrogen to plants.

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