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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.

Proteobacteria

Microbiologists attempt to classify bacteria into groups in order to make sense of the relationships between such a plentiful and diverse form of life.

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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.

Alphaproteobacteria

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.

Rickettsia

An infectious Alphaproteobacteria called Rickettsia infect their host through an insect or tick bite. Once the host is bitten, this rod-shaped bacterium enters their host’s cells by inducing phagocytosis. Phagocytosis is a process used by some cells in the immune system to ingest and digest bacteria.

Though phagocytosis is typically used to benefit the immune system in order to eradicate bacteria, Rickettsia uses this route to invade the cell. Once inside, it replicates using binary fission, a process in which the cell asexually divides into two daughter cells.This parasitic bacterium is responsible for many common diseases in humans. Typhus epidemics are caused by the species Rickettsia prowazekii and spread by lice.

Similarly, rat fleas spread the disease called endemic murine typhus. Rocky Mountain spotted fever is transmitted when ticks infect a host with the bacteria Rickettsia rickettsii. The distinctive red spots of someone who is infected with Rickettsia is the result of damaged blood capillaries.

Rocky Mountain spotted fever
Rocky Mountain spotted fever caused by Rickettsia

Lesson Summary

With such a numerous and diverse population, a modern way to classify bacteria is by looking for similarities in their rRNA.

One phylum, the Proteobacteria, is a gram-negative group that can display unique shapes. Alphaproteobacteria can have prosthecae, which are extensions that allow the bacteria to absorb nutrients readily. This bacteria’s ability to utilize energy in different ways can be beneficial to plants by supplying them with nitrogen. Alphaproteobacteria can also be harmful by infecting a wide variety of hosts sometimes using insects as disease-spreading agents, like in Rocky Mountain fever.

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