Important Bacterial Groups

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In addition to classifying bacteria based on their shape and reaction to Gram staining, biologists also classify bacteria by their biochemical properties and evolutionary relationships. Studies of DNA and rRNA similarities have improved biologists' understanding of how bacteria are related to each other. But questions about the flow of genes from one species to another and other issues remain. Prokaryotes can pick up genes from DNA in the environment by transformation. This passing of DNA from one species to another can sometimes confuse true evolutionary relationships. However, most rRNA comparison data support the bacterial lineages that are discussed below.


Proteobacteria make up one of the largest and most diverse groups of bacteria. This group contains several subgroups that have several nutritional needs and includes both aerobic and anaerobic bacteria.

Many proteobacteria live symbiotically with other organisms. For example, the nitrogen-fixing bacteria (genus Rhizobium) seen in Figure 23-5 live in nodules inside the roots of legumes. Legumes are plants of the pea and bean family. Legumes also include alfalfa and clover.

Nitrogen-fixing bacteria are important to the success of many ecosystems. Even though almost 80 percent of Earth's atmosphere is made up of nitrogen gas, N2, plants and animals cannot use it directly to make proteins, nucleic acids, and other nitrogen-containing molecules. In a process called nitrogen fixation, species of the genus Rhizobium convert nitrogen in the atmosphere to ammonia, which plants can use. These species are a major source of soil nitrogen for plants. The huge populations in root nodules can make about 250 kg (114 lb) of fixed nitrogen per hectare (2.5 acres) of alfalfa in a single year. Soil bacteria of the genus Nitrosomonas, another genus of proteobacteria, play an important part in the nitrogen cycle. Nitrosomonads form nitrite from ammonia in the soil. Nitrites are then converted into nitrates by other bacteria. Plants also use nitrates as a source of nitrogen.

Some proteobacteria cause diseases in plants and animals. For example, species of the genus Agrobacterium cause tumors in plants. Rickettsiae cause diseases such as Rocky Mountain spotted fever in humans. Helicobacter pylori, shown in the chapter opener photo, is a bacterium that commonly causes stomach ulcers.

Some species of proteobacteria are enteric bacteria—they live in human and animal intestines. Enteric bacteria such as Escherichia coli make vitamin K and help digestive enzymes in the breakdown of foods in the intestines. Some strains of E. coli and species of the genus Salmonella cause foodborne illnesses either by invading the cells that line the intestines or by making toxins.

Gram-Positive Bacteria

Most members of this group are Gram-positive, but not all are. Biologists place a few species of Gram-negative bacteria in this group because these species are genetically similar to Grampositive bacteria. This group is very large and has many members. Members include the streptococcal species that causes strep throat and Clostridium botulinum, the bacterium that makes the toxin that causes botulism. Botulinum toxin is used medicinally to treat painful muscle spasms and, more recently, to erase "frown lines" from the face. Lactic acid bacterial species, such as species of the genus Lactobacilli, which turn milk sour and make yogurt, are also members of this phylum. Anthrax is caused by the Grampositive rod Bacillus anthracis, shown in Figure 23-6. B. anthracis is often linked to its use as a biological weapon.

Actinomycetes (AK-tuh-noh-MIE-SEETS) are Gram-positive bacteria that form branching filaments of colonies. Many species of actin-omycetes grow in soil and make antibiotics, chemicals that inhibit the growth of or kill other microorganisms. Antibiotics kill neighboring bacteria and fungi that compete for resources. Streptomycin (which is made by species of the genus Streptomyces) and tetracycline are examples of antibiotics that are used medicinally. Members of the genus Mycobacteria, another genus of actinomycetes, cause tuberculosis and Hansen's disease (leprosy).

□ IntenMtcHvud Topic: Biological Weapons Keyword: HM61692

Maintained by the ifcrfc National Science t!\Kj. Teachers Association figure 23-6

Human infection by Gram-positive Bacillus anthracis (a) is usually through a cut in the skin (cutaneous anthrax). However, inhaling endospores may result in pulmonary anthrax. If endospores reach the intestine, gastrointestinal anthrax may result. Photo (b) shows the ulcerated lesion, called an eschar(ES-KAHR), of cutaneous anthrax on a shoulder.


figure 23-7

Cyanobacteria, such as this filament of cells of the genus Anabaena, play an important role in the carbon cycle because they take up large amounts of carbon from the atmosphere during photosynthesis. The orange cells are the heterocysts in which nitrogen fixation occurs.

figure 23-7

Cyanobacteria, such as this filament of cells of the genus Anabaena, play an important role in the carbon cycle because they take up large amounts of carbon from the atmosphere during photosynthesis. The orange cells are the heterocysts in which nitrogen fixation occurs.


Cyanobacteria use photosynthesis to get energy from sunlight and make carbohydrates from water and the carbon dioxide in the air. They give off oxygen as a waste product. Certain cyanobacteria, such as those in the genus Anabaena shown in Figure 23-7, grow in filaments. Some of the organisms in the genus Anabaena form specialized cells called heterocysts. Heterocysts have enzymes for fixing atmospheric nitrogen into ammonia, which plants can use. Nitrogen-fixing cyanobacteria have nature's simplest nutritional requirements. They need only light energy, carbon dioxide, nitrogen, water, and some minerals in order to grow.

Cyanobacteria are very numerous and offer a large amount of food to marine and freshwater ecosystems. They were Earth's first oxygen-producing organisms. They are believed to have been responsible for making the oxygen-rich atmosphere in which aerobic (oxygen-using) organisms evolved. Once called blue-green algae, cyanobacteria are now known to be bacteria because they lack a membrane-bound nucleus and chloroplasts.


Spirochetes (SPIE-roh-KEETS) are Gram-negative, spiral-shaped bacteria. Some are aerobic, and some are anaerobic. They move by means of a corkscrew-like rotation. Spirochetes live freely or as pathogens. The spirochete Treponema pallidum causes syphilis, a sexually transmitted infection (STI), and Borrelia burgdorferi causes Lyme disease.


Gram-negative, coccoid pathogens of the group Chlamydia live only inside animal cells. They depend on the animal cells for protection and nutrients. Unlike the cell walls of other bacteria, the cell walls of chlamydia do not have peptidoglycan. Chlamydia trachomatis causes nongonococcal urethritis, or chlamydia, an STI.

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