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Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Important Groups of Procaryotes (page 8)

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Rickettsias and chlamydiae are two unrelated groups of Bacteria that are obligate intracellular parasites of eucaryotic cells. Rickettsias cannot grow outside of a host cell because they have leaky membranes and are unable to obtain nutrients in an extracellular habitat. Chlamydiae are unable to produce ATP in amounts required to sustain metabolism outside of a host cell and are, in a sense, energy-parasites.

Rickettsias occur in nature in the gut lining of arthropods (ticks, fleas, lice, etc.). They are transmitted to vertebrates by an arthropod bite and produce such diseases as typhus fever, Rocky Mountain Spotted Fever, Q fever and canine ehrlichiosis. Chlamydiae are tiny bacteria that infect birds and mammals. They may colonize and infect tissues of the eye and urogenital tract in humans. Chlamydia trachomatis causes several important diseases in humans: chlamydia, the most prevalent sexually transmitted disease in the U.S., trachoma, a leading cause of blindness worldwide, and lymphogranuloma venereum.

Figure 21. Mammalian cells infected with rickettsial organisms. L. Bartonella bacilliformis infection of human erythrocytes and blood monoctyes. R. Ehrlichia canis infection of canine erythrocytes and blood monocytes. The distinct stained intracytoplasmic inclusion body in the monocyte is characteristic of the infection.

Mycoplasmas are a group of bacteria that lack a cell wall. The cells are bounded by a single triple-layered membrane. They may be free-living in soil and sewage, parasitic inhabitants of the mouth and urinary tract of humans, or pathogens in animals and plants. In humans, Mycoplasma pneumoniae causes primary atypical pneumonia ("walking pneumonia").

Mycoplasmas include the smallest known cells, usually about 0.2 - 0.3 micrometers in diameter. Mycoplasmas correspondingly have the smallest known genome of any cell. Their DNA is thought to contain about 650 genes, which is about one-fifth the number found in E. coli and other common bacteria. Mycoplasmas can survive without a cell wall because their cytoplasmic membrane is more stable than that of other procaryotes. In one group of mycoplasmas, the membrane contains sterols which seem to be responsible for the stability. Also, mycoplasmas tend to inhabit environments of high osmolarity wherein the risk of osmotic shock and lysis of the cells is minimized.

Plant-pathogenic bacteria. Many economically-important diseases of plants are caused by members of the Bacteria. It is estimated that one-eighth of the crops worldwide are lost to diseases caused by bacteria, fungi or insects. Almost all kinds of plants can be affected by bacterial diseases, and many of these diseases can be extremely destructive.

Almost all plant-pathogenic bacteria are Gram-negative bacilli, usually affiliated with the pseudomonads or enterics (above). The symptoms of bacterial disease in plants are described by a number of terms such as spots, blights, soft rots, wilts, and galls. Pseudomonas species, as well as species of  Burkholderia and Ralstonia (ex-Pseudomonas) are among the most important bacteria that are pathogens of plants. They cause economically significant crop disease and crop loss world-wide. Bacterial spots of various sizes on stems, leaves, flowers and fruits are usually caused by Pseudomonas or Xanthomonas species. Bacteria may cause spots by producing toxins that kill cells at the site of infection. Blights are caused by rapidly developing necrosis (dead, discolored areas) on stems, leaves and flowers. Fire blight in apples and pears, caused by Erwinia amylovora, can kill young trees within a single season. Bacterial soft rots occur most commonly in fleshy vegetables such as potatoes or onions or fleshy fruits such as tomatoes and eggplants. The most destructive soft rots are caused by Erwinia species that attack fruits and vegetables at the post-harvest stage.

Bacterial vascular wilts mainly affect herbaceous plants. The bacteria invade the vessels of the xylem, where they multiply, interfering with the movement of water and inorganic nutrients and resulting in the wilting and the death of the plants. The bacteria commonly degrade portions of the vessel walls and can even cause the vessels to rupture. Once the walls have ruptured, the bacteria then spread to the adjacent parenchyma tissues, where they continue to multiply. In some bacterial wilts, the bacteria ooze to the surface of the stems or leaves through cracks formed over cavities filled with cellular debris, gums, and bacteria. More commonly, however, the bacteria do not reach the surface of the plant until the plant has been killed by the disease. Wilts of alfalfa and bean plants are cause by species of Clavibacter; bacterial wilt of cucurbits, such as squashes and watermelons, are cause by Erwinia tracheiphila; the black rot of crucifers such as cabbage is caused by Xanthomonas campestris. The most economically-important wilt of plants is caused by Pseudomonas solanacearum which affects 44 genera of plants, including such major crops as bananas, peanuts, tomatoes, potatoes, eggplants and tobacco. This disease occurs worldwide in tropical, subtropical, and warm temperate areas.

Mycoplasmas (discussed above) have been identified in more than 200 plant species and associated with more than 50 plant diseases, many with symptoms of yellowing. Among these plant-pathogens are the spiroplasmas (genus Spiroplasma), which are pleomorphic, ovoid or spiral-shaped cells which are motile by means of a rotary or screw-like motion. Intracellular fibrils are thought to be responsible for their movement. The organisms have been isolated from the fluids of vascular plants and from the gut of insects that feed on these fluids. Some have been cultured on artificial media, including Spiroplasma citri, which is isolated from the leaves of citrus plants, where it causes citrus stubborn disease, and from corn plants suffering from corn stunt disease. A number of other mycoplasma-like organisms (sometimes called MLOs) have been detected in diseased plants by electron microscopy, which has been taken as evidence that these organisms may be more involved in plant disease than previously realized.

The causative agent of a common plant disease, termed crown gall, is Agrobacterium tumefaciens. The disease is characterized by large galls or swellings that form on the plant at the site of infection, usually near the soil line. Crown gall is a problem in nurseries, affecting ornamental plants and fruit stock, and it may be a serious disease in grapes. Because of their role in the genetic engineering of plants, the molecular biology of these bacteria is intensively studied.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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