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Gram‐Negative Rods and Cocci
Bdellovibrios. Bdellovibrios are aerobic Gram‐negative, curved rods that prey on other bacteria. The organism attaches to the surface of a bacterium, rotates, and bores a hole through the host cell wall. It then takes biochemical control of the host cell and grows in the space between the cell wall and plasma membrane. The host bacterium is killed in the process. The comma‐shaped Bdellovibrio bacteriovorus is the most thoroughly studied species of the group.
Pseudomonads. Pseudomonads are aerobic, Gram‐negative rods that are motile with polar flagella. Over 30 species are found in the group, and Pseudomonas fluorescens is a well‐known producer of a yellow‐green pigment. Another species, P. aeruginosa, causes urinary tract infections and infections of burned tissue.
Azotobacter and Rhizobium. Species of Azotobacter and Rhizobium are extremely important for their ability to fix nitrogen in the environment. These Gram‐negative rods live free in the soil (Azotobacter) or on the roots of legume plants (Rhizobium)and use their enzymes to convert atmospheric nitrogen into organic molecules useful to the plant. The plants then use the nitrogen compounds for the synthesis of amino acids and proteins, which serve as an extremely valuable food source for animals and humans. Members of the genus Azotobacter form a resting cell called a cyst, which withstands drying and environmental stresses.
Enterobacteria. Enterobacteria are facultatively anaerobic, Gram‐negative rods that inhabit the human intestine. Members of the enterobacteria group are members of the family Enterobacteriacae classified in section 5 of Bergey's Manual.
Over 25 genera of enterobacteria are recognized, many with pathogenic importance. Among the medically important enterobacteria areSalmonella species that cause intestinal disease known as salmonellosis;Yersinia pestis, the cause of plague; Klebsiella species, the causes of pneumonia, intestinal disease, and other infections; and species ofSerratia and Proteus. The well‐known organism Escherichia coli is also a member of this group. All enterobacteria have peritrichous flagella.
Vibrios. Vibrios are curved, Gram‐negative, facultatively anaerobic rods. They belong to the family Vibrionaceae. One species, Vibrio cholerae, is the cause of cholera in humans. Members of the genus Aeromonas andPlesiomonas are involved in human intestinal disease. Species ofPhotobacterium are marine organisms known for their ability to produce light as a result of chemical actions stimulated by the enzyme luciferase. This production of light is known as bioluminescence.
Pasteurellas. The pasteurellas belong to the family Pasteurellaceae. They are distinguished from vibrios and enterobacteria by their small size and inability to move. The genera Pasteurella, Haemophilus, and Actinobacillusare among the important members of the group. The species H. influenzaeis a cause of meningitis in children, while P. multocida causes cholera in fowl.
Streptococci. Streptococci are spherical bacteria that divide in parallel planes to produce chains. The bacteria are Gram‐positive, and certain species are aerobic, while others are anaerobic. On blood agar, certain species partly destroy the red blood cells and are said to be alpha‐hemolytic. Other species completely destroy the blood cells and are beta‐hemolytic. Those streptococci producing no blood cell destruction aregamma‐hemolytic.
One species of streptococcus (Streptococcus pneumoniae) is the cause of secondary bacterial pneumonias, while another species (Streptococcus pyogenes) causes strep throat and rheumatic fever. Other species are associated with dental caries. Harmless strains of streptococci are used in the production of yogurt, buttermilk, and cheese.
Staphylococci. Staphylococci are Gram-positive bacteria that divide in planes to produce clusters or packets. Normally associated with the skin and mucous membranes, certain species of staphylococci are involved in skin boils, abscesses, and carbuncles, especially if they produce the enzyme coagulase, which causes blood clotting. Staphylococcus aureus is involved in cases of food poisoning, toxic shock syndrome, pneumonia, and staphylococcal meningitis.
Lactobacilli. Lactobacilli are Gram-positive, rod-shaped bacteria occurring as single cells or chains. They produce lactic acid in their metabolism and are associated with the flora of the mouth and the vagina. Certain species are associated with the production of dairy products such as yogurt, sour cream, and buttermilk.
Bacillus and Clostridium species. Species of Bacillus and Clostridium are Gram-positive, rod-shaped bacteria able to produce highly resistant endospores (spores). The spores are found in the soil, air, and all environments of the body. Species ofBacillus grow aerobically, and Bacillus anthracis is the cause of anthrax. Clostridiumspecies grow anaerobically, and different species cause tetanus, botulism, and gas gangrene.
Bacillus and Clostridium species are also used for industrial purposes. Bacillus thuringiensis forms an insecticide useful against various forms of caterpillars, andClostridium species are used to produce various types of chemicals, such as butanol.
Corynebacteria. Corynebacteria are pleomorphic members of the genusCorynebacterium, which are Gram-positive rods found in various environments, including the soil. The bacteria contain cytoplasmic phosphate granules that stain as characteristic metachromatic granules. One species, Corynebacterium diphtheriae, causes human diphtheria.
Mycobacteria. The mycobacteria include species in the genus Mycobacterium. This group of rod‐shaped bacteria possesses large amounts of mycolic acid in the cell wall. The presence of mycolic acid makes the bacteria very difficult to stain, but when heat or other agents are used to force carbolfuchsin into the cytoplasm, the bacteria resist decolorization with a dilute acid‐alcohol solution. Therefore, they are said to be acid‐fast.
Many mycobacteria are free‐living, but two notable pathogens exist in the group: M. tuberculosis, the cause of tuberculosis in humans and cattle; and M. leprae, which causes leprosy.
Nocardioforms. Nocardioforms include nine genera of aerobic, acid‐fast rods, including members of the genus Nocardia. Nocardioforms have aerial hyphae which project above the surface of their growth medium as branching filaments. The hyphae fragment into rods and cocci. Nocardioforms are found throughout nature in many types of soil and aquatic environments. One species, N. asteroides, causes infection of the human lung.
Archaebacteria differ from all other bacteria (which are sometimes called eubacteria). Archaebacteria are so named because biochemical evidence indicates that they evolved before the eubacteria and have not undergone significant change since then. The archaebacteria generally grow in extreme environments and have unusual lipids in their cell membranes and distinctive RNA molecules in their cytoplasm.
One group of archaebacteria are the methanogens, anaerobic bacteria found in swamps, sewage, and other areas of decomposing matter. The methanogens reduce carbon dioxide to methane gas in their metabolism. A second group are the halobacteria, a group of rods that live in high‐salt environments. These bacteria have the ability to obtain energy from light by a mechanism different from the usual process of photosynthesis. The third type of archaebacteria are the extreme thermophiles. These bacteria live at extremely high temperatures, such as in hot springs, and are associated with extreme acid environments. Like the other archaebacteria, the extreme thermophiles lack peptidoglycan in their cell walls. Many depend on sulfur in their metabolism, and many produce sulfuric acid as an end‐product.
Rickettsiae. Rickettsiae are rod‐shaped and coccoid bacteria belonging to the order Rickettsiales. These bacteria cannot be seen with the light microscope, and therefore the Gram stain is not used for identification. However, their walls have the characteristics of Gram‐negative cell walls. Rickettsiae are obligate intracellular parasites that infect humans as well as arthropods such as ticks, mites, and lice. They are cultivated only with great difficulty in the laboratory and generally do not grow on cell‐free media. Tissue cultures and fertilized eggs are used instead.
Rickettsiae are very important as human pathogens. Various species cause Rocky Mountain spotted fever, epidemic typhus, endemic typhus, scrub typhus, Q fever, and ehrlichiosis.
Chlamydiae. Chlamydiae are extremely tiny bacteria, below the resolving power of the light microscope. Although the Gram stain is not used for identification, the bacteria have cell walls resembling those in Gram‐negative bacteria.
Chlamydiae display a growth cycle that takes place within host cells. The bacteria invade the cells and differentiate into dense bodies calledreticulate bodies. The reticulate bodies reproduce and eventually form new chlamydiae in the host cell called elementary bodies. Chlamydiae cause several diseases in humans, such as psittacosis, a disease of the lung tissues; trachoma, a disease of the eye; and chlamydia, an infection of the reproductive tract.
Mycoplasmas. Mycoplasmas are extremely small bacteria, below the resolving power of the light microscope. They lack cell walls and are surrounded by only an outer plasma membrane. Without the rigid cell wall, the mycoplasmas vary in shape and are said to be pleomorphic.Certain species cause a type of mild pneumonia in humans as well as respiratory tract and urinary tract diseases.
Spirochetes and Spirilla
Over 400 recognized genera of bacteria are known to exist. Bacterial species are listed in Bergey's Manual of Systematic Bacteriology. The entire kingdom of bacteria, including cyanobacteria, is entitled Prokaryotae. Four divisions of bacteria based on their cell wall characteristics are included in the Prokaryotae kingdom. Not all bacteria are assigned to a division, but all are assigned to one of 33 “sections.”
Spirochetes have a spiral shape, a flexible cell wall, and motility mechanisms based on structures called axial filaments. Each axial filament is composed of fibrils extending toward each other between two layers of the cell wall.
Spirochetes are very slender and difficult to see under the light microscope. They are cultivated with great difficulty (some cannot be cultivated), and their classification is based on their morphology and pathogenicity. Certain species inhabit water environments, while others are parasites of arthropods (such as ticks and lice) as well as warm‐blooded animals. Spirochetes include Borrelia burgdorferi, the agent of Lyme disease, Treponema pallidum, the cause of syphilis, and Leptospira interrogans, the agent of leptospirosis.
Spirilla have a spiral shape, a rigid cell wall, and motility mechanisms based on polar flagella. The genera Spirillum, Aquaspirillum, and Azospirillumare widely dispersed among and readily isolated from numerous environments. These organisms are aerobic bacteria wound like helices. Species S. minor is a cause of rat bite fever in humans. The genusCampylobacter contains several pathogenic species, including C. jejuni, which causes campylobacteriosis, an intestinal infection accompanied by diarrhea.