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Title Chapter 20 Viruses, Bacteria, and Archaea Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 20.1 Viruses • • • • • The viruses are a biological enigma. They have a DNA or RNA genome, but they can reproduce only by using the metabolic machinery of a host cell. Viruses are noncellular, and therefore cannot be assigned a two-part binomial name, as are organisms. In 1884, Pasteur suspected something smaller than bacteria caused rabies; he chose a Latin term for “poison.” In 1892, Russian biologist Dimitri Ivanowsky, working with the tobacco mosaic virus, confirmed Pasteur’s hypothesis that an infectious agent smaller than a bacterium existed. With the invention of the electron microscope, these infectious agents could be seen for the first time. Viral Structure • All viruses have at least two parts: – – An outer capsid is composed of protein subunits. An inner core contains either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), but not both. • The viral capsid may be surrounded by an outer membranous envelope; if not, the virus is said to be naked. Page 357 Tobacco mosaic virus Parasitic Nature • Viruses infect all sorts of cells, from bacteria to human cells, but they are host specific. • Virus Evolution – Some believe that viruses originated from the very cells that they infect. – For example, viral nucleic acids originated from the host cell genome. – Therefore, viruses evolved after cells came into existence; new viruses are probably evolving now. – Others suggest that viruses arose before the three domains. Reproduction of Bacteriophages • There are two types of bacteriophage life cycles, termed the lytic cycle and the lysogenic cycle. • In the lytic cycle, viral reproduction occurs, and the host cell undergoes lysis, a breaking open of the cell to release viral particles. • In the lysogenic cycle, viral reproduction does not immediately occur, but reproduction may take place sometime in the future. Fig. 20.3 The Bacteria • Characteristics of Bacterial Cells – Bacteria are the more common type of prokaryote. – Bacterial cell walls are protected by peptidoglycan, a complex of polysaccharides linked by amino acids. – The Gram stain procedure (developed in the late 1880s by Hans Christian Gram) differentiates bacteria. •Gram-positive bacteria stain purple, whereas Gram-negative bacteria stain pink. •This difference is dependent on the thick or thin (respectively) peptidoglycan cell wall. – Bacteria and archaea have three basic shapes. •A spirillum is spiral-shaped. •A bacillus is an elongated or rod-shaped bacteria. •Coccus bacteria are spherical. •Cocci and bacilli tend to form clusters and chains of a length typical of the particular species. • Bacterial Metabolism – Bacteria differ in their need for, and tolerance of, oxygen (O2). •Obligate anaerobes are unable to grow in the presence of O2; this includes anaerobic bacteria that cause botulism, gas gangrene, and tetanus. •Facultative anaerobes are able to grow in either the presence or absence of gaseous O2. •Aerobic organisms (including animals and most prokaryotes) require a constant supply of O2 to carry out cellular respiration. Major Groups of Bacteria • Proteobacteria – Alpha Proteobacteria Includes many symbionts of plants and animals, and some pathogens. Rhizobium species live symbiotically in root nodules of legumes (e.g., beans) and convert atmospheric nitrogen to a form usable by plants (nitrogen fixation). – Purple sulfur bacteria are photoautotrophs that do not produce oxygen. • Cyanobacteria – Gram negative; photosynthetic. Inhabit ponds, lakes, swimming pools, moist soil, dead logs, tree bark. Contain chlorophyll a and use a photosynthetic process similar to that of plants and algae. Many species also fix nitrogen. – They photosynthesize in the same manner as plants and are believed to be responsible for first introducing oxygen into the primitive atmosphere. – Cyanobacteria are common in fresh and marine waters, in soil, and on moist surfaces, but they are also found in harsh habitats, such as hot springs. – Lichens are a symbiotic relationship where the cyanobacteria provide organic nutrients to the fungus and the fungus protects and supplies inorganic nutrients. These "jelly balls" are colonies of the cyanobacteria species Nostoc pruniforme ("Mare's eggs"), living in freshwater ponds. (Determination microscopically confirmed by Prof. Dr. L. Kies, Hamburg). Anabaena Oscillatoria Lichen • Gram-positive Bacteria – Actinomycetes Several species of the genus Streptomyces produce antibiotics such as streptomycin, erythromycin, chloramphenicol, and the tetracyclines. Some actinomycetes cause serious lung disease or generalized infections in humans and other animals. – Lactic acid bacteria ferment sugar, producing lactic acids as the main end product. Inhabit decomposing plant material, milk, and other dairy products; responsible for the characteristic taste of yogurt, pickles, sauerkraut, and green olives. – Clostridia are anaerobic. One species causes tetanus; another causes gas gangrene. Clostridium botulinum can cause botulism, and often fatal type of food poisoning. Microscopic image of the bacterial spore formation of Bacillus subtilis (ATCC 6633) Spore staining, magnification:1,000. (green) spores, (red) vegetatives. – The mycoplasmas are a group of bacteria that lack cell walls. They inhabit soil and sewage; some are parasitic on plants or animals. Some inhabit human mucous membranes but do not generally cause diseases; one species causes a mild type of bacterial pneumonia in humans. • Chlamydias – Chlamydias lack peptidoglycan in their cell walls. Sexually transmitted chlamydias are the major cause of pelvic inflammatory disease in women. • Spirochetes – Spirochetes are spiral-shaped bacteria. Some species are free-living, whereas others form symbiotic associations; a few are parasitic. The spirochete of greatest medical importance is Treponema pallidum, which causes syphilis. Table 20.2 The Archaea Types of Archaea • Three main types of archaea are distinguished based on their unique habitats: methanogens, halophiles, and thermoacidophiles. These Archaea species live in extreme heat near deep sea vents. Extremophiles that survive in extremely cold environments. Three groups of Archaea • Crenarchaeota – Some extremely thermophilic (thrive at 70° C, or some thrive at temperatures greater than 100° C); others psychrophilic (can live at temperatures below 15° C). • Euryarchaeota – Includes methanogens, halophiles (not all extreme), and acidophiles. Some extreme thermophiles included in this phylum. • Nanoarchaeota