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Transcript
Chapter 27~ Prokaryotes and the Origins of Metabolic Diversity I. Phylogeny of Prokaryotes A. More prokaryotes inhabit a handful of fertile soil or the mouth or skin of a human than the total number of people who have ever lived. 1. No membrane bound organelles 2. Single celled 3. Classified by nutritional class and reactivity to oxygen 4. Not by Phyla/Divisions, but by Strains 5. Heterotrophs or Autotrophs (photoautotrophs or chemoautotrophs) B. Kingdom: Monera? Some books suggest two Domains: 1. Archaebacteria “ancient” 2. Eubacteria “true” C. Three domain system 1. Prokaryotes split early in the history of living things 2. Archaea are more closely related to Eukarya than bacteria 3. Eukarya are not directly related to Eubacteria 1. Archaebacteria Live in extreme environments “extremophiles” A. Halophiles- salt lovers B. Methanogens – produce methane as a by-product C. Thermoacidophiles - love hot, acidic places 2. Eubacteria Classified according to their mode of getting nutrients, mechanism of movement, and their shape D. Extreme thermophiles thrive in hot environments. 1. The optimum temperatures for most thermophiles are 60oC-80oC Sulfolobus oxidizes sulfur in hot sulfur springs in Yellowstone National Park Photo taken by Mrs. Brown! 1. 2. 3. 4. 5. Most prokaryotes are bacteria II. Structural characteristics A. Shape 1. cocci (sphere) 2. bacilli (rod) 3. helical (spiral) B. Cell wall: peptidoglycan (sugars & proteins) Gram +: with peptidoglycan Gram - : little peptidoglycan; impede drug action ; most pathogens Gram-positive bacteria have cell walls that are up to 10 times thicker than gram-negative bacteria. “Gram” named after Hans Christian Gram who first came up with a method of staining bacteria. Gram-positive = stains bright purple Gram-negative = no stain (light pink) Gram-Positive: Staphylococcus epidermidis, Streptococcus pyogenes, Clostridium tetani Gram-Negative: Escherichia coli, Salmonella typhi, Vibrio cholerae C. Capsule: adherence to substrate, causes tooth decay; protection from drying out & attack from immune system D. Pili: short, straight hair-like appendage; attachment to other bacteria Streptococcus mutans. The bacteria adhere to the surface of the tooth and then grow and synthesize a polysaccharide capsule which binds them to the enamel and forms a biofilm 300-500 microbial cells in thickness. The bacteria convert sugars in the diet into the dextran that forms plaque and cements the bacteria to tooth enamel. Within the plaque the bacteria convert sugars to lactic acid which can lead to dental caries or bacterial infection of the tooth. III. Motility A. Flagella – primary function = locomotion propels bacterium in straight line B. Filaments- found in spirochetes (helical shape) move through viscous liquid like a corkscrew (syphilis in mouth or genitals) Salmonella enterica Spirochete: Treponema pallidum C. Slime gliding- secretion of slimy threads D. Taxis- seek out favorable environments and avoid harmful ones 1. Chemotaxis- movement toward nutrients, away from toxins 2. Phototaxis- move or swim toward light 3. Magnetotaxis- move along magnetic flux lines IV. Form & Function A. Prokaryotes have smaller, simpler genomes than eukaryotes (about 0.001) B. Nucleoid region or genophore: noneukaryotic chromosome 1. Double-stranded DNA in the form of a ring C. Plasmids- smaller ring of DNA 1. provide genes for resistance to antibiotics 2. metabolism of unusual nutrients 3. Replicate independently of chromosome V. Growth, Reproduction and Genetic Exchange A. Asexual reproduction: binary fission (not mitosis) 1. Single cell produces a colony of offspring B. “Sexual” reproduction or combining genes (not meiosis): 1. Transformation: uptake of genes from surrounding environment 2. Conjugation: direct gene transfer from 1 prokaryote to another 3. Transduction: gene transfer by viruses C. Mutation is major source of genetic variation able to adapt to environment because generations made within hours constant changes D. Endospore Formation- withstand harsh conditions 1. Structure that withstands high heat, radiation, desiccation, toxins 2. Survival for hundreds of years 3. Form during unfavorable conditions 4. Low in water content (15% vs. 90%) 5. Thick, tough wall forms 6. When environment is hospitable, absorbs water and grows Parts of the Spore 1. Core - The core is dehydrated cytoplasm containing DNA, ribosomes, enzymes etc. Everything that is needed to function once returned to the vegetative state. 2. Cortex - The cortex is a modified cell wall/peptidoglycan layer that is not as cross-linked as in a vegetative cell. 3. Coats - Outside of the cortex are several protein layers that are impermeable to most chemicals. The coat is responsible for the spores resistance to chemicals VI. Nutrition & Metabolism A. Four major groups based on source of carbon and energy 1. Photoautotrophs- CO2 and light 2. Chemoautotrophs- CO2 + inorganic molecules (oxidize H2, H2S, NH3, Fe++) 3. Photoheterotrophs- organic molecules and light 4. Chemoheterotrophs- organic molecules A. Saprobes- decomposers, absorb nutrients from dead organisms B. Parasites- from the body fluids of living hosts B. Prokaryotes play an important role, responsible for key steps in the cycling of nitrogen (essential to proteins and nucleic acids) through environments. 1. Some bacteria convert ammonium (NH4+) to nitrite (NO2-) 2. Others “denitrify” nitrite or nitrate (NO3-) to N2, returning N2 gas to the atmosphere 3. Some (ex. Cyanobacteria) can use atmospheric N2 directly 4. During nitrogen fixation, they convert N2 to NH4+, making atmospheric nitrogen available to other organisms for incorporation into organic molecules C. Oxygen relationships: 1. obligate aerobes- require oxygen to survive 2. facultative anaerobes- can use it when available, not required 3. obligate anaerobes – must avoid oxygen or will die D. Evolution of metabolism 1. Early prokaryotes were probably heterotrophs feeding on the “primordial soup” of early Earth. 2. Glycolysis was probably the first metabolic process for gaining fuel from organic materials with no oxygen 3. Eventually, the food supply would be gone and the prokaryotes that were able to adapt to drive the synthesis of organic compounds would survive 4. Early prokaryotes split H2S to obtain electrons, before using water VI. The Importance of Prokaryotes A. Ecological cycles 1. Decomposers- unlock organics from corpses and waste products B. Symbiotic relationships – organisms in close contact with each other (symbiont and host) 1. Mutualism (+, +) - Nitrogen fixers and root nodules 2. Parasitism (+, -) - pathogenic strains 3. Commensalism (+, 0) – host not affected C. Pathogenic prokaryotes cause about half of all human disease, including pneumonia caused by Haemophilus influenzae bacteria. 1. Opportunistic: normal residents of host; only cause illness when defenses are weakened D. Louis Pasteur, Joseph Lister, and other scientists began linking disease to pathogenic microbes in the late 1800s E. Most pathogens cause illness by producing poisons, called exotoxins and endotoxins. 1. exotoxins: proteins that can produce disease w/o the prokaryote present a. Clostridium botulinum, which grows anaerobically in improperly canned foods, produces an exotoxin that causes botulism. b. An exotoxin produced by Vibrio cholerae causes cholera, a serious disease characterized by severe diarrhea. c. Even strains of E. coli can be a source of exotoxins, causing traveler’s diarrhea. 2. Endotoxins: They are originally from the lipid portion of outer membrane from gram negative bacteria. They are released in small amounts when the bacteria divide and in larger amounts when they die and disintegrate. They are called endotoxins because they are not secreted but are part of the cell itself. a. The endotoxin-producing bacteria in the genus Salmonella are not normally present in healthy animals. b. Salmonella typhi causes typhoid fever. c. Other Salmonella species, including some that are common in poultry, cause food poisoning. VIII. Prokaryotes in Research A. Much of what we know about metabolism and molecular biology has been learned using prokaryotes, especially E. coli, as simple model systems. B. With increasing use, prokaryotes are used to solve environmental problems. 1. organisms to remove pollutants from air, water, and soil is bioremediation C. Soil bacteria, called pseudomonads, have been developed to decompose petroleum products at the site of oil spills or to decompose pesticides. D. The chemical industry produces acetone, butanol, and other products from bacteria. E. The pharmaceutical industry cultures bacteria to produce vitamins and antibiotics. F. The food industry used bacteria to convert milk to yogurt and various kinds of cheese.