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Transcript
General Microbiology (Micr300) Lecture 7 Microbial Diversity: Bacteria (Text Chapter: 12.1-12.3; 12.5-12.11; 12.3; 12.4; 12.17; 12.19-12.25; 12.27) Phylogenetic Overview of Bacteria Nearly 7000 species of prokaryotes are known. Figure 12.1 gives a phylogenetic overview of Bacteria. These bacteria can be divided into at least 18 Phyla. We will discuss only a number of common Phyla. The Proteobacteria consist of five clusters. Each cluster is designated by a Greek letter: alpha, beta, gamma, delta, or epsilon (Table 12.1). Physiologically, Proteobacteria can be phototrophic, chemolithotrophic, or chemoorganotrophic. Phylum 1: Proteobacteria Five clusters: Alpha, Beta, Gamma, Delta and Epsilon Purple Phototrophic Bacteria Purple bacteria are anoxygenic phototrophs that obtain carbon from CO2 + H2S (purple sulfur bacteria) or organic compounds (purple nonsulfur bacteria). Purple nonsulfur bacteria are physiologically diverse, and most can grow as chemoorganotrophs in darkness. The purple bacteria reside in the alpha, beta, and gamma subdivisions of the Proteobacteria. The Nitrifying Bacteria Chemolithotrophs are prokaryotes that can oxidize inorganic electron donors and in many cases use CO2 as their sole carbon source. Several reactions are involved in the oxidation of inorganic nitrogen compounds by chemolithotrophic nitrifying bacteria (Figure 12.9). Oxidation of Nitrogen Compounds Hydrogen-Oxidizing Bacteria A wide variety of bacteria can grow with H2 as the sole electron donor and O2 as the electron acceptor; using H2 as their energy source. All hydrogen-oxidizing bacteria contain one or more hydrogenase enzymes that bind H2 and use it either to produce ATP or as reducing power for autotrophic growth. Methanotrophs and Methylotrophs Methylotrophs are prokaryotes able to grow on carbon compounds that lack carbon-carbon bonds. Some methylotrophs are also methanotrophs, able to grow on CH4. Two classes of methanotrophs are known, each having a number of structural and biochemical properties in common. Methanotrophs reside in water and soil and can also exist as symbionts of marine shellfish. Pseudomonas and the Pseudomonads Pseudomonads include many gramnegative chemoorganotrophic aerobic rods; many nitrogen-fixing species are phylogenetically closely related. Many species use a wide variety of organic compounds as carbon and energy sources; some species use over 100 different compounds. Pseudomonads are ecologically important in soil and water – degradation of pollutants. Free-Living Aerobic Nitrogen-Fixing Bacteria A variety of organisms inhabit soil and are capable of fixing N2 aerobically. Azotobacter species are large rods and obligately aerobic. But their nitrogenases are O2 sensitive. High respiration rate and abundant capsular slime help protect nitrogenases from damage of O2. Resting structures – cysts can form. They are resistant to dessiccation, UV but not to heat. Enteric Bacteria The enteric bacteria are a large group of facultative aerobic rods of medical and molecular biological significance. Table 12.14 gives the phenotypic characteristics used to separate the enteric bacteria from other bacteria of similar morphology and physiology. One important taxonomic characteristic separating the various genera of enteric bacteria is the type and proportion of fermentation products produced by anaerobic fermentation of glucose. Rickettsias The rickettsias are obligate intracellular parasites, many of which cause disease. Rickettsias are deficient in many metabolic functions and obtain key metabolites from their hosts. Spirilla Spirilla are spiral-shaped, chemoorganotrophic prokaryotes widespread in the aquatic environment. The genera Helicobacter and Campylobacter are pathogenic spirilla. Spirilla are distributed among all five subdivisions of the Proteobacteria. Gliding Myxobacteria The fruiting myxobacteria are rodshaped, gliding bacteria that aggregate to form complex masses of cells called fruiting bodies. Myxobacteria are chemoorganotrophic soil bacteria that live by consuming dead organic matter or other bacterial cells. The life cycle of a typical fruiting myxobacterium is shown in Figure 12.47. Phylum 2: Low GC GramPositive Bacteria Lactic Acid Bacteria Lactic acid bacteria and relatives belong to a group of Gram positve bacteria that are nonnsporulating and low GC content. The key genera include Staphylococcus, Micrococcus, Streptococcus, Lactobacillus and Sarcina Bacillus, Clostridium, and Relatives Bacillus, Clostridium, Sporosarcina and Heliobacterium are major genera that constitute endospore-forming, low GC, Gram positive bacteria. Table 12.25 lists major genera of endospore-forming bacteria. One group of endospore-formers, the heliobacteria, is phototrophic. Phylum 3: High GC GramPositive Bacteria (Actinobacteria) The Mycoplasmas The mycoplasma group contains organisms that lack cell walls and contain a very small genome. Many species require sterols to strengthen their membranes, and several are pathogenic for humans, other animals, and plants. Major characteristics of mycoplasmas are shown in Table 12.28. Coryneform and Propionic Acid Bacteria High GC, gram-positive Bacteria include such organisms as Corynebacterium, Arthrobacter, Propionibacterium, and Mycobacterium. They are mainly harmless soil saprophytes, with Mycobacterium species being the exceptions. The propionic acid bacteria were first discovered in Swiss cheese, where their fermentative production of CO2 results in the characteristic holes. Mycobacterium The genus Mycobacterium consists of rod-shaped organisms that at some stage of their growth cycle possess the distinctive staining property called acidfastness. This property results from the presence on the surface of the mycobacterial cell of unique lipids called mycolic acids, found only in the genus Mycobacterium Mycobacterium M. tuberculosis is the causative agent of the disease tuberculosis. M. tuberculosis cells have a lipid-rich, waxy outer surface layer that requires special staining procedures (the acid-fast stain) to observe the cells microscopically. Table 12.29 lists some characteristics of representative mycobacteria. Filamentous Actinobacteria The streptomycetes are a subset of actinomycetes, a large group of filamentous, gram-positive Bacteria that form spores at the end of aerial filaments. Many clinically useful antibiotics such as tetracycline and neomycin have come from Streptomyces species (Table 12.31) – ANTIBIOTIC PRODUCERS. Phylum 4: Cyanobacteria and Prochlorophytes Cyanobacteria Cyanobacteria comprise a large and morphologically heterogeneous group of phototrophic Bacteria. Cyanobacteria differ in fundamental ways from purple and green bacteria, most notably in that they are oxygenic phototrophs. Cyanobacteria represent one of the major phyla of Bacteria and show a distant relationship to grampositive Bacteria. Oxygen in Earth's atmosphere is thought to have originated from cyanobacterial photosynthesis. Phylum 5: Chlamydia Chlamydia Chlamydias are extremely small parasitic bacteria that cause a variety of human diseases. Figure 12.85 shows the infection cycle of chlamydia. Chlamydias contain a very small genome and are apparently deficient in many metabolic functions.