《环境工程微生物学》课程教学资源（微生物学教案）原核微生物 A survey of prokaryotic cells and microorganisms.doc

3. Distinguish among the types of external cell appendages.Bacteria often bear accessory appendages sprouting from their surfaces. Appendages canbe divided into two major groups: those that provide motility (flagella and axial filaments)andthosethatprovide attachments orchannels(fimbriaeandpili)4. Describe the structure and position of bacterial flagella and axial filaments, and theirattachmentpatterns.Flagella provide the power of motility or self-propulsion. This allows a cell to swimfreely through an aqueous habitat. The bacterial flagellum displays three distinct parts: thefilament, the hook (sheath),and the basal body.Flagella vary both in number andarrangement according to twogeneral patterns: (1)In a polar arrangement, the fl agellaare attached atone or both endsof thecell.Three subtypes of this pattern are:monotrichous with a singleflagellum;lophotrichous with small bunches or tufts offlagella emerging from the same site,and amphitrichous with flagella at both poles ofthe cell.(2) In a peritrichous * arrangement, flagella are dispersed randomly over thesurfaceofthecell.The periplasmicflagella oraxial filaments.Aperiplasmicflagellum isatype of internalflagellum that is enclosed in the space between the outer sheath and the cell wallpeptidoglycan. The filaments curl closely around the spirochete coils yet are free tocontractand imparta twisting orflexingmotion to thecell.Thisformoflocomotion mustbe seen in live cells such as thespirocheteof syphilistobetrulyappreciated5.Discuss the structure and functions of pili and fimbriaeThe structures termedfimbria and pilus bothrefertobacterial surface appendages thatare involved in interactions with other cells but do not provide locomotion, except forsomespecialized pili.Fimbriae are small, bristlelikefi bers emerging fromthe surface of many bacterial cells.Their exact composition varies, but most of them contain protein.Fimbriae have aninherent tendencytostick toeach other and tosurfaces.Theymayberesponsibleforthemutual clinging of cells that leads to biofilms and other thick aggregates of cells on thesurface of liquids and for the microbial colonization of inanimate solids such as rocksandglass.Apilus (also called a sex pilus)is an elongate, rigid tubular structure made ofa specialprotein, pilin.So far, true pili have been found only on gram-negative bacteria, wherethey are utilized primarily in a“mating”process between cells called conjugation,which involves a transfer of DNA from one cell to another.6.Define glycocalyx, and describe its different forms and functions.Thebacterial cell surface isfrequently exposedto severeenvironmental conditions.Theglycocalyx develops as a coating of macromolecules to protect the cell and, in some cases,help it adhere to its environment Glycocalyces differ among bacteria in thickness,organization,and chemical composition.Somebacteria arecovered with a loose shield calleda slime layer that evidently protects them from dehydration and loss of nutrients.Otherbacteria produce capsules composed ofrepeating polysaccharide units, of protein, or of both.2

2 3. Distinguish among the types of external cell appendages. Bacteria often bear accessory appendages sprouting from their surfaces. Appendages can be divided into two major groups: those that provide motility (flagella and axial filaments) and those that provide attachments or channels (fimbriae and pili). 4. Describe the structure and position of bacterial flagella and axial filaments, and their attachment patterns. Flagella provide the power of motility or self-propulsion. This allows a cell to swim freely through an aqueous habitat. The bacterial flagellum displays three distinct parts: the filament, the hook (sheath), and the basal body. Flagella vary both in number and arrangement according to two general patterns: (1) In a polar arrangement, the fl agella are attached at one or both ends of the cell. Three subtypes of this pattern are: monotrichous with a single flagellum; lophotrichous with small bunches or tufts of flagella emerging from the same site; and amphitrichous with flagella at both poles of the cell. (2) In a peritrichous * arrangement, flagella are dispersed randomly over the surface of the cell. The periplasmic flagella or axial filaments. A periplasmic flagellum is a type of internal fl agellum that is enclosed in the space between the outer sheath and the cell wall peptidoglycan. The filaments curl closely around the spirochete coils yet are free to contract and impart a twisting or flexing motion to the cell. This form of locomotion must be seen in live cells such as the spirochete of syphilis to be truly appreciated 5. Discuss the structure and functions of pili and fimbriae. The structures termed fimbria and pilus both refer to bacterial surface appendages that are involved in interactions with other cells but do not provide locomotion, except for some specialized pili. Fimbriae are small, bristlelike fi bers emerging from the surface of many bacterial cells. Their exact composition varies, but most of them contain protein. Fimbriae have an inherent tendency to stick to each other and to surfaces. They may be responsible for the mutual clinging of cells that leads to biofilms and other thick aggregates of cells on the surface of liquids and for the microbial colonization of inanimate solids such as rocks and glass. A pilus (also called a sex pilus) is an elongate, rigid tubular structure made of a special protein, pilin . So far, true pili have been found only on gram-negative bacteria, where they are utilized primarily in a “mating” process between cells called conjugation, which involves a transfer of DNA from one cell to another. 6. Define glycocalyx, and describe its different forms and functions. The bacterial cell surface is frequently exposed to severe environmental conditions. The glycocalyx develops as a coating of macromolecules to protect the cell and, in some cases, help it adhere to its environment. Glycocalyces differ among bacteria in thickness, organization, and chemical composition. Some bacteria are covered with a loose shield called a slime layer that evidently protects them from dehydration and loss of nutrients. Other bacteria produce capsules composed of repeating polysaccharide units, of protein, or of both

4.3The cell envelope: the boundary layer of bacteria.1.Explain the concept of the cell envelope, and describe its structureThe majority of bacteriahavea chemicallycomplex external coveringtermed the cellenvelopethat lies outside of the cytoplasm.It is composed of twomain layers:the cell walland the cell membrane.These layers are stacked together and often tightlybound into a unitlike the outer husk and casings of a coconut. Although each envelope layer performs adistinct function, together they act as a single unit that maintains cell integrity.2. Outline the structure andfunctions ofcell walls,and explain the role of peptidoglycanIn general, the cell wall helps determine the shape of a bacterium, and it also provides thekind of strong structural support necessary to keep a bacterium from bursting or collapsingbecause of changes in osmotic pressure.The cell walls of most bacteria gain their relativestrength and stability from a unique macromolecule called peptidoglycan (PG).Thiscompound is composed of a repeating framework of long glycan chains cross-linked byshort peptide fragments.The amount and exact composition of peptidoglycan vary amongthemajorbacterialgroups3. Contrast the major structure of gram-positive and gramnegative cell walls.The bulk of the gram-positive cell wall is a thick, homogeneous sheath of peptidoglycanrangingfrom20 to80nm in thickness.Italsocontains tightlybound acidicpolysaccharidesincluding teichoic acid directly attached to the peptidoglycan and lipoteichoic acid. Cellwall teichoic acid is a polymer of ribitol or glycerol and phosphate embedded in thepeptidoglycan sheath. Lipoteichoic acid is similar in structure but is attached to the lipids inthe plasma membrane.These molecules appear to function in cell wall maintenance andenlargement during cell division. They also move cations into and out of the cell andstimulate a specific immune response.The cell wall of gram-positivebacteria is oftenpressed tightly against the cell membrane with very little space between them, but in somecells, a thin periplasmic space is evident between the cell membrane and cell wall.Thegram-negativecell wall ismore complex inmorphologybecauseit is composed ofan outermembrane(OM)andathinner shell ofpeptidoglycan.Theoutermembraneissomewhat similar in construction to the cell membrane,except that it contains specializedtypes of lipopolysaccharides(LPS)and lipoproteins.Lipopolysaccharides are composedof lipid molecules bound to polysaccharides.The lipids form thematrix ofthetoplayer ofthe OM, and thepolysaccharide strands project from the lipid surface.The lipid portionmay become toxic when it is released during infections. The polysaccharides give rise tothe somatic(O)antigen in gram-negative pathogens and can beused in identification.Theymayalso function as receptors and in blocking host defenses.Twotypes of proteins arelocated in the OM. The porins are inserted in the upper layer of the outer membrane. Theyhave some regulatory control overmolecules entering and leaving the cell.Many qualitiesof the selective permeability of gram-negative bacteria tobile, disinfectants, and drugs aredue to the porins. Some structural proteins are also embedded in the upper layer of the OM.Thebottomlayerof theoutermembraneis similartothecell membraneinitsoverallstructure and is composed of phospholipids and lipoproteins. There is a well-developed3

3 4.3 The cell envelope: the boundary layer of bacteria. 1. Explain the concept of the cell envelope, and describe its structure. The majority of bacteria have a chemically complex external covering, termed the cell envelope that lies outside of the cytoplasm. It is composed of two main layers: the cell wall and the cell membrane. These layers are stacked together and often tightly bound into a unit like the outer husk and casings of a coconut. Although each envelope layer performs a distinct function, together they act as a single unit that maintains cell integrity. 2. Outline the structure and functions of cell walls, and explain the role of peptidoglycan. In general, the cell wall helps determine the shape of a bacterium, and it also provides the kind of strong structural support necessary to keep a bacterium from bursting or collapsing because of changes in osmotic pressure. The cell walls of most bacteria gain their relative strength and stability from a unique macromolecule called peptidoglycan (PG). This compound is composed of a repeating framework of long glycan chains cross-linked by short peptide fragments. The amount and exact composition of peptidoglycan vary among the major bacterial groups. 3. Contrast the major structure of gram-positive and gramnegative cell walls. The bulk of the gram-positive cell wall is a thick, homogeneous sheath of peptidoglycan ranging from 20 to 80 nm in thickness. It also contains tightly bound acidic polysaccharides, including teichoic acid directly attached to the peptidoglycan and lipoteichoic acid. Cell wall teichoic acid is a polymer of ribitol or glycerol and phosphate embedded in the peptidoglycan sheath. Lipoteichoic acid is similar in structure but is attached to the lipids in the plasma membrane. These molecules appear to function in cell wall maintenance and enlargement during cell division. They also move cations into and out of the cell and stimulate a specific immune response. The cell wall of gram-positive bacteria is often pressed tightly against the cell membrane with very little space between them, but in some cells, a thin periplasmic space is evident between the cell membrane and cell wall. The gram-negative cell wall is more complex in morphology because it is composed of an outer membrane (OM) and a thinner shell of peptidoglycan. The outer membrane is somewhat similar in construction to the cell membrane, except that it contains specialized types of lipopolysaccharides (LPS) and lipoproteins. Lipopolysaccharides are composed of lipid molecules bound to polysaccharides. The lipids form the matrix of the top layer of the OM, and the polysaccharide strands project from the lipid surface. The lipid portion may become toxic when it is released during infections. The polysaccharides give rise to the somatic (O) antigen in gram-negative pathogens and can be used in identification. They may also function as receptors and in blocking host defenses. Two types of proteins are located in the OM. The porins are inserted in the upper layer of the outer membrane. They have some regulatory control over molecules entering and leaving the cell. Many qualities of the selective permeability of gram-negative bacteria to bile, disinfectants, and drugs are due to the porins. Some structural proteins are also embedded in the upper layer of the OM. The bottom layer of the outer membrane is similar to the cell membrane in its overall structure and is composed of phospholipids and lipoproteins. There is a well-developed

periplasmic space above and below the peptidoglycan.This space is an importantreactionsite for a large and varied pool of substances that enter and leave the cell.TABLE 4.1Comparisonof Gram-PositiveandGram-Negative Cell WallsCharacteristicGram-PositiveGram-NegativeOneTwoNumberof major layersPeptidoglycanChemical compositionLipopolysaccharide (LPS)Teichoic acidLipoproteinLipoteichoic acidPeptidoglycanMycolic acids andPorin proteinspolysaccharides*Overall thicknessThicker (20-80 nm)Thinner (8-11 nm)NoYesOutermembraneNarrowExtensivePeriplasmicspacePermeabilitytomoleculesMorepenetrableLess penetrable4.Relate the characteristics ofother types ofcell walls and wall-freecells.Several bacterial groups lack the cell wall structure of gram-positive or gram-negativebacteria, and somebacteria haveno cell wall at all.Although theseexceptional forms can stainpositive or negative in the Gram stain, examination of their fine structure and chemistry showsthat they do not fit the descriptions for typical gram-negative or positive cells.5. Describe the structure of the cell membrane, and explain several ofits major roles in bacterialcells.Appearing just beneath the cell wall is the cell, or cytoplasmic, membrane, a very thin (5 -10nm),flexible sheetmolded completelyaroundthecytoplasm.Ingeneralcomposition,itisalipid bilayer with proteins embedded to vary ing degrees. In some locations, the cell membraneforms internal folds in the cytoplasm called mesosomes.These areprominent in gram-positivebacteria but are harder to see in gram-negative bacteria because of their relatively small sizeThe cell membraneprovidesa siteforenergy reactions,nutrientprocessing,andsynthesis.1.4Bacterial internalstructure.1.Listthecontentsofthecell cytoplasmThe cell membrane surrounds a complex solution referred to as cytoplasm, or cytoplasmicmatrix.This chemical“pool"is aprominent siteformanyof the cell'sbiochemical andsynthetic activities. Its major component is water (70%-80%), which serves as a solventfor a complex mixture of nutrients including sugars, amino acids, and other organicmoleculesand salts.The components of this pool serveasbuildingblocksforcell synthesisor as sources of energy.The cytoplasm also holds larger, discrete bodies such as thechromosome, ribosomes, granules, and actin strands.2.Describefeaturesof the bacterial chromosomeandplasmids.The hereditary material of most bacteria exists in the form of a single circular strand of4

4 periplasmic space above and below the peptidoglycan. This space is an important reaction site for a large and varied pool of substances that enter and leave the cell. 4. Relate the characteristics of other types of cell walls and wall-free cells. Several bacterial groups lack the cell wall structure of gram-positive or gram-negative bacteria, and some bacteria have no cell wall at all. Although these exceptional forms can stain positive or negative in the Gram stain, examination of their fine structure and chemistry shows that they do not fi t the descriptions for typical gram-negative or positive cells. 5. Describe the structure of the cell membrane, and explain several of its major roles in bacterial cells. Appearing just beneath the cell wall is the cell, or cytoplasmic, membrane, a very thin (5– 10 nm), flexible sheet molded completely around the cytoplasm. In general composition, it is a lipid bilayer with proteins embedded to varying degrees. In some locations, the cell membrane forms internal folds in the cytoplasm called mesosomes. These are prominent in gram-positive bacteria but are harder to see in gram-negative bacteria because of their relatively small size. The cell membrane provides a site for energy reactions, nutrient processing, and synthesis. 1.4 Bacterial internal structure. 1. List the contents of the cell cytoplasm. The cell membrane surrounds a complex solution referred to as cytoplasm, or cytoplasmic matrix. This chemical “pool” is a prominent site for many of the cell’s biochemical and synthetic activities. Its major component is water (70%–80%), which serves as a solvent for a complex mixture of nutrients including sugars, amino acids, and other organic molecules and salts. The components of this pool serve as building blocks for cell synthesis or as sources of energy. The cytoplasm also holds larger, discrete bodies such as the chromosome, ribosomes, granules, and actin strands. 2. Describe features of the bacterial chromosome and plasmids. The hereditary material of most bacteria exists in the form of a single circular strand of

DNA designated as thebacterial chromosome.Although the chromosome is the minimalgenetic requirement for bacterial survival, many bacteria contain other, nonessential piecesof DNAcalledplasmids.Thesetiny strands existapartfromthechromosome,although attimes they can become integrated into it.Duringbacterial reproduction,they are duplicatedandpassedontooffspring.Theyarenot essential to bacterial growthandmetabolism,buttheyoften conferprotectivetraits suchas resisting drugs and producingtoxins and enzymesBecause they can be readily manipulated in the laboratory and transferred from onebacterial cell to another,plasmids are an important agent in modern genetic engineeringtechniques.3.Characterizethebacterial ribosomes and cytoskeletonA bacterial cell contains thousands of ribosomes, which are made of RNA and proteinWhen viewed even by very high magnification, ribosomes show up as fine, sphericalspecks dispersed throughout the cytoplasm that often occur in chains (polysomes).Manyarealso attachedtothecell membrane.Chemically,a ribosome isacombination ofaspecialtypeofRNAcalled ribosomalRNA,orrRNA(about60%),andprotein(40%)4.Describe inclusion bodies and granules,andexplaintheirimportanceto cells.Most bacteria are exposed to severe shifts in the availability of food. During periods ofnutrient abundance, some can compensate by storing nutrients as inclusion bodies,orinclusions, of varying size,number,and content As theenvironmental source of thesenutrients becomes depleted, the bacterial cell canmobilize its own storehouse as required.Some inclusion bodies contain condensed, energy-rich organic substances, such asglycogen and poly b-hydroxybutyrate (PHB), within special single-layered membranes. Aunique type of inclusion found in some aquatic bacteria is gas vesicles that providebuoyancy and flotation.Otherinclusions,also calledgranules, contain crystalsof inorganiccompoundsandare notenclosed bymembranes5.Describe the life cycle of endospore-forming bacteria including the fomation andgerminationofendospores.These bacteria have a two-phase life cycle that shifts between a vegetative cell and anendospore. The vegetative cell is the metabolically active and growing phase. Whenexposed to certain environmental signals,itforms an endosporeby a process termedsporulation.The spore exists in an inert, resting condition that is capable of high resistanceandverylong-term survival.6.Discusstheresistanceand significanceofendospores.Bacterial endospores are the hardiest of all life forms, capable of withstanding extremes inheat, drying,freezing,radiation, and chemicals that would readily kill ordinary cells.Although themajorityof spore-formingbacteria are relativelyharmless, several bacterialpathogens are sporeformers. In fact, some aspects of the diseases they cause are related tothe persistence andresistanceoftheir spores.Hospitals and clinicsmust takeprecautionstoguard againstthepotential harmful effectsof endospores in wounds.Endospore destruction is aparticular concern of the5

5 DNA designated as the bacterial chromosome. Although the chromosome is the minimal genetic requirement for bacterial survival, many bacteria contain other, nonessential pieces of DNA called plasmids. These tiny strands exist apart from the chromosome, although at times they can become integrated into it. During bacterial reproduction, they are duplicated and passed on to offspring. They are not essential to bacterial growth and metabolism, but they often confer protective traits such as resisting drugs and producing toxins and enzymes. Because they can be readily manipulated in the laboratory and transferred from one bacterial cell to another, plasmids are an important agent in modern genetic engineering techniques. 3. Characterize the bacterial ribosomes and cytoskeleton. A bacterial cell contains thousands of ribosomes, which are made of RNA and protein. When viewed even by very high magnification, ribosomes show up as fine, spherical specks dispersed throughout the cytoplasm that often occur in chains (polysomes). Many are also attached to the cell membrane. Chemically, a ribosome is a combination of a special type of RNA called ribosomal RNA, or rRNA (about 60%), and protein (40%). 4. Describe inclusion bodies and granules, and explain their importance to cells. Most bacteria are exposed to severe shifts in the availability of food. During periods of nutrient abundance, some can compensate by storing nutrients as inclusion bodies, or inclusions, of varying size, number, and content. As the environmental source of these nutrients becomes depleted, the bacterial cell can mobilize its own storehouse as required. Some inclusion bodies contain condensed, energy-rich organic substances, such as glycogen and poly b-hydroxybutyrate (PHB), within special single-layered membranes. A unique type of inclusion found in some aquatic bacteria is gas vesicles that provide buoyancy and flotation. Other inclusions, also called granules, contain crystals of inorganic compounds and are not enclosed by membranes. 5. Describe the life cycle of endospore-forming bacteria, including the formation and germination of endospores. These bacteria have a two-phase life cycle that shifts between a vegetative cell and an endospore. The vegetative cell is the metabolically active and growing phase. When exposed to certain environmental signals, it forms an endospore by a process termed sporulation. The spore exists in an inert, resting condition that is capable of high resistance and very long-term survival. 6. Discuss the resistance and significance of endospores. Bacterial endospores are the hardiest of all life forms, capable of withstanding extremes in heat, drying, freezing, radiation, and chemicals that would readily kill ordinary cells. Although the majority of spore-forming bacteria are relatively harmless, several bacterial pathogens are sporeformers. In fact, some aspects of the diseases they cause are related to the persistence and resistance of their spores. Hospitals and clinics must take precautions to guard against the potential harmful effects of endospores in wounds. Endospore destruction is a particular concern of the