微生物学英文教学课件：chapter3 Procaryotic Cell Structure and Function(continue)

1、CHAPTER 3 Procaryotic Cell Structure and Function(Continue)3.6 Components external to the cell wall Fimbriae and pili Flagella Capsules and slime layersFimbriae can be evenly distributed over the entire surface of the cell. They can number anywhere from a few to several hundred per cell. Fimbriae al

2、low a cell to adhere to surfaces including the surfaces of other cells. Pili are usually longer than fimbriae and number only one or two per cell. Pili function to join bacterial cells prior to the transfer of DNA from one cell to another. F pilusMotility allows the cell to reach different regions o

3、f its environment. In the struggle for survival, movement to a new location may mean the difference between survival and death of the cell. But, as in any physical process, cell movement is closely tied to an energy expenditure, and the movement of flagella is no exception. Many prokaryotes are moti

4、le, and this ability to move independently is usually due to a special structure, the flagellum (plural, flagella). Four basic types of flagellar arrangementsa, monotrichousb, amphitrichousc, lophotrichousd, peritrichousFlagella are arranged differently on different bacteria. In polar flagellation t

5、he flagella are attached at one or both ends of the cell. Occasionally a tuft (group) of flagella may arise at one end of the cell, an arrangement called lophotrichous. In peritrichous flagellation the flagella are inserted at many places around the cell surface (peri means around). The type of flag

6、ellation, polar or peritrichous, is often used as a characteristic in the classification of bacteria.The flagellum of a Gram-negative bacteriumThe filament of bacterial flagella is composed of subunits of a protein called flagellin. The base of the flagellum is different in structure from that of th

7、e filament. There is a wider region at the base of the flagellum called the hook. The hook consists of a single type of protein and functions to connect the filament to the motor portion of the flagellum. The basal body is anchored in the cytoplasmic membrane and cell wall. The basal body consists o

8、f a small central rod that passes through a system of rings.In gram-negative Bacteria, an outer ring is anchored in the lipopolysaccharide layer and another in the peptidoglycan layer of the cell wall, and an inner ring is located within the cytoplasmic membrane.In gram-positive Bacteria, which lack

9、 the outer lipopolysaccharide layer, only the inner pair of rings is present. Surrounding the inner ring and anchored in the cytoplasmic membrane are a pair of proteins called Mot . These proteins actually drive the flagellar motor causing a torque that rotates the filament. A final set of proteins,

10、 called the Fli proteins function as the motor switch, reversing rotation of the flagella in response to intracellular signals.The movement of a procaryotic flagellum results from rotation of its basal body and is similar to the movement of the shaft of an electric motor. Bacterial cells can alter t

11、he speed and direction of rotation of flagella and thus are capable of various patterns of motility.Many prokaryotic organisms secrete on their surfaces slimy or gummy materials. A variety of these structures consist of polysaccharide, and a few consist of protein. The terms capsule and slime layer

12、are frequently used to describe polysaccharide layers.Demonstration of the presence of a capsule is usually by means of negative staining Capsules and Slime Layers (a) capsule capsule (b) (b) slime layerslime layer FlocMany prokaryotes contain a cell surface layer composed of a two-dimensional array

13、 of protein. These layers are called S-layers. S-layers have been detected in representatives of virtually every phylogenetic grouping of Bacteria and are nearly universal among Archaea. In some species of Archaea the S-layer is also the cell wall. The major function of S-layers is unknown. However,

14、 as the interface between the cell and its environment it is likely that in cells that produce them the S-layer at least functions as an external permeability barrier, allowing the passage of low-molecular-weight substances while excluding large molecules. Paracrystalline Surface Layers (S-Layers)3.

15、7 Chemotaxis3.7 Chemotaxis Bacteria do not always swim aimlessly but are attracted by such nutrients as sugars and amino acids, and are repelled by many harmful substances and bacterial waste products. (Bacteria also can respond to other environmental cues such as temperature, light, and gravity). M

16、ovement toward chemical attractants and away from repellents is known as chemotaxischemotaxis. . Such behavior is of obvious advantage to bacteria. Chemotaxis may be demonstrated by observing bacteria in the chemical gradient produced when a thin capillary tube is filled with an attractant and lower

17、ed into a bacterial suspension. As the attractant diffuses from the end of the capillary, bacteria collect and swim up the tube. The number of bacteria within the capillary after a short length of time reflects the strength of attraction and rate of chemotaxis. Positive and negative chemotaxis also

18、can be studied with petri dish cultures (figure 3.37figure 3.37). If bacteria are placed in the center of a dish of agar containing an attractant, the bacteria will exhaust the local supply and then swim outward following the attractant gradient they have created. The result is an expanding ring of

19、bacteria. Bacteria can respond to very low levels of attractants (about 10-8 M for some sugars), the magnitude of their response increasing with attractant concentration. Usually they sense repellents only at higher concentrations. If an attractant and a repellent are present together, the bacterium

20、 will compare both signals and respond to the chemical with the most effective concentration. It should be noted here that a similar mechanism is used to respond to other environmental factors such as oxygen (aerotaxis), light (phototaxis), temperature (thermotaxis),and osmotic pressure (osmotaxis).

21、3.8 Bacterial endospores Certain species of bacteria produce special structure called endospores. They are very resistant to heat and can not be destroyed easily, even by harsh chemicals. Endospores are also resistant to other harmful agents such as drying, radiation, acids and chemical disinfectant

22、s.Sporulating cellCentral coreCortexSpore coat / membraneexosporiumMicrograph of a endosporeVegetative cell Containing abundant DPA (dipicolinic acid) which is combined with calcium ions. Lower water content only 10-30% of the water content of the vegetative cell. Low pH value, and contains small ac

23、id-soluble spore proteins (SASPs)Structure of DPA Properties of endospore and its resistance In summary, endospore heat resistance probably is due to several factors: calcium-dipicolinate and acid-soluble protein stabilization of DNA, protoplast dehydration,the spore coat, DNA repair, the greater st

24、ability of cell proteins in bacteria adapted to growth at high temperatures,and others.Sporulation involves a very complex series of events in cellular differentiation. Bacterial sporulation does not occur when cells are dividing exponentially but only when growth ceases owing to the exhaustion of a

25、n essential nutrient. Thus, cells of Bacillus cease vegetative growth and begin sporulation when a key nutrient such as the carbon or nitrogen source becomes limiting.Endospore formatiom1, Axial filament formation2, Septum formatiom3, Engulfment of forespore4, Cortex formation5, Coat synthesis6, Com

26、pletion of coat synthesis, Increase in refractility and heat resistance7, Lysis of sporangium, spore liberation Stages in endospore formationSpore germination1. Activation Usually results from treatments like heating.2. Germination Breaks spores dormant state. This process is characterized by spore

27、swelling, loss of resistance to heat and other stresses, loss of refractility and increase in metabolic activity.3. Outgrowth The spore protoplast makes new components and develops once more into an active bacterium. Parasporal Crystal(Spore-companioned crystal) Several Bacillus species, most notabl

28、y B.popilliae and B. thuringiensis, produce intracellular crystals of toxic glycoproteins when they sporulate.Toxic crystals3.9 ActinomycetesActinomycetes The actinomycetes actinomycetes s.,actinomycete are aerobic, gram-positive bacteria that form branching filaments or hyphae that do not normally

29、undergo fragmentation and produce asexual spores. Although they are of bacterial dimensions,they closely resemble fungi in overall morphology. When growing on a solid substratum such as agar, the branching network of hyphae developed by actinomycetes grows both on the surface of the substratum and i

30、nto it to form a substrate mycelium. Septa usually divide the hyphae into long cells (20m and longer)containing several nucleoids. Many actinomycetes also have an aerial mycelium that extends above the substratum and forms asexual,thin-walled spores called conidia conidia s., conidium or conidiospor

31、es conidiospores on the ends of filaments. If the spores are located in a sporangium, they are called sporangiospores. sporangiospores. The spores can vary greatly in shape. Actinomycete spores develop by septal formation at filament tips, usually in response to nutrient deprivation. Most are not pa

32、rticularly heat resistant but do withstand desiccation well and thus have considerable adaptive value. Most actinomycetes are not motile. When motility is present,it is confined to flagellated spores. Actinomycete cell wall composition varies greatly among different groups and is of considerable tax

33、onomic importance. Some other taxonomically valuable properties are the morphology and color of mycelia and sporangia, the surface features and arrangement of conidiospores, the percent G+C in DNA, the phospholipid composition of cell membranes, and spore heat resistance. Actinomycetes have consider

34、able practical significance. They are primarily soil inhabitants and are very widely distributed. They can degrade an enormous number and variety of organic compounds and are extremely important in the mineralization of organic matter. Actinomycetes produce most of the medically useful natural antib

35、iotics (90% produced by Streptomyces). Although most actinomycetes are free-living microorganisms, a few are pathogens of humans, other animals, and some plants. CyanobacteriaThe cyanobacteria have typical prokaryotic cell structures and a normal gram-negative cell wall. They range in diameter from

36、about 1 10 m and may be unicellular or form filaments. They have chlorophyll and carry out oxygen-producing photosynthesis, much as plants and the eukaryotic algae do. Filamentous Cyanobacterium, Anabaena sp. (SEM x5,000) Nonfilamentous cyanobacteriaThe morphological diversity of the cyanobacteria i

37、s considerable. Both unicellular and filamentous forms are known, and considerable variation within these morphological types occurs. Heterocysts have intercellular connections with adjacent vegetative cells, and there is mutual exchange of materials between these cells, with products of photosynthe

38、sis moving from vegetative cells to heterocysts and products of nitrogen fixation moving from heterocysts to vegetative cells. Photosynthesis Nitrogen fixationMain function of CyanobacteriaThe nutrition of cyanobacteria is simple. Vitamins are not required, and nitrate or ammonia is used as nitrogen

39、 source. Nitrogen-fixing species are common. Most species tested are obligate phototrophs, However, some cyanobacteria are able to grow in the dark on organic compounds, using the organic material as both carbon and energy source.Physiology of cyanobacteria:Problems: Many cyanobacteria produce poten

40、t neurotoxins, and during water blooms when massive accumulations of cyanobacteria may develop, animals ingesting such water may succumb rapidly. Other prokaryotesRickettsia Chlamydia Mycoplasma Bdellovirio1. 0.2-0.5m in diameter. obligate intracellular parasites. The majority of them are gram-negat

41、ive and multiply only within host cells. 2. Binary fission within host cells.They lack the enzymatic capability to produce sufficient amounts of ATP to support their reproduction. They obtain the ATP from host cells.3. Many species of them cause disease in humans and other animals.RickettsiaThey are

42、 obligate intracellular parasites, unable to generate sufficient ATP to support their reproduction.gram-negative and cell divides by binary fissionThey Chlamydia cause human respiratory and genitourinary tract disease, and in birds they cause respiratory disease.ChlamydiaDiameter=0.1-0.25 m. They la

43、ck cell wall, are bounded by a single triple-layered membrane.They are the smallest organisms capable of self-reproduction. The colony is “fried egg” appearance.Several of them cause diseases in humans. (pneumonia, respiratory tract disease)MycoplasmaBdellovirioSee Movie for the detail of infection

44、and reproductionMovieClassification of bacteria MORPHOLOGICAL CHARACTERISTICS DIFFERENTIAL STAINING NUCLEIC ACID HYBRIDIZATION GENETIC RECOMBINATION NUMERICAL TAXONOMYFungi Plants Animals ProtistaProkaryotaeFive-kingdom system is a commonly accepted system of classificationArchaebacteriaEubacteriaEu

45、karyotesThe taxonomic classification scheme for bacteria may be found in Bergeys Manual of Systematic Bacteriology. In Bergeys Manual, bacteria are divided into four divisions. Three divisions consist of eubacterial cells, and the fourth division consists of the archaeobacteria. Each division is div

46、ided into classes; Classes are divided into orders;families; genera; species.Bacterial species is defined simply as a population of cells with similar characteristics. strain is a group of cells all derived from a single cell. MORPHOLOGICAL CHARACTERISTICS Morphological characteristics are useful in

47、 identifying bacteria. For example, differences in such structures as endospores or flagella can be helpful.However, many microorganisms appear too similar to be classified by their structures. DIFFERENTIAL STAINING(for example Gram staining) Most bacteria are either gram-positive or gram-negative.

48、But not useful in identifying either the wall-less bacteria or the archaeobacteria with unusual walls.NUCLEIC ACID HYBRIDIZATION GENETIC RECOMBINATIONSummary1, Prokaryotic genetic material is located in an area called the nucleoid and is not enclosed by a membrane.2, Most bacteria have a cell wall o

49、utside the plasma membrane to give them shape and protect them from osmotic lysis.Bacterial walls are chemically complex and usually contain peptidoglycan or murein.3, Bacteria often are classfied as either gram positive or gram negative based on differences in cell wall structure and their response

50、 to Gram staining. Gram-positive walls have thick,homogeneous layers of peptidoglycan and teichoid acid. Gram-negative bacteria have a thin peptidoglycan layer surrounded by a complex outer membrane containing lipopolysaccharides and other components4, Some bacteria are motile, usually by means of f

51、lagella, and bacterial species differ in the number and distribution of their flagella.5, Structures such as capsules, fimbriae, and sex pili are found outside of some bacterial cell wall.6, Some bacteria survive adverse environmental conditions by forming endospores, dormant structures resistant to

52、 heat,desiccation and many chemicals7. Actinomycetes are aerobic, gram-positive bacteria that form branching, usually nonfragmenting, hyphae and asexual spores.8. The asexual spores borne on aerial mycelia are called conidiospores or conidia if they are at the tip of hyphae and sporangiospores whent

53、hey are within sporangia.9. Actinomycetes have several distinctively different types of cell walls and often also vary in terms of the sugars present in cell extracts. Properties such as color and morphology are also taxonomically useful.10. Streptomycetes are important in the degradation of more re

54、sistant organic material in the soil and produce many useful antibiotics. A few cause diseases in plants and animals.HOMEWORKI.MULTIPLE-CHOICE QUESTIONS1. The major locomotor structures in bacteria are a.flagella b. fimbriae c. pili d. cilia2. Which of the following is a primary bacterial cell wall function?a. transport b. supportc. motility d. adhesion3. A bacterial arrangement in packets of eight cells is described as a ( ).a. micrococcus b. tetradc. diplococcus d. sarcina4. Which of the following is not found in all bact