微生物学史微生物学史 (21).ppt

Viral MultiplicationAfter reading this section,you should be able to:Compare and contrast the major steps of the life cycles of virulent phages and temperate phages Infections of Bacterial:Lysis and Lysogeny Bacteriophages can be either virulent or temperate.A virulent phage is one that has only one option:to begin multiplying immediately upon entering its bacterial host,followed by release from the host by lysis.T4 is an example of a virulent phage.Temperate phages have two options:upon entry into the host,they can multiply like virulent phages and lyse the host cell,or they can remain within the host without destroying it.Bacteriophage T4:A Virulent Bacteriophage The life cycle of T4 bacteriophage serves as our example of a virulent dsDNA phage.Virulent(lytic)bacteriophages are capable only of the lytic cycle.That is,their infection of a host always ends with cell lysis.T4 bacteriophage-Injection baseplate As with most viruses(except plant viruses),the first step of viral infection is attachment (adsorption)to the host cell surface.T4 attachment begins when a tail fiber contacts either the lipopolysaccharide or certain proteins in the outer membrane of its Escherichia coli host.As more tail fibers make contact,the baseplate settles down on the surface.After the baseplate is seated firmly on the cell surface,the baseplate and sheath change shape,and the tail sheath reorganizes so that it shortens from a cylinder 24 rings long to only 12 rings.As the sheath becomes shorter and wider,the central tube is pushed through the bacterial cell wall.The baseplate contains the protein gp5,which has lysozyme activity.Lysozyme is an enzyme that breaks the bonds linking the sugars of peptidoglycan together(see figure 33.4).Thus this protein aids in the penetration of the tube through the peptidoglycan layer.Finally,the linear DNA is extruded from the head probably through the tail tube,and into the host cell(figure 27.4e,f).The tube may interact with the plasma membrane to form a pore through which DNA passes.DNA injectionEarly mRNA madePhage DNA replicatedLate RNA madeHead and tails madeHead filledpackasomeVirions formedHost cell lysisT4 lysozymeHolinFigure 27.5 The Life Cycle of Bacteriophage T4.A diagram depicting the life cycle with the minutes after DNA injection given for each stage.Electron micrograph showing a bacterium that contains numerous completed phage virions about 30 minutes after injection.Virions lysogens or lysogenic bacteriasame virusCultivation and Enumeration of Viruses Because they are unable to reproduce outside of living cells,viruses cannot be cultured in the same way as cellular microorganisms.It is relatively simple to culture lytic bacterial and archaeal viruses as long as the host cell is easily grown in culture.Cells and virus particles are simply mixed together in a broth culture.Over time,more and more cells are infected and lysed,releasing virions into the broth.Culturing temperate viruses requires an additional step of inducing the lytic phase of their life cycles.Animal viruses are often cultivated by inoculating suitable host animals or embryonated eggs-fertilized chicken eggs incubated about 6 to 8 days after laying.Animal viruses also are grown in tissue(cell)culture on monolayers of animal cells.If the virus causes its host cell to lyse,plaques often are formed.Viruses that cause cytopathic effects are also grown and detected in tissue culture.Wordlist1.virulent phages 烈性噬菌体烈性噬菌体2.temperate phages 温和噬菌体温和噬菌体3.Lysis 裂解裂解4.Lysogeny溶原现象溶原现象5.Virulent(lytic)bacteriophages 烈烈性噬菌体性噬菌体6.attachment(adsorption)吸附吸附7.tail fiber尾丝尾丝8.the baseplate基板基板9.Sheath尾鞘尾鞘10.Lysozyme溶菌酶溶菌酶11.tail tube尾管尾管12.DNA polymerase.DNA聚合酶聚合酶13.DNA replication DNA复制复制14.Bidirectional双向的双向的15.structural proteins结结构蛋白构蛋白16.Packasome组装粒子组装粒子17.Holin孔蛋白孔蛋白18.Prophage前噬菌体前噬菌体19.Superinfection双重感双重感染染20.progeny cell子代细胞子代细胞21.Enumeration计数计数22.embryonated egg鸡鸡胚胚23.plaques 噬菌斑噬菌斑