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Bacterial Viruses

Introduction--Viral infections--acute infectious illnesses


            Obligate intracellular parasite--animals, plants & microbes

                        lack metabolic machinery  (vital function)

                        viral genes--DNA or RNA enclosed in protein coat

            Virion--structurally complete infectious virus

            Bacteriophage--bacterial viruses

            Frederick W. Twort 1915--filterable bacteriolytic agent

            Felix d’Herelle  (Pasteur Institute)  1917--bacteriophage

General Characteristics

            Wide distribution in nature--phages exist for practically all bacteria

            Nucleic acid core surrounded by protein  (various shapes)

            Two main types of viruses

                        lytic  (virulent)--infected cells produce new phages

                                    lytic cylce

                        temperate  (avirulent)--lysogenic--viral nucleic acid

                                    reproduced from bacterial generation to generation

            Nomenclature--coded symbols  (researcher specific)

                        oX174 or lambda (  )

Morphology and Structure

            Nucleic acid core

                        Capsid--protein subunits--capsomeres--protomers

            Morphological types:

A.  complex--hexagonal head, rigid tail, contractile

            sheath and tail fibers

B.  similar to A--lacks sheath, tail if flexible and

            may or may not possess tail fibers

C.  similar to B--shorter tail than head

D.  Head of large capsomeres, “tail-less”

E.  similar to D--small capsomers

F.  Filamentous

Phage structure

            Cubic  (polyhedra)

                        icosahedra  (20 facets--equilateral triangles

                                    form 12 vertices)  capsomeres at each vertex

            Helical  (rod-shaped)

                        helically arranged capsomeres

Phage nucleic acids

            Phage morphology is dependent upon nucleic acid type

            tailed phages--double stranded DNA

            large capsomeres  (D) and filamentous  (F)

                        single stranded DNA

            small capsomeres  (E)  single strand RNA

            circular and linear arrangements based on

                        conditions of host and virion

Bacteriophages of E. coli  (strain B)  T1 to T7  (coliophages)

            Composed of DNA and protein in 1:1 proportions

            Morphology types A, B, and C

                        length  65 nm to 200 nm  (T3 & T7)

                        width  50 nm to 80 nm

                        type F--f2, fd, and f1 smaller than T phages

single strand RNA  (linear)

Circular single strand DNA--icosahedral phage oX174

Filamentous single strand DNA  (non-lytic)

Isolation and Cultivation of Bacterial Viruses

            Isolated and cultured from young active bacterial cultures

                        broth or agar plate media

                        plaque formation on agar plate culture

            Principle requirement for isolation and cultivation of

                        phages is the optimal conditions for growth of the host

            Phages most easily isolated from hosts environment

            Medical application of virulent phages

                        phage typing--detection and identification of pathogenic bacteria

characterized by host resistance or susceptibility to infection

 by phages

Reproduction of Bacterial Viruses

            Absorption and penetration  (receptor site recognition)

                        lysozyme may facilitate cell wall penetration

                                    phage tail associated enzyme

                        penetration achieved by

1)      tail fibers attach to cell wall

2)      tail sheath contracts--driving tail core though cell wall and membrane

3)      injection of viral DNA

RNA and filamentous coliphages are absorbed on pili

            pilus retraction initiates nucleic acid injection

Replication, Assembly and Lysis

            Injection of vial nucleic acid causes bacteria to

                        synthesize viral nucleic acids

            Growth curve

                        latent period--no infectious virus demonstrated

                        rise period--cell lysis and release of phages

            Lysogeny--viral DNA  (temperate phage)  is incorporated into bacterial DNA to

 from a prophage

                                    Viral DNA is transmitted to each successive generation as a “gene”

of the bacterial chromosome

            Only one gene of phage genes is expressed

                        Repressor molecule synthesized which

 resists lysis or lytic infection of

 host cell


                                    Medical implications of lysogeny

                                                lysogenic conversion--prophage changes properties of host

                                                            Corynebacterium diphtheriae--toxins

                                                            Streptococcus pyogenes--scarlet fever

                                                                        erythrogenic toxin

                                                            Clostrodium botulinum--toxin

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