microbio 2125 unit 6 part 2

Flexible cell membrane is penetrated by whole virus or its nucleic acid by:

Fusion or Endocytosis

Fusion

envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm

Endocytosis

entire virus is engulfed and enclosed in vacuole or vesicle

Synthesis and Assembly

Require DNA or RNA replication and protein production

• Mechanism varies depending on whether virus is DNA or RNA virus

DNA viruses generally are replicated and assembled in…

nucleus, from which they bud

RNA viruses generally are replicated and assembled in…

cytoplasm, from which they bud

• Positive-sense RNA contain message for translation

• Negative-sense RNA must be converted into positive-sense message

release: Budding

exocytosis; nucleocapsid binds to membrane which pinches off and sheds viruses gradually; cell is not immediately destroyed

release: Lysis

nonenveloped and complex viruses released when cell dies and ruptures

Number of viruses released is variable

• 3,000-4,000 released by poxvirus

• >100,000 released by poliovirus

Cytopathic Effects

• virus-induced damage to cells

  • Changes in size and shape

  • Cytoplasmic and nuclear inclusion bodies

  • Cells fuse to form giant multi-nucleated cells (syncitia)

  • Cell lysis

  • Alter DNA

  • Transform cells into cancerous cells

Persistent infections

cell harbors virus and is not immediately lysed

• Can last weeks or host’s lifetime; several can periodically reactivate (chronic latent state)

examples of persistent infections

• Measles virus: may remain hidden in brain cells for many years

• Herpes simplex virus: cold sores and genital herpes

• Herpes zoster virus: chickenpox and shingles

Transformation

some animal viruses enter host cell and permanently alter genetic material resulting in cancer

Transformed cells have increased rate of…

growth, alterations in chromosomes, and capacity to divide for indefinite time periods resulting in tumors

Mammalian viruses capable of initiating tumors are called…

oncoviruses

• Papillomavirus: cervical cancer

• Epstein-Barr virus: Burkitt’s lymphoma

Bacteriophages

bacterial viruses (phages)

• Most widely studied are those that infect Escherichia coli – complex structure, DNA

• Only nucleic acid enters cytoplasm - uncoating is not necessary

Phage replication step 1 Adsorption

binding of virus to specific molecule on host cell via tail fibers

Phage replication step 2 Penetration

genome enters host cell

Phage replication step 3 Replication

viral components produced

Phage replication step 4 Assembly

viral components assembled

Phage replication step 5 Maturation

completion of viral formation

Phage replication step 6 Release

viruses leave cell to infect other cells

Lysogeny

The Silent Virus Infection

• Not all phages complete lytic cycle

Some DNA phages, called ______ _____, undergo adsorption and penetration but don’t replicate

temperate phages

Viral genome inserts into bacterial genome and becomes…

inactive prophage – cell is not lysed

Prophage is retained and copied during normal cell division resulting in transfer of temperate phage genome to all host cell progeny

lysogeny

Induction

can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis

Lysogeny results in?

spread of virus without killing host cell

Phage genes in bacterial chromosome can cause production of toxins or enzymes that cause pathology

lysogenic conversion

• Corynebacterium diphtheriae 

• Vibrio cholerae

• Clostridium botulinum

Bacteriophage versus Animal Viral Multiplication table 6.4

Viruses are limited to….

particular host or cell type

• Many viruses are strictly human in origin, others are zoonoses transmitted by vectors

Viral infections range from..

asymptomatic to mild to life-threatening

Several viruses can cross placenta causing developmental disturbances and permanent defects

teratogenic

Course of disease

invasion at portal of entry and primary infection; some viruses replicate locally, others enter circulation and infect other tissues

Common manifestations

rashes, fever, muscle aches, respiratory involvement, swollen lymph nodes

Body defenses

combined action of interferon, antibodies, and cytotoxic T cells; frequently results in lifelong immunity

Diagnosis of viral diseases

symptoms, isolation in cell or animal culture, serological tests for antibodies; some tests for antigens

Some viruses establish long-term persistent infections that last many years or lifetime

• 2 types of persistent infections:

Many viral infections have rapid course; lytic cycle

• Chronic infections

• Latent infections

• Some persistent viruses are oncogenic

Chronic infections

virus is detectable in tissue samples, multiplying at slow rate; symptoms mild or absent

Latent infections

after lytic cycle, virus enters dormant phase; generally not detectable, no symptoms; can reactivate and result in recurrent infections

Interferon (IFN) – A Natural Antiviral Protein

Potent antiviral cytokine

Antiviral Mechanism 1

Infected cell releases IFN

Antiviral Mechanism 2

IFN signals neighboring cells to produce other factors that suppress transcription and increase RNase expression in infected cell

Antiviral Mechanism 3

IFN also increases MHC production, increasing presentation of viral antigens to T-cells

Animal viruses are categorized according to..

nucleic acid, capsid, and presence or absence of envelope

Survey of DNA Viruses

7 DNA families

• DNA viruses causing human disease: 

  • Enveloped DNA viruses

  • Nonenveloped DNA viruses

  • Nonenveloped ssDNA viruses

Smallpox - History

Throughout history, smallpox has devastated populations, particularly naïve (never exposed before) populations

• One of deadliest diseases of past, now only found in government labs (U.S.A. & Russia)

1999 - World Health Organization recommended that all smallpox stores be destroyed, but…

gov’ts scoffed, maintaining smallpox should be tested and used in research

Russia has reported developing resistant strain that would..

withstand delivery by bomb or missile

There is concern that all Russian stores may not be accounted for and CDC in US had scare with…

discovery of long-term inappropriate storage of smallpox samples in the 2010’s