Microbiology Exam 5

Obligate intracellular parasites

• only replicate in host cell

• few or no enzymes of own for metabolism (mainly use host cell)

Host range

• viruses infect every known organism

• most can only infect specific cells of one host species

• viruses that infect bacteria are called bacteriophage

• phage therapy or phage

• determined by receptor specificity

• phage therapy

Phage therapy

Use phage to treat bacterial infections

Problem with phage

Immune system attacks it before it can be beneficial

Structure

• nucleic acid

• protein capsid

• may contain an envelope around capsid - made up of lipids, proteins, and carbohydrates

Nucleic acid

Either DNA or RNA

Protein capsid

Protein coat that surrounds the nucleic acid

General characteristics of viruses

• obligate intracellular parasites

• host range

• size

Taxonomy (old system)

• based on symptoms (human had) or similar morphologies

• may cause more than one disease state

• look the same but dramatically different in action

Taxonomy (new system)

1. nucleic acid

2. strategy for replication

3. morphology

4. classification is “new” and changingT

Taxonomy (viral species)

1. same genetic information

2. same host range

3. no specific epithet - use common name

Lytic cycle

1. attachment - phage attaches to host cell wall

2. penetration - penetrates cell via phage lysozyme and injects DNA

3. Biosynthesis - phage DNA directs synthesis of viral components by host cell (eclipse period)

4. maturation - components assembled into virions

5. release - cell lyses (phage lysozyme) and virions released

Lysogeny cycle

1. attachment (cell wall) and penetration (inject)

2. can continue with lytic cycle or

3. DNA sometimes integrates with bacterial DNA (prophage)

• bacteria can reproduce normally (divide) - prophage genes repressed

• phage conversion can happen

• occasionally, viral DNA excises from bacterial DNA and then continues with lytic cycle

Prophage

DNA sometimes integrates with bacterial DNA

Life cycles of bacteriophage

• lytic cycle

• lysogeny cycle

Life cycles of animal viruses

• lytic cycles

• persistent infections

Persistent infections

• not completely cleared

• latent

• chronic

Latent persistent infections

Virus remains in host for long periods without producing disease (repressed state) - reactivates

Chronic persistent infections

Small amounts of virus always found

Animal virus attachment

• use a receptor on plasma membrane of host cell

• no tails - attachment sites over whole surface virus

Animal virus penetration

• endocytosis

• fusion

Endocytosis

Plasma membrane folds inward to form vesicle virus which virus is in - loses the envelope

Fusion

Envelopes fuses with plasma membrane and releases capsid

Animal virus uncoating

Enzymatic removal of capsid proteins

Animal virus biosynthesis/assembly

• animal viruses don’t always express genes using the normal flow of genetic information (DNA → RNA → protein)

• assembly varies from happening in the nucleus to happening in the cytoplasm

• retrovirus - RNA turned into DNA that integrates and follows dogma

Animal virus maturation and release

• lysis: nonenveloped

• budding: enveloped

Bacteria

• plasma membrane

• binary fission

• possess both DNA and RNA

• make ATP

• ribosomes

• sensitive to antibiotics

Virus

Intracellular parasite

Latent viruses

• Vercella-Zoster

• Herpes Simplex

• Epstein-Barr

Chronic viruses

• Hepatitis C

• HIV

Size of viruses

20-300 nm

Viruses and cancer

• hard to diagnose

• most do not cause cancer

• cancer may not develop immediately

• cancers are not contagious

Oncogenes in animal cells

• any gene that cause the cell to grow/divide again when mutated causes cancer

• causes for activation

-mutagenic chemicals

-UV light

-oncogenic viruses

Oncogenic viruses

• 10% of all cancers

• activated by virus integration into DNA

• transformation of the cells (tumors)

Characteristics of tumors

• uncontrolled growth

• no contact inhibition

• decreased cell adhesion

Antiviral drugs

• viral disease

• nucleoside analogs

• enzyme inhibitors

• fusion or exit inhibitors

• interferon

Viral disease

1. 60% of infectious diseases world-wide are caused by viruses and only 15% is bacterial

2. 90% US population suffers from viral disease/year

3. very few antiviral therapies

Nucleoside analogs

Interfere with DNA and RNA synthesis

Acyclovir (nucleoside analog)

• used for herpesvirus and shingles treatment

• activity - virus infected cells use drug in place of normal nucleoside which leads to DNA synthesis issues

• administered orally, topically, or injected

Ribavirin (nucleoside analog)

• Hepatitis C and respiratory syncytial virus

• induces high mutation rate of RNA virus

Zidovudine - AZT (nucleoside analog)

• HIV treatment

• competitive analog blocks synthesis by reverse transcriptase

• fairly toxic

Nevirapine (enzyme inhibitor)

• HIV treatment

• inactivates reverse transcriptase

Indinavir and saquinavir (enzyme inhibitor)

• HIV treatment

• protease inhibitor

Raltegravir and elvitegravir (enzyme inhibitor)

Integrase inhibitors

Fusion or exit inhibitors

1. Enfuvirtide: stops HIV fusion

2. Oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab): prevent influenza virus release

Interferon

• natural product of the immune system which stimulates cells to produce antiviral proteins

• alpha interferon

• Hepatitis C treatment

Prions structure/description

1. Proteinaceous infectious particle

2. causes rare neurodegenerative disorders

• diseases with long incubation periods

• always fatal (progressive)

Prion diseases

• Scrapie (sheep)

• Mad cow disease

• Kuru (humans)

• Creutzfeldt-Jakob disease (CJD) (humans)

• Gerstmann-Straussler-Scheinker syndrome (humans)

• Fatal Familial Insomnia (humans)

• Chronic Wasting Disease (deer)  

Prion transmission

• eating CNS from an infected animal

• transplanting nerve tissue

• contaminated surgical instruments (hard to degrade via enzyme or heating)

Prion mode of action

• can induce abnormal folding of NORMAL cellular prion proteins in the brain

• causes large vacuoles in the brain (spongiform encephalopathy)

Viroids structure/description

• short pieces of naked circular RNA - 300 to 400 nucleotides long, no protein coat

• causes plant disease

Types of influenza

• Influenza A

• Influenza B

• Infleunza C

Influenza A

• broken into subtypes: based on glycoprotein present - hemagglutination (H) and neuraminidase (N)

• cause for major pandemics

• found in many different animals, including ducks, chickens, pigs, whales, horses, seals, cats, and humans

• 16 H subtypes and 9 N subtypes (available)

Influenza B

• infects humans only

Influenza C

• no epidemics (mild disease)

• infects humans and pigs