Micro Lecture 4: Cellular and Acellular Microbes: The Viruses PDF

🦠 Lecture 4: Cellular and Acellular Microbes: The Viruses Key Vocabulary Bacteriophage (bacteria infecting virus) Enveloped (covers, surrounds) Naked (no cover) Capsid (protein shell, pro...

🦠 Lecture 4: Cellular and Acellular Microbes: The Viruses Key Vocabulary Bacteriophage (bacteria infecting virus) Enveloped (covers, surrounds) Naked (no cover) Capsid (protein shell, protects genetic information of of Virus) Nucleocapsids ( release nucleic acid genome of virus) Reverse Transcriptase (enzyme that copies RNA into DNA) Budding (asexual mode of making a new organism) Coated Pit Adsorption SARS Influenza HIV West Nile Role of Phage in the Gut SARS (2003) and SARS-CoV-2 SARS-CoV: Severe Acute Respiratory Syndrome Coronavirus SARS-CoV-2 (COVID-19): Structure analyzed under electron microscope Lecture 4: Cellular and Acellular Microbes: The Viruses 1  : A coronavirus with its spike proteins protruding, credit to CDC/Fred Murphy Viral Genome Organization Viruses can have different genome structures: Some viruses contain a single molecule of RNA or DNA Others, like influenza, have eight single-stranded RNA segments Replication site: cytoplasm or nucleus The Viral Genome Refers to the nucleic acid molecule in the virion Packaged inside the capsid after replication Exists in intermediates during viral replication All of the above are correct Virus Reproduction A) Attachment (Adsorption) First step in infection Not random; specific viral component binds to host receptor Lecture 4: Cellular and Acellular Microbes: The Viruses 2  Host cells must have the receptor to be infected Receptors are located on the surface of a host cell If receptor is removed, the virus cannot attach, but this could harm the host cell Examples of Receptors in Bacteria: E. coli Bacteriophage lambda → LamB protein (used for sugar uptake) Bacteriophage R17, M13, f1, fd → Specific sites on pili (used for DNA exchange) B. subtilis Bacteriophage spo → Flagella structures (used for motility) Examples of Receptors in Multicellular Organisms: Virus Receptor Location Polio ICAM Nasopharynx, gut, spinal cord Rhinovirus ICAM Nasopharynx (common cold) Measles CD46 Various tissues HIV CD4 Leukocytes (WBCs) Rabies Acetylcholine receptors Neuronal plasma membrane SARS ACE-2 Blood pressure regulation (lungs) Influenza Hemagglutinin Sialic acid residues on host cells Lecture 4: Cellular and Acellular Microbes: The Viruses 3  Receptors determine host and tissue specificity Used in gene therapy for targeted gene delivery B) Penetration into Host Cells Second step of viral infection Different mechanisms for different viruses How Bacteriophage T4 Infects E. coli Virus injects DNA into the bacterial cell How Animal Viruses Enter Cells Receptor-mediated endocytosis Caveolin-mediated entry Basic endocytosis Fusion and delivery of capsid- encoded virus C) Replication, Packaging, and Exit Viral genome is released from the nucleocapsid Lecture 4: Cellular and Acellular Microbes: The Viruses 4  Genome replication occurs first (exception: +RNA viruses can directly make proteins) Viral genome is transcribed New copies of the genome are packaged into capsids Viruses leave the cell and infect other cells Virus Replication Strategies: Viruses can not replicate on their own b/c they lack necessary mechanisms (i.e. ribosomes for protein synthesis) Some viruses bring enzymes for replication Others rely entirely on host cell enzymes Nucleotides (make new viral DNA or RNA) Ribosomes & Amino Acids (build viral proteins, like capsid proteins) Viral Exit Mechanisms the new copies of genetic material are packaged & in newly formed capsids than exit host cell… 1. Enveloped viruses Budding from plasma membrane (e.g., Influenza) Budding from nucleus (e.g., Herpes virus) Exocytosis from ER/Golgi Viruses steal a pieces of host’s membrane as they leave Viral envelope have special viral proteins to help w/ infection 1. Naked viruses (viruses w/o a membrane coat) these do not bud Lyse (burst) the host cell, killing it in the process Reason a virus infects a bacteria = they do not have ribosomes or other components needed to express genes ; so they hijack a host to do it for them. Lecture 4: Cellular and Acellular Microbes: The Viruses 5  Phage Life Cycles Lytic Cycle: Virus replicates and destroys host cell Lysogenic Cycle: Virus integrates into host genome and remains dormant Feature Lytic Cycle Lysogenic Cycle Viral Replication Immediate Delayed (Dormant) Host Cell Fate Destroyed (lysis) Lives until activation Viral DNA Separate from host Integrated into host genome New Viruses Produced? Yes No (until activation) Example Viruses Bacteriophage T4 Bacteriophage Lambda Examples of Viral Replication HIV Replication (Retrovirus) Uses reverse transcriptase to convert RNA into DNA (Converting into DNA allows it to hide itself & make new viruses overtime) Integrates into host chromosomal DNA Produces new virions that bud from host cells Summary of HIV Replication Steps Step Process HIV binds to CD4 receptor & coreceptor, fuses with 1. Entry membrane, and enters. 2. Reverse Transcription Reverse transcriptase converts viral RNA → DNA. Integrase inserts viral DNA into the host genome 3. Integration (provirus). 4. Transcription & Host cell makes viral RNA & proteins. Translation Viral components self-assemble into immature HIV 5. Assembly particles. 6. Budding & Maturation HIV buds from the cell, matures, and infects other cells. Lecture 4: Cellular and Acellular Microbes: The Viruses 6  Influenza Virus Uses hemagglutinin (HA) → Helps the virus attach and enter. It is a antigen Uses neuraminidase (NA) → Helps the virus exit and spread. It is a antigen Antiviral drugs (e.g., Tamiflu) block NA, preventing virus release and stopping infection spread. Types of Influenza Influenza A: responsible for pandemics (spread over large geographical areas) this is due to variations in HA & NA Influenza B: Generally caused localized outbreaks or epidemics & have no clear subtypes. Influenza C: cause only mild symptoms & no clear subtypes Summary of Influenza Replication Steps Key Protein Step Process Involved Hemagglutinin 1. Attachment Virus binds to sialic acid receptors (HA) Virus is engulfed into an endosome 2. Entry (when inside, protected from immune HA detection). Lecture 4: Cellular and Acellular Microbes: The Viruses 7  HA triggers fusion, releasing RNA into 3. Genome Release HA cytoplasm 4. Replication & Viral RNA enters nucleus, replicates, and RNA polymerase Protein Synthesis makes proteins New viral RNA and proteins are packaged 5. Assembly Capsid proteins into new viruses Virus buds from cell but stays attached to 6. Budding & Exit HA & NA sialic acid Neuraminidase (NA) cleaves sialic acid, Neuraminidase 7. Virus Release freeing new virus (NA) Antigenic Shift vs. Antigenic Drift Antigenic Shift (Major Change) Reassortment of viral segments creates a new subtype Occurs in Influenza A (infects multiple species) Influenza A can infect multiple species (humans, birds, pigs, etc) increasing the chance of different strains mixing. Antigenic Drift (Minor Change) Random mutations in surface proteins to evade immunity continues process of small genetic changes in virus (mutation) Small changes causes virus to change its antigen Changed virus can’t be recognized by immune system of host, making harder to fight off. Occurs in Influenza A, B, and C Example: SARS-CoV-2 variants: (like delta & Omicron) area result of antigenic drift. Lecture 4: Cellular and Acellular Microbes: The Viruses 8  Types of Viral Infections 1. Acute Infections Rapid onset, short duration (e.g., Influenza) 2. Persistent Infections Chronic infections: Virus always present (e.g., Hepatitis B, HIV) Latent infections: Virus is dormant and reactivates later (e.g., Herpes) No, symptoms, antibodies, or virus detectable Herpes Simplex Virus (HSV): member of Herpesviridae cold sores/ genital herpes infections Contagious particularly at outbreak & incurable Latency triggered by stress, sunburn, or illness Shingles occurs when Varicella-Zoster virus (chickenpox) reactivates Transmission of Viruses Lecture 4: Cellular and Acellular Microbes: The Viruses 9  1. Direct Contact: w/ infected host (e.g., bodily fluids, saliva) 2. Indirect Contact: on a non-living object or aerosols (e.g., surfaces, airborne particles) 3. Vector Transmission: from a living organism may or may not be the host (e.g., mosquitoes spreading West Nile virus) Treatment Strategies Vaccines (e.g., COVID-19 vaccine) Antivirals (e.g., Tamiflu, Zanamivir) Gene Therapy Viruses used as delivery vectors for genetic material Successfully used to treat SCID ("bubble baby syndrome") Key Takeaways Viruses require specific host receptors for infection Different viruses have distinct replication and exit strategies Mutations (drift/shift) contribute to viral evolution and immune evasion Gene therapy is a promising medical application of viruses Lecture 4: Cellular and Acellular Microbes: The Viruses 10