Overview
The lecture examines key characteristics of foodborne bacterial illnesses and provides an in-depth overview of viruses, their structures, replication cycles, host interactions, and the unique properties of prions.
Foodborne Illness Case Study
- Patient experienced rapid-onset diarrhea after eating fried rice, suggesting bacterial contamination.
- Gram-positive, endospore-forming rods identified as cause, likely Bacillus cereus.
- Only Bacillus and Clostridium genera produce endospores; both are gram-positive rods.
- Differentiation from other diarrhea-causing bacteria (Salmonella, E. coli, Staphylococcus) based on endospore production and cell shape.
Introduction to Viruses
- Viruses are acellular infectious agents debated to be non-living due to lack of independent replication.
- Viruses infect all cellular life forms, including plants, animals, and bacteria.
- Viral genetic material can become part of host genomes, influencing evolution and causing inherited effects.
- A significant portion (40–80%) of the human genome consists of viral remnants.
Viral Structure and Classification
- All viruses have a nucleic acid genome (DNA or RNA, single/double-stranded, positive/negative sense) and a protein coat (capsid).
- Viruses can be "naked" (capsid only) or "enveloped" (capsid plus host-derived membrane).
- Capsid structures are typically helical or icosahedral in shape; subunits are called capsomeres.
- Surface spikes (often glycoproteins) mediate host cell entry, determining host and tissue specificity.
Viral Replication and Life Cycle
- Viruses are obligate intracellular parasites—require host cells for replication.
- Animal virus replication includes: attachment (absorption), penetration, uncoating, synthesis, assembly, and release (by budding or exocytosis).
- Bacteriophages (phages) inject their DNA into bacteria and often cause cell lysis.
- Some viruses integrate their genome into host DNA, forming latent infections (provirus for animal cells, prophage for bacteria).
Special Topics: Reverse Transcription, Antiviral Therapy, and Viral Effects
- Retroviruses (e.g., HIV) use reverse transcriptase enzyme to convert RNA into DNA, integrating into host genome.
- Antiviral drugs are highly specific, often target unique viral enzymes/proteins.
- Cytopathic effects: viruses can disrupt host cell function, cause cell fusion (syncytia), detachment, or death.
- Some viruses are linked to cancer via genome integration (e.g., HPV, hepatitis B).
Non-Cellular Infectious Agents: Prions
- Prions are infectious misfolded proteins (no nucleic acid) causing spongiform encephalopathies (e.g., Creutzfeldt-Jakob disease).
- Prion diseases are always fatal and can result from transmission or genetic predisposition.
Key Terms & Definitions
- Endospore — a tough, dormant bacterial cell form produced by genera like Bacillus and Clostridium.
- Capsid — protein shell surrounding the viral genome.
- Envelope — lipid membrane derived from the host cell, surrounding some viruses.
- Capsomere — protein subunit of a viral capsid.
- Spike (glycoprotein) — surface protein that mediates host cell entry.
- Obligate intracellular parasite — organism/agent that must reproduce inside host cells.
- Provirus/Prophage — viral genome integrated into host DNA (animal/bacterial).
- Reverse transcriptase — enzyme synthesizing DNA from RNA (retroviruses).
- Syncytia — multinucleated cell mass from viral-induced cell fusion.
- Prion — infectious, misfolded protein causing neurodegenerative disease.
Action Items / Next Steps
- Review Chapter 7 on viruses.
- Prepare for detailed discussion on viral genetics and replication in the next class.
- Stay current with assigned readings and class notes.