Viruses vs Bacteria

Study Notes

Viruses are the smallest infectious agents in nature, capable of infecting humans, animals, plants, insects, or bacteria.

Viruses are small in size
Contain only a single Nucleic acid (NA), either DNA or RNA
Have no metabolic activity (e.g., no ribosomes for protein synthesis)
Are obligatory intracellular (replicate only inside the cell)
Are not affected by antibiotics
Cannot grow on artificial media and are cultured only on living cells (e.g., tissue culture)

Measure in nanometers (nm), with sizes ranging from 20-300 nm
Can pass through bacterial filters and are seen only by an electron microscope (EM)

NA core (DNA or RNA): the infective part of the virus, can be single or double stranded
Capsid: a protein coat that surrounds the NA, formed of several capsomeres
Capsid symmetry: can be cubic or icosahedral, helical, or complex
Functions of capsid: responsible for viral symmetry, protection of NA, antigenicity, and adsorption to host cell
Envelope: a lipoprotein present outside the capsid in some viruses, responsible for adsorption to host cell receptors

Prions: protein agents without detectable NA, cause slow virus diseases with CNS manifestations
Viroids: RNA without a protein coat, affect plants
Defective Viruses: have a mutation that makes them unable to synthesize their own capsid protein, require the help of another virus to replicate
Pseudoviruses: cellular DNA incorporated into viral capsid, can infect a cell but cannot replicate

Definition: collection of virus particles (or viral protein) inside the cell, visible by light microscope with certain stains
Significance: used in the identification of some viruses
Types of IBs:
- Non-viral: intracytoplasmic basophilic, e.g. in Chlamydia
- Viral:
  - Intracytoplasmic acidophilic: e.g. in Rabies (Negri bodies), Smallpox (Guarnieri bodies)
  - Intranuclear: e.g. in Yellow fever (Torres bodies), Polio, Herpes simplex, Adenovirus
  - Both Intracytoplasmic and Intranuclear: e.g. in measles and CMV