Microscopy, Domains of Life, Viral Envelopes, and Gram Staining

Recognizing Microscopy Images

• Brightfield: Standard microscopy with a light background.
• Darkfield: Produces a bright specimen against a dark background.
• Phase contrast: Enhances contrast in transparent specimens, making them more visible.
• Fluorescence: Uses dyes that glow under UV light to visualize specific structures.
• Electron microscopy (TEM/SEM): Provides high-resolution imaging but produces black and white images.

Domains of Life

• Domains: Bacteria, Archaea, Eukarya
• Classification is based on rRNA sequences, cell membrane lipids, and the presence or absence of a nucleus.
  • Bacteria and Archaea are prokaryotes, meaning they lack a nucleus.
  • Eukarya are eukaryotes, characterized by having a nucleus and other membrane-bound organelles.

Viral Envelopes

• Origin: Viral envelopes are derived from the host cell membrane or internal membranes.
• Occurrence:
  • Animal viruses: commonly have envelopes, acquired via budding from the host cell.
  • Plant viruses: typically do not have envelopes.
  • Bacteriophages: do not possess envelopes.
• Naked virus: A virus without an envelope, which tends to be more resistant to drying.

Gram Staining

• Gram-Positive:
  • Stain Color: Purple (retains crystal violet)
  • Feature: Retains crystal violet dye due to a thick peptidoglycan layer in the cell wall.
• Gram-Negative:
  • Stain Color: Pink (safranin counterstain)
  • Feature: Does not retain crystal violet but is counterstained pink by safranin, indicating a thinner peptidoglycan layer and an outer membrane.