Chapter-3-notes-8th-edition (1)

Chapter 3: Microscopy and Cell Structure

Tools of the Trade

1. Light Microscopes

• Basic tool in any microbiological lab

• Uses focused light to magnify objects up to 1000 times

• Types of Light Microscopes:

  • Bright Field Microscope

    • Illuminates the entire field of view

  • Phase-Contrast Microscopes

    • Utilizes differences in light refraction through specimens.

    • Ideal for viewing unstained specimens, preserving live microbes.

  • Dark-field Microscopes

    • Only illuminates the specimen against a dark background,

    • Useful for observing poorly stained bacteria.

2. Electron Microscopes

• Utilize a focused beam of electrons rather than light.

• Can magnify specimens up to 100,000 times, essential for studying very small microbes.

• Types of Electron Microscopes:

  • TEM (Transmission Electron Microscope)

    • Requires thin sections and vacuum.

    • Produces high magnification images.

  • SEM (Scanning Electron Microscope)

    • Scans surface of specimens coated with metal.

    • Generates high-detail 3D images.

Dyes and Stains

• Many microbes are transparent; staining is essential for visibility.

  • Staining enhances specimen visibility (common stains include methyl blue, malachite green).

  • Fixing immobilizes specimens (usually involves heat fixing).

Differential Staining

• Distinguishes between groups of bacteria based on chemical structure.

• Common Differential Stains:

  • Gram Stain

    • Developed by Dr. Hans Gram.

    • Stains bacteria based on cell wall structure; Gram positive (purple) vs. Gram negative (red).

  • Acid-Fast Stain

    • Identifies Mycobacterium.

    • Retains stain despite acid/alcohol rinse, crucial for tuberculosis and leprosy diagnostics.

Prokaryotic Cell Morphology

1. Shapes

• Three basic bacterial shapes:

  • Coccus - round

  • Bacillus - rod

  • Spirillum - spiral

• Additional shapes include:

  • Coccobacillus - short rods

  • Vibrios - curved rods

  • Spirochetes - tightly spiraled

  • Pleomorphic - variable shapes.

2. Groupings

• Bacteria can form clusters:

  • Diplococci - pairs

  • Chains - long strands

  • Packets - cuboidal arrangements

  • Clusters - random bunches.

Parts of a Prokaryotic Cell

a. Plasma Membrane

• Surrounds the cell, composed of a phospholipid bilayer.

  • Imbedded with transmembrane proteins that:

    • Function in transport, receptor activities, and as enzymes.

b. Cell Wall

• Rigid structure outside the plasma membrane.

  • Provides structure and protection, mainly composed of peptidoglycans.

  • Gram Positive vs. Gram Negative:

    • Gram Positive: Thick peptidoglycan layer, retains purple stain.

    • Gram Negative: Complex structure with thin peptidoglycan layer and outer membrane; does not retain purple stain.

  • Peptidoglycans and antibiotics:

    • Penicillin disrupts peptidoglycan assembly.

    • Lysozyme breaks bonds in peptidoglycan chains.

c. Glycocalyx

• External layer for adhesion and protection.

  • Forms capsules (distinct and jelly-like) or slime layers (irregular).

d. Filamentous Appendages

• Flagella - long protein fibers for mobility.

  • Pili - short filaments; can aid in adhesion or genetic exchange (conjugation).

Internal Parts of a Prokaryotic Cell

a. Nucleoid

• Region with tightly packed DNA, not membrane-bound.

b. Plasmids

• Small, circular DNA molecules separate from the nucleoid, often exchanged during conjugation.

c. Ribosomes

• Structures vital for protein synthesis, varying between prokaryotic and eukaryotic structures.

d. Endospores

• Resistant structures formed during stress, can persist until conditions are favorable again.

Eukaryotic Cells

• Less focus but includes:

  • Cell Wall: Present in plants (cellulose), fungi (chitin), and some protists.

  • Plasma Membrane: Composed of phospholipid bilayer with various functions (transport, receptors, adhesion).

  • Nucleus: Encloses DNA in the form of chromosomes.

  • Mitochondria: Energy production site.

  • Chloroplasts: Responsible for photosynthesis in plants and algae.

  • Lysosomes: Digestive structure for waste materials.

  • Endomembrane System: Involves rER, sER, and Golgi apparatus for protein processing and transport.