Bacterial Identification and Gram Staining

Introduction

• The lecture discusses methods used to differentiate bacteria under a microscope.
• Focuses on shape and color of bacteria as seen through staining.

Observations Under the Microscope

• Shapes
  • First Row: Circular or spherical shapes (cocci).
  • Second Row: Stretched out rod-like shapes (bacilli).
  • Third Row: Squiggle shapes (spirochetes).

Bacterial Shapes

• Coccus
  • Singular: Coccus
  • Plural: Cocci
• Bacillus
  • Singular: Bacillus
  • Plural: Bacilli
• Spirochete
  • Singular: Spirochete
  • Plural: Spirilla

Gram Staining

• Developed to differentiate bacteria using color.
• Gram Positive: Stains purple.
• Gram Negative: Stains pink.

Gram Stain Mechanism

• Stain affects the external layers of bacteria.
• Initial Stain: Purple across all bacteria.
• Washing and Restaining:
  • Gram-negative: Thin peptidoglycan layer allows initial stain to wash off, then restained to pink.
  • Gram-positive: Thick peptidoglycan retains purple stain.

Bacterial Cell Wall Structure

• Gram Positive Bacteria

  • Composed of plasma membrane, thick peptidoglycan layer, and capsule.
  • Peptidoglycan Layer
    • Consists of long sugar chains (glycan) connected by proteins (peptides).

• Gram Negative Bacteria

  • Composed of inner plasma membrane, thin peptidoglycan layer, outer membrane, LPS layer, and capsule.
  • Lipopolysaccharide (LPS) Layer
    • Contains lipid bottom and polysaccharide (sugar) chains on top.

• Periplasmic Space

  • Space located next to the plasma membrane.
  • Present between two plasma membranes in Gram-negative bacteria and above the plasma membrane in Gram-positive.

Conclusion

• Key aspects of bacterial identification include shape (cocci, bacilli, spirochetes) and Gram stain color (purple or pink).
• Understanding cell wall structure is crucial to interpreting Gram stain results.