Microbial World and Cell Structure

Microbial World and Bacterial Cell Structure

• Microorganisms are classified into two groups: prokaryotes and eukaryotes
  • Prokaryotes include bacteria and blue-green algae. These are single-celled organisms with DNA and RNA, capable of all life processes like growth, reproduction, and metabolism.
  • Eukaryotes include fungi, algae(not blue-green), protozoa, and slime molds. These organisms also have DNA and RNA, but are more complex in structure.
• Microorganisms of medical interest include protozoa, fungi, bacteria, and viruses. All but viruses are visible under a light microscope.

Differences Between Prokaryotic and Eukaryotic Cells

• | Characteristic | Prokaryotic cell | Eukaryotic cell |
• |---|---|---|
• | Nucleus | No | Yes |
• | DNA associated with histones | No | Yes |
• | Membrane-bound organelles | No | Yes |
• | Cell membrane composition | Peptidoglycan (except in mycoplasma) | Sterols present |
• | Cell wall composition | Peptidoglycan (except in mycoplasma)| No peptidoglycan |
• | Size of ribosomes | 70S | 80S |
• | Number of cells | Unicellular | Multicellular |
• | Size of cell | Smaller (0.5–2 µm) | Larger (10–100 µm) |
• | Mitotic division | No | Yes |

Bacteria

• Bacteria are a large group of single-celled prokaryotic microorganisms.
• They vary in size from 0.1–10 µm.
• They have a simple cell structure, containing both DNA and RNA.
• Most medically important bacteria will grow on artificial culture media in the lab and reproduce by binary fission.

Shapes of Bacteria

• Bacteria have various shapes:
  • Cocci (round or oval)
  • Bacilli (rod-shaped)
  • Vibrios (curved or comma-shaped rods)
  • Spirilla (spiral)
  • Spirochetes (slender and flexible spirals)
• Bacterial shape is further categorized by their staining reactions (e.g., Gram staining, acid-fast staining).

Bacterial Cell Structure

• A. Cell Envelope:
  • 1. Cell Wall: A rigid layer surrounding the cytoplasmic membrane, composed of peptidoglycan, which is crucial for bacterial classification. Important for maintaining cell shape and resisting pressure. Gram-positive has a thick layer of peptidoglycan while Gram-negative has a thinner layer. Gram-negative has an outer membrane with lipopolysaccharides.
  • 2. Cytoplasmic membrane: A thin, elastic, semipermeable membrane consisting of a phospholipid bilayer that controls the movement of substances into and out of the cell.
• B. Cytoplasm and its components:
  • 1. Ribosomes: Granules with 70S ribosomes involved in protein synthesis.
  • 2. Bacterial nucleoid: The bacterial chromosome, a single, circular DNA molecule.
  • 3. Plasmid: Small, circular DNA molecules that are separate from the bacterial chromosome. They often contain genes for antibiotic resistance.
  • 4. Inclusion granules: Granules that store nutrients. These include lipid, glycogen, starch, sulfur, or polyphosphates.
• C. External structures:
  • 1. Capsule: A mucogelatinous layer surrounding the cell wall, which helps to protect the bacteria from phagocytosis and antibiotics.
  • 2. Flagella: Long, filamentous appendages for locomotion.
  • 3. Fimbriae/Pili: Hair-like projections helping in attachment to surfaces.

Gram Staining

• Gram staining is a differential staining technique that differentiates bacteria into Gram-positive and Gram-negative based on their cell wall structure.

Functions of Cell Wall

• 1. Responsible for the shape of the bacterial cell.
• 2. It can stand high internal pressure.
• 3. LPS is the site of O antigen and endotoxin in Gram-negative bacteria.
• 4. InGram-positive bacteria, teichoic acid is the major surface antigen.
• 5. Site of actions of many antibiotics.
• 6. Contains specific receptors for bacterial viruses.
• 7. Plays a role in cell division.

Explore the fascinating world of microorganisms with this quiz focused on microbial classification and cellular structures. Understand the key differences between prokaryotic and eukaryotic cells, and learn about their unique characteristics. Test your knowledge on the microorganisms that are crucial in medical science.