Microbes are tiny living organisms or infectious agents that include bacteria, viruses, fungi, protozoa, algae, prions, and viroids.

Bacteria

• Structure: Single-celled, prokaryotic organisms (no true nucleus).
• Cell Wall Composition:
  • Made of peptidoglycan (a mesh-like polymer of sugars and amino acids).
  • Gram-positive bacteria have thick peptidoglycan walls.
  • Gram-negative bacteria have thin peptidoglycan layers and an outer membrane.
• Size: 0.2 to 10 micrometers.
• Features:
  • Live in diverse environments (soil, oceans, extreme heat/cold).
  • Reproduce by binary fission.
  • Can be autotrophs or heterotrophs.
  • Some are helpful (probiotics, nitrogen fixation), others harmful (pathogens).

Viruses

• Structure: Genetic material (DNA or RNA) inside a protein coat (capsid); some have lipid envelopes
• Cell Wall Composition:
  • No cell wall.
  • Outer structure is a protein coat (and sometimes a lipid membrane from the host cell).
• Size: 20 to 300 nanometers.
• Features:
  • Obligate parasites; cannot live without a host.
  • Cause diseases like influenza, HIV, COVID-19.

Fungi

• Structure: Eukaryotic cells; unicellular (yeast) or multicellular (molds, mushrooms).
• Cell Wall Composition:
  • Made of chitin (a tough, flexible material also found in insect exoskeletons).
• Features:
  • Act as decomposers.
  • Cause infections (e.g., athlete’s foot, ringworm).
  • Also used in food production (bread, cheese, alcohol).

Protozoa

• Structure: Single-celled, eukaryotic organisms.
• Cell Wall Composition:
  • Generally no rigid cell wall.
  • Some have a flexible membrane called pellicle for protection.
• Features:
  • Mostly free-living in aquatic environments.
  • Some are parasitic (e.g., malaria by Plasmodium).

Algae

• Structure: Eukaryotic organisms; unicellular (like Chlorella) or multicellular (like seaweeds).
• Cell Wall Composition:
  • Mainly cellulose, sometimes mixed with other substances like pectin or silica.
• Features:
  • Photosynthetic; produce oxygen.
  • Found mostly in water.

Special Infectious Agents

Prions

• Definition: Infectious misfolded proteins.
• Cell Wall Composition:
  • None (only protein, no cellular structure).
• Features:
  • Cause neurodegenerative diseases (e.g., Mad Cow Disease).

Viroids

• Definition: Small infectious RNA molecules.
• Cell Wall Composition:
  • None (just naked RNA).
• Features:
  • Infect plants (e.g., potato spindle tuber disease).

Updated Summary Table

Microbe Type	Structure	Cell Wall Composition	Living/Non-living	Diseases Caused
Bacteria	Prokaryotic	Peptidoglycan	Living	TB, Cholera
Virus	DNA/RNA + protein coat	No cell wall	Non-living outside host	Flu, HIV
Fungi	Eukaryotic	Chitin	Living	Ringworm, Yeast infections
Protozoa	Eukaryotic	Usually none (pellicle sometimes)	Living	Malaria
Algae	Eukaryotic	Cellulose (sometimes silica)	Living	Rarely pathogenic
Prions	Protein only	None	Infectious protein	Mad Cow Disease
Viroids	Naked RNA	None	Infectious RNA	Plant diseases

Written by Kasiban Parthipan