Pathogens: Cellular vs Non-Cellular

Pathogen Definition

• Agents that cause disease after infection.

• May be cellular (prokaryotic/eukaryotic) or non-cellular (acellular).

Cellular vs Non-Cellular Pathogens

• Cellular: contain cells; self-replicate (e.g.
  bacteria, fungi, protozoa, worms).

• Non-cellular: no cells; replicate only inside host (e.g.
  viruses, prions).

Cellular Pathogens – Bacteria

• Prokaryotic; circular DNA , ribosomes, no membrane-bound organelles.

• Gram-positive: plasma membrane + peptidoglycan wall (total 2 layers).

• Gram-negative: plasma membrane + peptidoglycan wall + outer lipopolysaccharide membrane (total 3 layers).

• Toxins
  • Exotoxins—secreted while alive (both gram types).
  • Endotoxins—lipopolysaccharide fragments released only when gram-negative cells die.

Non-Cellular Pathogens – Viruses

• Structure: nucleic acid (DNA or RNA) inside protein capsid.

• No ribosomes/organelles; metabolically inert outside host.

• Cannot replicate their own, rely on host cell.

• Replication cycle (simplified)

  1. Attachment to host cell.

  2. Injection/entry of viral nucleic acid.

  3. Host transcribes/translates viral genes → viral proteins.

  4. Assembly of new virions.

  5. Release; infection of new cells.

Non-Cellular Pathogens – Prions

• Causes by changes to the secondary structure of a protein (alpha helix, beta, pleated sheet)

• Misfolded host protein; altered secondary/tertiary structure.

• Induce normal proteins to misfold → autocatalytic chain reaction.

• Undetected by immune system (recognized as self protein).

• Cause neurodegenerative diseases (e.g.
  mad cow disease); diagnosis often late, treatment limited.

Quick Comparison

• Self-replication possible: Cellular – Yes; Non-cellular – No (needs host).

• Cellular structure present: Cellular – Yes; Non-cellular – Absent.

• Major examples: Bacteria vs Viruses/Prions.

• Key danger mechanisms: bacterial toxins, viral hijacking, prion misfolding cascade.