Figure - Koch Postulates Flow

Purpose

Show classical causality steps and where modern molecular diagnostics break or extend them.

Used In

• Koch’s Postulates
• Germ Theory
• Robert Koch
• PCR
• Learning Media Hub

Diagram

flowchart TD
  A[1. Associate<br/>Found in diseased hosts] --> B[2. Isolate<br/>Pure culture]
  B --> C[3. Inoculate<br/>Reproduce disease in host]
  C --> D[4. Re-isolate<br/>Same microbe again]
  D --> E[Causal link accepted]

  A -. exception .-> X1[Asymptomatic carriers]
  B -. exception .-> X2[Unculturable organisms]
  C -. exception .-> X3[Ethics / no animal model]
  A -. modern .-> M1[PCR / WGS detect DNA ≠ prove disease]
  B -. modern .-> M2[Molecular Koch: knock out virulence gene]

Teaching Legend

Step	Classical tool	Modern analogue
Associate	Microscopy / pathology	NAAT, metagenomics
Isolate	Culture and Isolation	Sometimes impossible → genes/markers
Causation	Animal challenge	Gene knockout / gain-of-function evidence
Confirm	Re-culture	Sequence identity / phylogeny

Quick Self-Check

1. Which postulate fails for viruses that cannot be cultured in cell-free media historically?
2. Why can a PCR+ nasal swab fail postulate-style causation?