Culture Media
Culture media support, select, or differentiate organisms so the lab can recover pathogens and recognize useful colony patterns.
Core idea
Choosing the right plate affects recovery. Reading the plate correctly affects the whole identification path.
Principle
Media can be enriched, selective, differential, or specialized. Each plate answers a different bench question.
Common Media Roles
| Medium | Type | What it helps show |
|---|---|---|
| Blood agar | Enriched and differential | General growth and hemolysis |
| Chocolate agar | Enriched | Fastidious respiratory organisms such as Haemophilus and pathogenic Neisseria |
| MacConkey agar | Selective and differential | Gram-negative rod growth and lactose reaction |
| CNA or PEA agar | Selective | Helps recover Gram-positive organisms by suppressing many Gram-negative rods |
| Thioglycollate broth | Enrichment broth | Supports low numbers or mixed oxygen-tolerance organisms |
| BCYE agar | Specialized | Supports Legionella when clinically suspected |
Basic workflow
- Match media to specimen source and suspected organisms.
- Use enrichment when fastidious organisms may be present.
- Use selective media to suppress background flora.
- Use differential reactions to separate colony groups.
- Interpret growth with the specimen source and incubation conditions.
How to read it
- Blood agar shows growth quality and hemolysis.
- Chocolate agar supports more fastidious respiratory organisms.
- MacConkey selects for many Gram-negative rods and differentiates lactose reaction.
High-yield trapCommon mistakes
- Thinking one plate can answer every question.
- Ignoring atmosphere and incubation time.
- Overcalling normal flora from nonsterile specimens.
ShortcutStudent shortcut
Ask what the plate is doing: feeding, selecting, differentiating, or protecting a special organism.