Culture Media Classification
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Direct answer
Culture media are classified by their purpose into basal, enriched, selective, differential or indicator, transport and enrichment media. Basal media such as nutrient agar grow non-fastidious organisms, enriched media such as chocolate agar add blood or serum for fastidious species, and selective media such as MacConkey agar suppress unwanted flora. Transport media preserve viability in transit, and enrichment broths boost a target organism before it is plated.
What you must remember
- Basal media: nutrient broth and nutrient agar, which support non-fastidious organisms such as Escherichia coli.
- Enriched media: blood agar and chocolate agar, where chocolate agar is heated blood that supports Haemophilus and Neisseria.
- Selective media: MacConkey agar, Thayer-Martin medium and TCBS agar, which inhibit unwanted flora while the target grows.
- Differential or indicator media: MacConkey separates lactose fermenters, and mannitol salt agar identifies mannitol-fermenting Staphylococcus aureus.
- Transport media: Stuart's and Amies media keep fastidious organisms such as Neisseria gonorrhoeae viable between collection and plating.
- Enrichment broths: selenite F and tetrathionate broths for Salmonella and Shigella, and alkaline peptone water for Vibrio cholerae.
- Tellurite media: Tinsdale medium reduces tellurite, so Corynebacterium diphtheriae colonies turn black.
- Lowenstein-Jensen medium: an egg-based medium containing malachite green, used to culture Mycobacterium tuberculosis.
The six classes and how they are used
A single medium often belongs to more than one class, so the examination answer depends on the purpose named in the question. The table lists the six classes, with the purpose and one named example for each.
| Class | Purpose | Example |
|---|---|---|
| Basal | Grows non-fastidious bacteria | Nutrient agar |
| Enriched | Adds blood or serum for fastidious bacteria | Chocolate agar |
| Selective | Suppresses unwanted flora | Thayer-Martin agar |
| Differential | Shows biochemical differences by colony | MacConkey agar |
| Transport | Preserves viability in transit | Stuart's medium |
| Enrichment broth | Boosts the target before plating | Selenite F broth |
MacConkey agar is both selective and differential, because bile salts and crystal violet inhibit Gram-positive organisms and lactose fermenters colour the colonies. Choosing a medium therefore means naming its purpose first and only then giving the example, which is the logic that examiners test.
Enrichment broths are liquid and used before plating, whereas selective agars are solid and used to isolate colonies, and confusing the two is a common error. Transport media are deliberately non-nutrient, so they keep organisms alive without letting the normal flora overgrow the specimen.
Where the exam tries to catch you
Culture media questions usually describe a specimen or an organism and ask which medium is most suitable.
- Trap: calling blood agar selective Correct: blood agar is enriched and differential for haemolysis, because most organisms grow on it.
- Trap: using MacConkey agar to isolate Neisseria gonorrhoeae Correct: Thayer-Martin medium is needed, since fastidious gonococci need an enriched base with selective antibiotics.
- Trap: plating a Salmonella-suspect stool directly on a selective agar only Correct: an enrichment broth such as selenite F raises the yield before plating on a selective agar such as XLD.
Frequently asked questions
Which medium is used to culture Neisseria gonorrhoeae from genital specimens?
Thayer-Martin medium is used to culture Neisseria gonorrhoeae from genital specimens. It is an enriched base containing antibiotics such as vancomycin, colistin and nystatin, which suppress the normal flora while the gonococci grow.
Why is TCBS agar used for stool cultures in cholera?
TCBS agar is used for stool cultures in cholera because its alkalinity and bile salts inhibit most enteric bacteria while Vibrio cholerae grows. Vibrio cholerae ferments sucrose, so its colonies appear yellow on this medium.
Which medium is used to isolate Corynebacterium diphtheriae from throat swabs?
Tinsdale medium or potassium tellurite agar is used to isolate Corynebacterium diphtheriae from throat swabs. The organism reduces the tellurite to metallic tellurium, so its colonies turn black while many commensal bacteria are inhibited.
What is the purpose of Stuart's transport medium?
Stuart's transport medium preserves the viability of fastidious organisms such as Neisseria gonorrhoeae between collection and plating. It is a non-nutrient medium that protects the organisms in transit without allowing them to multiply.
What is the difference between selective and differential media?
Selective media suppress unwanted organisms while allowing the target to grow, whereas differential media let the target be told apart by its colony appearance. MacConkey agar performs both roles, because bile salts inhibit Gram-positive bacteria and lactose fermenters colour the colonies.