Selective & differential media
Selective and differential agar for Gram-negative bacteria. Bile salts and crystal violet suppress Gram-positives; lactose fermenters make acid and turn pink, non-fermenters stay colourless.
Our ruleSimulations prepare you for the bench but don't replace hands-on practice.
MacConkey
Selective and differential agar for Gram-negative bacteria. Bile salts and crystal violet suppress Gram-positives; lactose fermenters make acid and turn pink, non-fermenters stay colourless.
How it differs from the standard method
- Read at 18–24 h: earlier, fermenters are still colourless; later, pink acid zones spread over neighbouring colonies.
- Pick colourless colonies only when they stand clear of pink ones, and subculture to non-selective agar before spot tests such as oxidase.
- Check the formulation: MacConkey without crystal violet lets staphylococci and enterococci grow.
- Some organisms ferment lactose slowly and may look colourless at 24 h; an identification test, not colour alone, decides.
1Who grows, and what they look like
A plain nutrient agar lets almost anything grow and makes most bacteria look alike: cream or white colonies of slightly different sizes. Two additions change that.
MacConkey agar
Bile salts and crystal violet suppress Gram-positives; lactose and neutral red turn lactose fermenters pink while non-fermenters stay colourless.
Blood agar
5% sheep blood in a rich base. Colonies that lyse the red cells completely give clear β-zones, partial lysis gives green α-zones, none is γ.
Antibiotic plates
After a transformation, only cells carrying the resistance plasmid should grow. Selection holds only while the drug stays above the sensitive cells' MIC.
2MacConkey: acid makes pink
On MacConkey agar a Gram-negative that ferments lactose makes acid, which turns the neutral red indicator pink to red and precipitates bile salts round the colony as a hazy pink zone. A non-fermenter uses the peptones instead, so its colonies stay colourless or take the pale colour of the agar.
3Blood agar: reading the zone
Haemolysis is read by holding the plate against a light and looking through it. A β-zone is truly clear; an α-zone is a greenish or brownish discoloration where haemoglobin has been oxidised but red-cell ghosts remain.
4Antibiotic selection: a moving target
Ampicillin plates select for cells carrying a bla gene, which encodes a β-lactamase the cells secrete into the agar. That enzyme is the weakness of ampicillin selection: given enough time, the drug round each resistant colony is destroyed and sensitive cells grow there as small satellite colonies, which carry no plasmid.
- Lactose fermenter (LF)
- An organism that ferments lactose to acid; pink to red colonies on MacConkey.
- Non-lactose fermenter (NLF)
- An organism that does not, or does so too slowly to show; colourless colonies on MacConkey.
- Satellite colonies
- Small colonies of plasmid-free cells growing next to a resistant colony that has destroyed the antibiotic round itself.
Key terms
Practise these as flashcards →A medium with an agent that suppresses part of the microbial population, such as bile salts, crystal violet, high salt or an antibiotic.
A medium that makes different organisms look different, usually through a substrate and an indicator or through red blood cells.
Complete lysis of red cells round a colony, a clear zone seen against the light.
Partial lysis with oxidation of haemoglobin, a green-brown zone with intact red-cell ghosts.
An oxygen-labile haemolysin of Streptococcus pyogenes; it acts fully only below the surface or with reduced oxygen.
Reference organisms with known reactions streaked on each new batch of medium to prove it selects and differentiates as it should.
An enzyme that hydrolyses penicillins such as ampicillin; secreted by cells carrying a bla gene.
When to use it
- Isolating Gram-negative enteric bacteria from a mixed sample and flagging the non-lactose fermenters for further identification: MacConkey agar.
- Recognising β-haemolytic streptococci in a throat culture, or describing the haemolysis of an unknown: blood agar.
- Recovering only cells that took up a plasmid or carry a resistance marker after transformation, conjugation or gene editing: antibiotic selection plates.
Compared with alternatives
A Gram stain sorts cells by wall type in minutes, from a single colony or a specimen; selective plates sort them by growth overnight and keep them alive for the next tests.
The step after the plate: catalase, oxidase, biochemical strips or MALDI-TOF identify the organism behind a pink, colourless or β-haemolytic colony.
How the cells go onto the plate; selective and differential media change what the agar does to them.
Where antibiotic selection plates are used most: getting the plasmid in, recovering the cells and choosing the selection.
Measures how much drug it takes to stop a particular isolate, rather than using a fixed dose to select resistant cells.
Sub-techniques
Choosing one here switches the whole page, including the protocol and the lab, to it.
Selective and differential agar for Gram-negative bacteria. Bile salts and crystal violet suppress Gram-positives; lactose fermenters make acid and turn pink, non-fermenters stay colourless.
- Read at 18–24 h: earlier, fermenters are still colourless; later, pink acid zones spread over neighbouring colonies.
- Pick colourless colonies only when they stand clear of pink ones, and subculture to non-selective agar before spot tests such as oxidase.
- Check the formulation: MacConkey without crystal violet lets staphylococci and enterococci grow.
- Some organisms ferment lactose slowly and may look colourless at 24 h; an identification test, not colour alone, decides.