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Selective and Differential Media

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Petri plates are commonly used to grow colonies of bacteria from samples that are a mix of several to hundreds of different species.

such as dirt, sea water, a plaque sample, throat swab or skin infection

In many cases there is a need to selectively grow one type of bacteria while inhibiting the growth of other types

grow Gram Positive bacteria, while inhibiting Gram Negative bacteria for example

It is also useful to be able to differentiate one species from another when several different species can grow on the same media type.

Differentiate S. aureus from S. epidermidis for example

Examples of Common Selective Media

Media Selective For
Eosin Y Methylene Blue (EMB) Gram Negative Bacteria
MacConkey Agar Gram Negative Bacteria
Mannitol Salt Agar (MSA) Gram Positive Bacteria

Escherichia coli grown on an Eosin Y Methylene Blue (EMB) plate

E coli on EMB

EMB is both selective and differential

EMB is Selective

The addition to the media of the dyes Eosin Y and Methylene Blue inhibit the growth of Gram Positive bacteria and allow for the growth (or select for) Gram Negative bacteria.

EMB is also Differential

In the presence of Eosin Y and Methylene Blue, enteric bacteria such as E. coli that ferment lactose produce colonies that have a dark nucleated appearance and sometimes a metallic green sheen.

EMB Selective Compenents

  • Eosin Y
  • Methylene Blue

EMB Differential Components

  • lactose

MacConkey Agar is Selective

The addition to the media of the dye Crystal Violet and bile salts inhibit the growth of Gram Positive bacteria and allow for the growth (or select for) Gram Negative bacteria.

MacConkey Agar is also Differential

Enteric bacteria such as E. coli that ferment lactose will lower the pH of the media as a result of the fermentation. A pH indicator neutral red which is added turns pink in acidic conditions. Colonies of enterics such as E. coli develop a characteristic pink color on the media which makes the media differential.

MacConkey Agar Selective Compenents

  • Crystal Violet
  • Bile Salts

MacConkey Agar Differential Components

  • Neutral Red
  • lactose

Two species grown on a Mannitol Salt Agar (MSA) plate

Mannitol Salt Agar

MSA is both selective and differential

Mannitol Salt Agar is Selective

The addition to the media of a high salt concentration inhibits the growth of Gram Negative bacteria and allows for the growth (or selects for) Gram Positive bacteria.

Mannitol Salt Agar is also Differential

Some Gram Positive species (Staphylococcus aureus) are capable of fermenting mannitol and in the process lower the pH of the media. The pH indicator phenol red in the media turns yellow when the media is acidified, thus making the media differential.

Mannitol Salt Agar Selective Compenents

  • NaCl (7.5-10%)

Mannitol Salt Agar Differential Components

  • phenol red
  • mannitol

Beta-hemolysis on a blood agar plate

blood agar beta hemolysis

Blood agar plates are differential