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
|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
EMB is both selective and differential
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.
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.
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.
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.
Two species grown on a Mannitol Salt Agar (MSA) plate
MSA is both selective and differential
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.
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.
Beta-hemolysis on a blood agar plate
Blood agar plates are differential