Selective and Differential Media

Selective media contains certain compounds or chemicals that favor the growth of certain species of bacteria.  Selective media is often used when one is trying to isolate a specimen of interest from a mixed culture.  Let us say that the specimen of interest is one cell in a million.  It is unlikely that anyone could isolate such a rare specimen from a mixed sample.  Yet, if one enriches for the specimen of interest with a selective media, an isolation may become much more likely if not probable.  Selective media is often used when trying to find the rare pathogen from a patients numerous mixed normal flora.  As some organisms inhibit or grow faster than others, using selective media may be required.  Selective media can even sometimes be used to tentatively identify an organism, such as if it grows on an MSA plate, it is likely a Staphylococcus species.  (MSA is a selective media.)

Differential media contains certain indicators (compounds) that allow one to differentiate between different species of bacteria.  The test is often positive or negative, allowing one to differentiate between two species.  For example, Staphylococcus aureus when grown on MSA ferments the sugar mannitol, but Staphylococcus epidermidis does not.  (MSA is both a differential media and a selective media.)

It is recommended that students streak their unknown culture, E. coli, and Staphylococcus epidermidis onto the following selective and differential media, but other species may be used.  E. coli is positive for growth and lactose metabolism on MAC and is negative for growth on PEA.  Staphylococcus epidermidis is negative for growth on MAC, is positve for growth on PEA and MSA.  

1.  Inoculate MacConkey agar (MAC) plates, Atlas, pg. 18.

The selective and differential aspects are due to crystal violet, the bile salts, the lactose & indicators in the medium.   Selective:  it encourages some bacteria to grow while inhibiting others.  Crystal violet and bile salts generally inhibit Gram positive bacteria (some will grow) and generally allows growth of Gram negative organism (some will not grow).  

Differential:  the media contains indicators (lactose and a pH indicator) that expose differences between species.  These plates are used for distinguishing enteric coliforms (diarrheal disease, stool samples, etc.) from enteric noncoliforms.  These plates are differential due to presence of lactose:

One predicts that if organism grows on MAC it is most likely Gram negative, BUT YOU HAVE TO CONFIRM THIS with a GRAM STAIN, as a few Gram positive organisms will grow on Mac plates.   (We do not discuss lactose negative (or positive) of these few Gram positives.)  For pictures & further discussion, see Atlas, pg. 18.  Incidentally, the difference between a coliform and a noncoliform is simply whether the enteric can metabolize lactose.

2.   Inoculate a TSA or NA Plate as a Control

3.  Inoculate a Phenylethyl Alcohol plate (PEA).

PEA is a Selective Media
Selective -phenylethyl alcohol inhibits the growth of most Gram negative organisms (but not all of them) and encourages the growth of Gram positive organisms (not all of them, but staphlococci, streptococci, enterococci, lactococci & others do grow).  

4.  Inoculate Mannitol Salt Agar (MSA) plate, Atlas, p 18-19, Fig, 2-18 & 19.

MSA is a selective & differential media that favors the growth of Staphylococcus
Selective - high sodium chloride concentration inhibits the growth of most organisms, but not Staph.

Differential -the sugar mannitol and a pH indicator make this media differential.  If the organism ferments mannitol, an acid is produced turning the indicator yellow.  Nonpathogenic Staph do not ferment mannitol, some pathogenic Staph do.

5.  How to Plate the Above Quickly


You will need to view your plates to make your observations in one to two days when incubated at 35-37 C.  Occasionally with slow growing organisms, you may need to incubate them for a little longer.  Look for the results as detailed above.