Culture Media

Phenylethyl Alcohol Agar – Composition, Preparation, Uses, Limitation

A selective medium called phenylethyl alcohol (PEA), is used to cultivate Gram positive organisms, especially cocci, in a sample containing pathogens. Phenylethyl...

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Sourav Bio
This article writter by Sourav Bio on January 12, 2022

Writer and Founder of Microbiologynote.com. I am from India and my main purpose is to provide you a strong understanding of Microbiology.

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Phenylethyl Alcohol Agar - Composition, Preparation, Uses, Limitation
Phenylethyl Alcohol Agar - Composition, Preparation, Uses, Limitation

A selective medium called phenylethyl alcohol (PEA), is used to cultivate Gram positive organisms, especially cocci, in a sample containing pathogens. Phenylethyl alcohol is the active ingredient that inhibits or significantly reduces growth of Gram-negative organisms. It interferes with DNA synthesis. Staphylococcus aureus is a Gram positive organism that grows on PEA, while Serratia marcescens is a Gram negative organism.

Principle of Phenylethyl Alcohol Agar

A selective medium, Phenylethyl alcohol (PEA), allows the growth of gram positive cocci and inhibits most gram negative organisms. PEA changes the membrane permeability of Gram-negative bacteria, allowing for the influx of other molecules. This causes large amounts of cellular potassium to leak into the cells, which ultimately leads to disruption or inhibition in DNA synthesis in Gram-negative bacteria.

Composition of PEA

Suppliers can purchase PEA agar medium as a premixed product. To prepare the plates, you should follow the manufacturer’s instructions. Premade agar plates can also be made from this media.

IngredientsAmounts (gm/L)
Pancreatic digest of casein15.0 g
Papic digest of soybean meal5.0 g
Sodium chloride5.0 g
Agar15.0 g
Sterile defibrinated sheep blood50.0 ml
β-Phenylethyl alcohol2.5 g
Distilled water1 liter

Final pH at 25°C 7.3 ± 0.2.

Preparation of the Media

You can make phenylethyl alcohol (PEA) with or without 5% sheep’s blood. To enhance the growth anaerobic bacteria, five percent sheep blood can be added to the base medium.

  1. Mix the ingredients in 1 liter of distilled Water.
  2. To dissolve the mixture completely, heat with constant stirring.
  3. Autoclave for 15 minutes at 121°C and 15 lb/in2.
  4. Keep it at 45 degrees Celsius
  5. Mix well.
  6. Allow the melted medium to cool in sterilized Petri dishes.

Step 5. is required when preparing PEA with 5% sheepblood supplementation.

Note:

CharacteristicsPEA without sheep bloodPEA with 5% sheep blood
Color of the prepared mediaclear to slightly hazy and pale yellow.firm, opaque, and red in color.
Shelf liveprepared plates can be stored in the refrigerator for up to 4 weeks before use.prepared plates could be stored in the refrigerator up to 1 week before use.

Culture Technique

Inoculation

Before inoculation, store media should be at room temperature.

Aseptically transfer potential mixed cultures onto the surface using a spreading or streaking technique depending on the purpose of the study.

Incubation

Plates should be incubated for between 24 and 48 hours at 35degC +-2 degC in an appropriate environment. According to the purpose of the study, PEA blood-agar plates can either be incubated under anaerobic or aerobic conditions. High CO2 conditions allow bacteria to be detected and can also promote the growth of most other pathogens.

Interpretation of results

The colonies will form on the agar plates after proper incubation. While Gram-positive bacteria can grow well, most gram-negative bacteria cannot or are only partially inhibited.

Interpretation of results
PEA agar plates with 5% sheep blood incubated at 48 hours at 35°C
Left: Escherichia coli. Right: Staphylococcus aureus
Image source: Cheeptham and Farday, ASM MicrobeLibrary

Growth Response

Growth response to some gram positive and gram negative bacteria on PEA agar

OrganismGram reactionGrowth response
Escherichia coliGram-negativeInhibited
Enterobacter aerogenesGram-negativeInhibited
Proteus mirabilisGram-negativeMarkedly inhibited. Swarming inhibition.
Pseudomonas aeruginosaGram-negativePartially inhibited
Salmonella enteritidisGram-negativeInhibited
Bacillus sp.Gram-positiveGood
Clostridium perfringensGram-positivePartially inhibited
Enterococcus faecalisGram-positiveGood
Micrococcus luteusGram-positiveGood
Staphylococcus aureusGram-positiveGood
Streptococcus pneumoniaeGram-positiveGood
Streptococcus pyogenesGram-positiveGood

Quality Control 

After checking for correct pH, colour, depth, and sterility, the following organisms are used to determine the growth performance of the completed medium. 

OrganismExpected Result
Streptococcus pyogenes
ATCC 19615
Growth
Staphylococcus aureus
ATCC 25923
Growth
Proteus mirabilis
ATCC 12453
Partial Inhibition

Uses of Phenylethyl Alcohol Agar (PEA)

  • If the source of the samples contains multiple pathogens, such as peritoneal fluid or gastrointestinal contents, PEA agar can be used. Gram staining indicates that the culture has Gram-negative rods. In specimens with mixed bacterial flora, phenylethyl alcohol-agar inhibits Gram-negative bacteria, particularly Proteus species. It is used to selectively grow Streptococcus and Staphylococcus in mixed cultures.
  • PEA agar containing 5% sheep blood can be used to isolate most gram positive and gram negative anaerobes from enteric specimens. It prevents facultative gram negative rods from growing, which is important for Enterobacteriaceae to avoid overgrowing the anaerobes. Also, it inhibits the swarming by Clostridium septicum and Proteus.

Limitations

  • Some gram-positive cocci can be inhibited slightly by PEA. Many require incubation for up to 48 hours to allow sufficient growth to be visible.
  • This medium does not inhibit Pesudomonas Aeruginosa, a gram-negative bacteria. While some gram-negatives may produce tiny colonies, they are usually limited to the first quadrant of a streak plate.
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Writer and Founder of Microbiologynote.com. I am from India and my main purpose is to provide you a strong understanding of Microbiology.

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