Phenylethyl Alcohol Agar: Defined Or Complex?

Phenylethyl Alcohol Agar, also known as PEA, is a type of selective medium used to cultivate and isolate specific types of bacteria, particularly gram-positive cocci, while inhibiting the growth of most gram-negative bacteria. The active ingredient, phenylethyl alcohol, plays a crucial role in disrupting the growth of gram-negative organisms by interfering with their DNA synthesis. This medium is often used when dealing with samples that contain a mixture of pathogens, such as gastrointestinal content or peritoneal fluid. The question arises: is PEA considered a defined or undefined medium?

Phenylethyl Alcohol Blood Agar (PEA) is a selective medium

PEA agar is used when the sample source contains a mixture of pathogens (e.g., gastrointestinal content or peritoneal fluid) or when a Gram stain indicates the presence of Gram-negative rods. It inhibits Gram-negative bacteria, specifically Proteus species, in specimens containing mixed bacterial flora. It also inhibits the swarming of certain Clostridium species and Proteus species from clinical specimens.

The active ingredient in PEA, phenylethyl alcohol, inhibits or significantly reduces the growth of Gram-negative organisms by interfering with DNA synthesis. It alters the membrane permeability of Gram-negative bacteria, allowing the influx of otherwise blocked molecules. This results in the leakage of large amounts of cellular potassium, ultimately disrupting or inhibiting DNA synthesis.

PEA agar is prepared with and without 5% sheep blood supplement. The blood is added to enhance the growth of anaerobic bacteria. If used properly, PEA supports the growth of most anaerobes found in clinical infections. It is recommended that a non-selective medium, such as Brucella Blood Agar, also be inoculated from the same clinical specimen to ensure the recovery of all species present, as some organisms that would typically grow on PEA agar may be inhibited.

PEA inhibits the growth of gram-negative bacteria

Phenylethyl Alcohol (PEA) agar is a selective medium used for the inhibition of gram-negative bacteria. It is particularly effective against facultative anaerobic gram-negative rods such as Escherichia coli and Proteus species. PEA agar has a high concentration of phenylethyl alcohol, which acts as the selective agent. This high concentration inhibits the growth of gram-negative bacteria by disrupting their lipid structure and reducing protein synthesis.

PEA agar is often used to inhibit common contaminants such as Escherichia coli and Proteus species, which can interfere with the growth of desired organisms. By inhibiting the growth of these gram-negative bacteria, PEA agar helps promote the growth of gram-positive organisms, particularly cocci. This characteristic makes it especially useful for the selective isolation of gram-positive Staphylococcus and Streptococcus species from clinical specimens or specimens with mixed bacterial flora.

The ability of PEA agar to inhibit gram-negative bacteria is attributed to its disruption of their cell membranes. Specifically, PEA disrupts the lipid structure of gram-negative bacterial membranes, affecting their integrity and function. This disruption can lead to a loss of cell contents and the leakage of essential ions and metabolites, ultimately inhibiting bacterial growth.

Furthermore, PEA agar also inhibits DNA synthesis in gram-negative bacteria. By reversibly inhibiting DNA synthesis, PEA prevents these bacteria from replicating and dividing, effectively stalling their growth. This inhibition of DNA synthesis is a key mechanism by which PEA agar selectively suppresses the proliferation of gram-negative bacteria while allowing the growth of gram-positive organisms.

While PEA agar is effective against gram-negative bacteria, it is important to note that it may not inhibit all species within this group. Some gram-negative bacteria may still grow on PEA agar, depending on their specific characteristics and adaptations. Therefore, when using PEA agar, it is recommended to use it in conjunction with a non-selective medium to ensure the recovery of all species present in a clinical specimen. This complementary approach helps provide a comprehensive analysis and identification of bacterial isolates.

PEA is used for the selective growth of Staphylococcus and Streptococcus

Phenylethyl Alcohol Agar (PEA) is a nutrient-rich medium used for the selective growth of Staphylococcus and Streptococcus. It is selective as it inhibits the growth of gram-negative microbes and associated fungi in test samples. The active ingredient, phenylethyl alcohol, disrupts the membrane permeability of gram-negative bacteria, allowing the influx of otherwise blocked molecules. This results in the leakage of large amounts of cellular potassium, ultimately disrupting or inhibiting DNA synthesis.

PEA is particularly useful for cultivating Gram-positive organisms, specifically cocci, from a sample containing a mixture of pathogens. It is often used when the sample source contains a mixture of pathogens, such as gastrointestinal content or peritoneal fluid, or when a Gram stain indicates the presence of Gram-negative rods.

PEA agar is especially effective in the selective isolation of anaerobes from mixed populations that contain rapidly growing gram-negative bacteria, such as Proteus species. It can also be used to inhibit the swarming of certain Clostridium species, specifically Clostridium septicum.

PEA agar with 5% sheep blood is used to isolate most gram-positive and gram-negative anaerobes from enteric samples. The blood is added to the base medium to enhance the growth of anaerobic bacteria. However, PEA agar may slightly inhibit some gram-positive cocci, and many require incubation for up to 48 hours for sufficient growth to be visible.

PEA agar can be purchased as a premixed powder

Phenylethyl Alcohol Agar (PEA) is a selective medium used to cultivate Gram-positive organisms, particularly cocci, from a sample containing a mixture of pathogens. It is also used to selectively grow Streptococcus and Staphylococcus in mixed cultures. PEA agar is especially useful for the inhibition of certain Clostridium species and Proteus species from clinical specimens.

PEA agar with 5% sheep blood can be used to isolate most gram-positive and gram-negative anaerobes from enteric specimens. The 5% sheep blood is added to the base medium to enhance the growth of anaerobic bacteria. To prepare PEA agar with 5% sheep blood supplementation, the ingredients must be mixed in one litre of distilled water. The mixture should be heated with constant stirring until it is completely dissolved. It should then be autoclaved for 15 minutes at 121°C and 15 lb/in2. The melted medium should then be allowed to cool in sterilized Petri dishes. Before inoculation, the media should be at room temperature.

PEA agar plates can be incubated under aerobic, anaerobic, and 5% CO2 atmosphere. Incubation in a high CO2 atmosphere allows the detection of bacteria that require an increased CO2 concentration and also results in better growth of almost all of the other pathogens. Gram-positive bacteria demonstrate good growth while most gram-negative bacteria do not grow or are partially inhibited.

PEA is prepared with and without 5% sheep blood supplement

Phenylethyl Alcohol Agar (PEA) is a selective medium used to cultivate Gram-positive organisms, particularly cocci, from a sample containing a mixture of pathogens. The active ingredient, phenylethyl alcohol, inhibits or reduces the growth of Gram-negative organisms by interfering with DNA synthesis.

PEA agar is prepared by heating the nutritive base—consisting of casein, soy peptone, meat peptone, yeast extract, dextrose, hemin, and vitamin K1—with frequent agitation, boiling for one minute to dissolve it completely. Once the base medium has cooled, 5% sterile defibrinated sheep blood can be added to enhance the growth of anaerobic bacteria. The addition of sheep blood is optional, and PEA can be prepared without it. The melted medium is then poured into sterilized Petri plates and allowed to solidify.

When preparing PEA with 5% sheep blood supplementation, the following additional step is required: aseptically add sterile 5% sterile defibrinated sheep blood and mix well. The blood stimulates the growth of anaerobes. Sodium chloride is also added as a source of essential electrolytes and to maintain osmotic equilibrium.

PEA agar plates can be incubated under aerobic, anaerobic, and 5% CO2 atmosphere depending on the objectives of the study. For the cultivation of anaerobic bacteria, the plates are typically incubated anaerobically to create an oxygen-free environment. This is important as PEA is designed to inhibit the growth of facultative anaerobic gram-negative rods, such as Escherichia coli, and the swarming of Proteus mirabilis.

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