Theo Nash
Phenylethyl alcohol agar, also known as PEA agar, is a selective medium used to cultivate gram-positive bacteria, particularly cocci, while inhibiting most gram-negative bacteria and fungi. PEA agar is often used to selectively isolate Staphylococcus and Streptococcus species from clinical specimens or samples containing mixtures of bacterial flora. It is also useful for inhibiting common contaminants such as Escherichia coli and Proteus species. PEA agar's ability to alter the membrane permeability of gram-negative bacteria, leading to the disruption of DNA synthesis, makes it a valuable tool in the cultivation and isolation of specific bacterial species.
| Characteristics | Values |
|---|---|
| Selective for | Gram-positive organisms, particularly cocci |
| Selective against | Most gram-negative bacteria and fungi |
| Used for | Isolation of gram-positive Staphylococcus and Streptococcus species from clinical specimens or specimens that contain mixtures of bacterial flora |
| Inhibits | Common contaminants such as Escherichia coli and Proteus species |
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What You'll Learn
Inhibition of gram-negative bacteria
Phenylethyl alcohol agar (PEA) is a selective medium used to cultivate and isolate gram-positive bacteria, particularly cocci, from a sample containing a mixture of pathogens. It is effective in inhibiting the growth of most gram-negative bacteria, such as Escherichia coli, Proteus species, and Klebsiella pneumoniae.
The inhibition of gram-negative bacteria by PEA occurs through altering their membrane permeability. This alteration allows the influx of molecules that would usually be blocked, leading to a leakage of cellular potassium. Consequently, this disruption inhibits DNA synthesis in gram-negative bacteria, preventing their growth.
PEA is particularly useful in the selective isolation of anaerobes from mixed populations containing rapidly growing gram-negative bacteria. It effectively inhibits the swarming of Proteus species and Clostridium septicum, which are common contaminants. However, it's important to note that PEA may not inhibit all gram-negative bacteria, as Pseudomonas aeruginosa, for instance, is not inhibited on this medium.
While PEA is selective, it may not provide complete information for the identification of bacterial isolates. Therefore, additional tests and non-selective media are often necessary to ensure the recovery of all species present in a sample. This includes the use of media like Brucella Blood Agar to complement the use of PEA agar.
In summary, PEA agar is a valuable tool for selectively cultivating and isolating gram-positive bacteria while effectively inhibiting most gram-negative bacteria through the disruption of their DNA synthesis. However, it is not a comprehensive solution, and additional testing is typically required for comprehensive bacterial identification and analysis.
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Selective growth of Staphylococcus
Phenylethyl alcohol agar plates are selective for gram-positive organisms, particularly gram-positive cocci such as Micrococcus luteus and Staphylococcus species, including Staphylococcus epidermidis and Staphylococcus aureus.
Staphylococcus aureus is a Gram-positive, coccus-shaped bacterium, a facultative anaerobe that can grow without oxygen. It is a common member of the human body's microbiota, frequently found in the upper respiratory tract, nasal passages, and on the skin. While S. aureus usually acts as a commensal organism in the human microbiota, it can also become an opportunistic pathogen, causing a range of illnesses. These range from minor skin infections, such as pimples, impetigo, boils, and abscesses, to life-threatening diseases, including pneumonia, meningitis, endocarditis, toxic shock syndrome, sepsis, and more.
Staphylococcus epidermidis is another significant species within the Staphylococcus genus. It is often found on the skin and can cause infections, particularly in individuals with weakened immune systems or those with indwelling medical devices.
To selectively grow Staphylococcus species, such as S. aureus or S. epidermidis, phenylethyl alcohol agar plates can be utilized. This selective medium inhibits the growth of common contaminants and gram-negative bacteria, such as Escherichia coli and Proteus species. The growth conditions for S. aureus should be considered when using phenylethyl alcohol agar plates. S. aureus can grow at temperatures between 15°C and 45°C, but extended exposures above 42°C or below 10°C are not recommended. Additionally, S. aureus is resistant to high osmolarity, detergents, and alcohol, which can inform the preparation of the phenylethyl alcohol agar plates for optimal selectivity.
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Isolation of gram-positive cocci
Phenylethyl alcohol (PEA) agar is a selective medium used to isolate and cultivate Gram-positive cocci, particularly from samples containing a mixture of pathogens or flora. PEA agar allows for the growth of Gram-positive organisms while inhibiting most Gram-negative bacteria and fungi. This is because PEA alters the membrane permeability of Gram-negative bacteria, allowing the influx of molecules that would usually be blocked. This results in a leakage of cellular potassium and the disruption or inhibition of DNA synthesis.
PEA agar is especially useful for isolating Staphylococcus and Streptococcus species from clinical specimens or mixed cultures. For example, PEA agar can be used to isolate Staphylococcus epidermidis and Micrococcus luteus from a sample also containing the Gram-negative Klebsiella pneumoniae. PEA agar can also be used to isolate Staphylococcus saprophyticus from infected human urine.
PEA agar is also useful for the selective isolation of anaerobes from mixed populations that contain rapidly growing Gram-negative bacteria. The medium can be used to isolate most Gram-positive and Gram-negative anaerobes from enteric samples. For example, PEA agar with 5% sheep blood can be used to isolate Enterococcus faecalis from a sample.
It is important to note that PEA agar will not provide complete information for the identification of bacterial isolates. Additional test procedures and media are required for a full identification. Furthermore, some Gram-positive cocci may be slightly inhibited by PEA and require longer incubation periods for sufficient growth.
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Inhibition of Proteus species
Phenylethyl alcohol (PEA) agar is a selective medium used for the isolation of gram-positive Staphylococcus and Streptococcus species. It is particularly effective in inhibiting gram-negative bacteria, such as Proteus species, which include P. mirabilis and P. vulgaris.
Proteus species are Gram-negative, rod-shaped, and aerobic or facultatively anaerobic bacteria. They are known for their distinctive swarming motility, which is driven by flagella and allows them to migrate across surfaces. This swarming behaviour is a key factor in their ability to cause infections, particularly in wounds, the urinary tract, and the kidneys. P. mirabilis, for example, is responsible for 90% of Proteus infections in humans and is commonly associated with urinary tract infections, kidney stones, and wound infections.
The inhibition of Proteus species, specifically their swarming motility, is crucial in preventing and treating infections caused by these bacteria. Phenylethyl alcohol agar is an effective tool in this regard, as it inhibits the growth and swarming of Proteus species. This inhibition is attributed to the bacteriostatic properties of phenylethyl alcohol against gram-negative bacteria, which reversibly inhibit DNA synthesis.
Triclosan, an antimicrobial agent, has also been found to effectively inhibit the growth and swarming of Proteus mirabilis and Proteus vulgaris. In vitro studies have shown that low concentrations of triclosan can successfully impede the growth and movement of these bacteria, even in the presence of blood, which typically enhances their swarming behaviour.
Additionally, the distinctive characteristics of Proteus species, such as their ability to produce urease and their unique growth patterns, can be utilised for identification and differentiation from other bacteria. For example, the bulls-eye pattern exhibited by Proteus mirabilis during growth on solid media is a distinguishing feature. These identification methods aid in the development of targeted treatments and contribute to our understanding of Proteus species' behaviour and impact on human health.
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Use in mixed bacterial flora
Phenylethyl alcohol agar plates are selective for Gram-positive bacteria and are often used to isolate and identify specific genera or species within this group. This selectivity is due to the presence of phenylethyl alcohol, which inhibits the growth of most Gram-negative bacteria, as well as certain Gram-positive bacteria that are sensitive to its antimicrobial effects.
When used in the context of mixed bacterial flora, such as in You may want to see also Phenylethyl alcohol agar plates are selective for gram-positive organisms, particularly gram-positive cocci. Phenylethyl alcohol agar plates can be used for the isolation of gram-positive Staphylococcus species such as Staphylococcus epidermidis and Streptococcus species from clinical specimens or specimens that contain mixtures of bacterial flora. Phenylethyl alcohol agar plates inhibit most gram-negative bacteria, including Escherichia coli, Proteus species, and Klebsiella pneumoniae.Illinois Alcohol Transportation: Do You Need a License?
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