[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"$fxLN3MUwXCdr5RPjwZYIDpOj8CHyjOmngWTgoKXPtZbg":3,"$fBUSu0o6_axgVj_kCOPBczKCABVS_VidWABIxm7keojk":32,"$f3Ft0rKFJHppdzE-vuveecxx1BUcg9iOlMLtyzf_MJDg":57},[4,8,12,16,20,24,28],{"title":5,"slug":6,"path":7},"About Microbeonline.com","about-microbeonline-com","\u002Fabout-microbeonline-com\u002F",{"title":9,"slug":10,"path":11},"About Me","about-me","\u002Fabout-microbeonline-com\u002Fabout-me\u002F",{"title":13,"slug":14,"path":15},"Advertise with Us","advertise-us","\u002Fadvertise-us\u002F",{"title":17,"slug":18,"path":19},"Privacy Policy","privacy-policy","\u002Fprivacy-policy\u002F",{"title":21,"slug":22,"path":23},"Abbreviations","abbreviations","\u002Fabbreviations\u002F",{"title":25,"slug":26,"path":27},"Microbes","microbes","\u002Fmicrobes\u002F",{"title":29,"slug":30,"path":31},"Books","recommended-books","\u002Frecommended-books\u002F",{"type":33,"data":34},"blog",{"slug":35,"title":36,"description":37,"seoTitle":38,"seoDescription":38,"author":39,"createdDate":40,"lastUpdatedDate":41,"draft":42,"category":43,"image":38,"body":44,"faq":45,"tags":55,"related":56},"phenylethyl-alcohol-agar-pea-principle-composition-and-preparation","Phenylethyl Alcohol (PEA) Agar: Composition, Principle, Uses, and Applications","PEA agar selects for gram-positive organisms and anaerobes by inhibiting gram-negative bacteria through membrane permeability disruption. Learn when to use PEA vs Columbia CNA, and its critical role in anaerobic culture.",null,"Acharya Tankeshwar","2021-05-09","2026-06-28",false,"culture-media","A peritoneal fluid specimen from a patient with abdominal sepsis is plated on blood agar, MacConkey agar, and phenylethyl alcohol blood agar. MacConkey grows several enteric gram-negatives. PEA blood agar, incubated anaerobically, grows *Bacteroides fragilis* — the clinically most important anaerobe in intra-abdominal infection — that was completely obscured on blood agar by the swarming *Proteus mirabilis* growing in the same specimen.\n\nThis is PEA agar's most important clinical role: suppressing the swarming of *Proteus* and *Clostridium septicum* to prevent them from overgrowing slow-growing anaerobes and gram-positive organisms in mixed anaerobic cultures.\n\nPhenylethyl 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 markedly reduces the growth of Gram-negative organisms by interfering with DNA synthesis. *Staphylococcus aureus*, a Gram-positive organism, grows on PEA while Serratia marcescens, a Gram-negative organism, does not.\n\n## Principle\n\nPhenylethyl alcohol agar (PEA) is a selective medium that permits the growth of gram-positive cocci while inhibiting most gram-negative organisms. PEA alters the membrane permeability, of Gram-negative bacteria allowing influx of otherwise blocked molecules. This results in leakage of large amounts of cellular potassium that ultimately results in disruption or inhibition of DNA synthesis of Gram-negative bacteria.\n\n## Composition of PEA\n\nPEA agar medium can be purchased as a premixed powder from suppliers. The manufacturer’s instructions should be followed to prepare the plates. This media can also be purchased as premade agar plates.\n\n| Ingredients | Amounts (gm\u002FL) |\n| --- | --- |\n| Pancreatic digest of casein | 15.0 g |\n| Papic digest of soybean meal | 5.0 g |\n| Sodium chloride | 5.0 g |\n| Agar | 15.0 g |\n| Sterile defibrinated sheep blood | 50.0 ml |\n| β-Phenylethyl alcohol | 2.5 g |\n| Distilled water | 1 liter |\n\nFinal pH at 25°C 7.3 ± 0.2.\n\n## Preparation of the Media\n\nPhenylethyl alcohol agar (PEA) may be prepared with and without 5% sheep blood supplement. Five percent sheep blood is added to the base medium to enhance the growth of anaerobic bacteria.\n\n| Characteristics | PEA without sheep blood | PEA with 5% sheep blood |\n| --- | --- | --- |\n| Color of the prepared media | clear to slightly hazy and pale yellow. | firm, opaque, and red in color. |\n| Shelf live | prepared 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. |\n\n> Step no. 5 is required while preparing PEA with 5% sheep blood supplementation.\n\n**Note:**\n\n## Culture\n\n### Inoculation\n\n> Stored media should be brought to room temperature before inoculation.\n\nAseptically transfer potentially mixed cultures onto the surface of the agar using a streaking or [spreading technique](\u002Fspread-plate-technique\u002F), depending on the objectives of the study.\n\n### Incubation\n\nIncubate plates for 24 to 48 hours at 35°C ± 2 °C in an appropriate atmosphere. Depending on the objectives of the study, PEA blood 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.\n\n### Interpretation of results\n\nAfter proper incubation growth of bacteria will appear as colonies on the surface of agar plates. Gram-positive bacteria demonstrate good growth while most gram-negative bacteria do not grow or are partially inhibited.\n\n![ - PEA agar plates with 5% sheep blood incubated at 48 hours at 35°CLeft:Escherichia coli.Right:Staphylococcus aureusImage source: Cheeptham and Farday, ASM MicrobeLibrary](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FPEA-Blood-Agar.jpg)Figure: 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\n\n## Growth Response\n\n**Growth response of some gram-positive and gram-negative bacteria on PEA agar**\n\n| Organism | Gram reaction | Growth response |\n| --- | --- | --- |\n| Escherichia coli | Gram-negative | Inhibited |\n| Enterobacter aerogenes | Gram-negative | Inhibited |\n| Proteus mirabilis | Gram-negative | Markedly inhibited. Swarming inhibition. |\n| Pseudomonas aeruginosa | Gram-negative | Partially inhibited |\n| Salmonella enteritidis | Gram-negative | Inhibited |\n| Bacillus sp. | Gram-positive | Good |\n| Clostridium perfringens | Gram-positive | Partially inhibited |\n| Enterococcus faecalis | Gram-positive | Good |\n| Micrococcus luteus | Gram-positive | Good |\n| Staphylococcus aureus | Gram-positive | Good |\n| Streptococcus pneumoniae | Gram-positive | Good |\n| Streptococcus pyogenes | Gram-positive | Good |\n\n![ - PEA agar plates without blood incubated at 48 hours at 35°CLeft:Escherichia coli.Right:Enterococcus faecalisImage source: Cheeptham and Farday, ASM MicrobeLibrary](https:\u002F\u002Fassets.microbeonline.com\u002Fblogs\u002FPEA-Plain-growth-and-no-growth.jpg)Figure: PEA agar plates without blood incubated at 48 hours at 35°C Left: *Escherichia coli.* Right: *Enterococcus faecalis* Image source: Cheeptham and Farday, ASM MicrobeLibrary\n\n## Uses\n\n- PEA agar is used if the sample source contains a mixture of pathogens (e.g., gastrointestinal content or peritoneal fluid) or Gram stain indicates that the culture contains Gram-negative rods. Phenylethyl alcohol agar inhibits gram-negative bacteria, specifically *Proteus* species, in specimens containing mixed bacterial flora. It is used for the selective growth of *Staphylococcus* and *Streptococcus* inmixed cultures.\n- PEA agar with 5% sheep blood is used to isolate most gram-positive and gram-negative anaerobes from enteric samples. It inhibits facultative gram-negative rods, preventing [Enterobacteriaceae](\u002Fenterobacteriaceae\u002F) from overgrowing the anaerobes and inhibiting the swarming of *Proteus* and *Clostridium septicum.*\n\n## PEA vs Columbia CNA Agar — Which to Choose?\n\nBoth PEA and Columbia CNA select for gram-positive organisms, but they use different mechanisms and have different strengths:\n\n| Feature | PEA Agar | Columbia CNA Agar |\n| --- | --- | --- |\n| Selective agents | Phenylethyl alcohol (disrupts membrane) | Colistin + Nalidixic acid (antibiotic inhibition) |\n| Mechanism | Membrane permeability disruption → K⁺ leakage → DNA synthesis failure | Colistin destroys outer membrane; nalidixic acid inhibits DNA gyrase |\n| *Pseudomonas* growth? | **Yes** — *P. aeruginosa* is NOT inhibited by PEA | Generally no — colistin inhibits most *Pseudomonas* |\n| Anaerobic use? | **Yes** — blood-supplemented PEA is standard for anaerobic gram-positive isolation | Limited — CNA primarily used aerobically |\n| Anti-swarming (*Proteus*) | **Yes** — phenylethyl alcohol specifically inhibits *Proteus* swarming | Less effective for swarming inhibition |\n| Primary use | Mixed anaerobic cultures; *Proteus* swarming specimens | Wound\u002Fgenital specimens; aerobic gram-positive isolation |\n\n**Key rule:** Use PEA for anaerobic cultures and specimens with swarming *Proteus*. Use Columbia CNA for aerobic gram-positive isolation from mixed aerobic specimens.\n\n## Limitations\n\n- Some gram-positive cocci may be slightly inhibited by PEA and many require incubation up to 48 hours for sufficient growth to be visible.\n- *Pseudomonas aeruginosa* (a gram-negative bacteria) is not inhibited on this medium. Other gram-negatives sometimes may give tiny observable colonies but they are often confined to the first quadrant on a streak plate.\n\n## Key Exam Facts in One Table\n\n| Feature | Detail |\n| --- | --- |\n| Type | Selective medium |\n| Selective agent | Phenylethyl alcohol (PEA) — disrupts gram-negative outer membrane |\n| Organisms that grow | Gram-positive cocci and anaerobes |\n| *Pseudomonas aeruginosa* | NOT inhibited — grows on PEA (key exception) |\n| *Proteus* swarming | Specifically inhibited — important clinical use |\n| Blood supplement | 5% sheep blood added for anaerobic version — enhances anaerobe growth |\n| Primary clinical use | Anaerobic cultures from mixed specimens; *Proteus*-contaminated specimens |\n| Incubation (aerobic) | 35°C, 24–48h |\n| Incubation (anaerobic) | 35°C, 48–72h in anaerobic atmosphere |\n| vs Columbia CNA | CNA preferred for aerobic gram-positive; PEA preferred for anaerobic and anti-swarming |\n\n**References and further readings**\n\n1. Lal A, Cheeptham N. Phenylethyl alcohol agar protocol. American Society for Microbiology; 2011.\n2. Garcia LS. Clinical Microbiology Procedures Handbook. 4th ed. ASM Press; 2016.\n3. Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 14th ed. Elsevier; 2023.\n4. Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2020.",[46,49,52],{"question":47,"answer":48},"How does phenylethyl alcohol agar selectively inhibit gram-negative bacteria?","Phenylethyl alcohol (PEA) disrupts the outer membrane of gram-negative bacteria by increasing membrane permeability — causing leakage of intracellular potassium ions and inhibiting DNA synthesis. Gram-positive bacteria lack the outer membrane that PEA targets and are therefore not inhibited at the concentrations used in the medium. An important exception is Pseudomonas aeruginosa, which is intrinsically resistant to PEA at standard concentrations and may grow on PEA agar — a key difference from Columbia CNA agar, where colistin usually suppresses Pseudomonas.",{"question":50,"answer":51},"What is the most important clinical use of PEA agar that distinguishes it from Columbia CNA agar?","PEA agar's most important distinguishing use is for anaerobic cultures from mixed specimens. Blood-supplemented PEA agar incubated anaerobically is a standard medium for isolating gram-positive anaerobes (Actinomyces, Peptostreptococcus, anaerobic streptococci) and the clinically important anaerobe Bacteroides fragilis from specimens with mixed aerobic and anaerobic flora. PEA also specifically inhibits the swarming of Proteus mirabilis, preventing it from overgrowing other organisms in wound and urine specimens. Columbia CNA is primarily used aerobically and is less effective for these applications.",{"question":53,"answer":54},"Why is PEA agar used in specimens with Proteus mirabilis contamination?","Proteus mirabilis has a characteristic swarming motility on moist agar surfaces — the organism spreads in concentric waves across the plate, overgrowing all other colonies and making isolation of other pathogens impossible. Phenylethyl alcohol specifically inhibits this swarming motility without completely killing Proteus (it may still grow as individual colonies rather than spreading sheets). This makes PEA agar valuable for wound swabs, urines, and any specimen where Proteus contamination threatens to obscure clinically important gram-positive organisms such as streptococci, enterococci, or staphylococci.",[],[],[58,64,71,76,80,84,89,94,98,102],{"slug":59,"name":39,"description":60,"image":61,"body":62,"postCount":63},"acharya-tankeshwar","Editor-in-chief","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Ftankeshwar-acharya-author-microbeonline.jpg","***Tankeshwar Acharya, MSc (Medical Microbiology)***\n\n*Tankeshwar Acharya is an Assistant Professor in the Department of Microbiology at Patan Academy of Health Sciences (PAHS), Nepal, where he has been teaching and practicing clinical microbiology for over 14 years. He is the founder of Microbe Online, one of the leading free microbiology education resources on the web, covering bacteriology, mycology, parasitology, immunology, and clinical laboratory diagnostics written from direct experience in both the classroom and the diagnostic laboratory.*",433,{"slug":65,"name":66,"description":67,"image":68,"body":69,"postCount":70},"ashma-shrestha","Ashma Shrestha","SEO Copywriter and Science Communicator\nKathmandu, Nepal","https:\u002F\u002Fassets.microbeonline.com\u002Fauthors\u002Fashma-shrestha.png","Ashma Shrestha holds a Master of Science in Medical Microbiology from the Institute of Science and Technology (IOST), Tribhuvan University, Nepal, where she developed a strong foundation in virology, molecular biology, and diagnostic microbiology.\n\nShe now works as an SEO Copywriter at Resolution Digital, where she combines her scientific training with research-driven content strategy. She is certified in Google Analytics and Google Business Profile (GBP), and brings a data-informed approach to science communication writing content that is not only accurate but structured to reach and serve the students who need it most.\n\nAt microbeonline, Ashma contributes articles primarily in virology and molecular biology, areas she finds most compelling for their mechanistic depth and their growing clinical relevance. Her writing reflects the same standard the site is built on: factual rigor, clear explanation of the *why* behind microbiology concepts, and content that helps students move from memorization to genuine understanding.\n\nShe is passionate about making complex microbiological concepts accessible without sacrificing accuracy; a skill that sits at the intersection of her scientific training and her professional work in content and SEO.",81,{"slug":72,"name":73,"description":74,"image":38,"body":38,"postCount":75},"sushmita-baniya","Sushmita Baniya","Author \u002F Contributor",32,{"slug":77,"name":78,"description":74,"image":38,"body":38,"postCount":79},"samikshya-acharya","Samikshya Acharya",20,{"slug":81,"name":82,"description":74,"image":38,"body":38,"postCount":83},"alisha-tripathi","Alisha Tripathi",6,{"slug":85,"name":86,"description":87,"image":38,"body":38,"postCount":88},"aastha-shrestha","Aastha Shrestha"," Author \u002F Contributor",10,{"slug":90,"name":91,"description":92,"image":38,"body":38,"postCount":93},"guest-author","Guest Author","Guest Author \u002F Contributor",2,{"slug":95,"name":96,"description":74,"image":38,"body":38,"postCount":97},"srijana-khanal","Srijana Khanal",18,{"slug":99,"name":100,"description":92,"image":38,"body":38,"postCount":101},"dr-poonam-acharya","Dr. Poonam Acharya",1,{"slug":103,"name":104,"description":74,"image":38,"body":105,"postCount":106},"nisha-rijal","Nisha Rijal","**Nisha Rijal** is a microbiologist and quality assurance specialist. She served for nearly 12 years as a microbiologist at the National Public Health Laboratory (NPHL), Nepal's national reference laboratory, and continues to work as a consultant microbiologist in international public health organization. ",51]