Why PEA Agar Doesn't Grow E. coli: Medium Selectivity Explained

PEA (Phenylethyl Alcohol) Agar is a selective medium for Gram-positive organisms—primarily staphylococci and streptococci. It inhibits Gram-negative bacteria like E. coli.

Mechanism of Selectivity

• Phenylethyl alcohol disrupts DNA synthesis in Gram-negative bacteria
• Gram-positive organisms are relatively resistant
• E. coli lacks the metabolic pathways to overcome this inhibition

Intended Uses

• Isolation of Gram-positive cocci from mixed cultures
• Suppression of Gram-negative "swarming" (Proteus, Pseudomonas)
• Not for fungi (despite original content claim)—use Sabouraud or PDA for fungi

Media for E. coli Growth

• Non-selective: Nutrient Agar, TSA, Blood Agar
• Selective/Differential: MacConkey Agar (lactose fermentation → pink colonies), EMB Agar (metallic green sheen), CHROMagar
• Enrichment: Tryptic Soy Broth (TSB), Luria-Bertani (LB) broth

PEA agar's purpose is Gram-positive selection; it actively inhibits E. coli and other Gram-negatives.