E. Coli vs Serratia Marcescens: Key Differences, Risks, and Lab ID
Escherichia coli (E. coli) and Serratia marcescens are both gram-negative, rod-shaped bacteria. E. coli normally lives in human intestines and most strains are harmless, while Serratia marcescens is an opportunistic pathogen that can thrive on soap scum and in hospital environments, often producing a red pigment at room temperature.
People confuse them because both appear as pink-red colonies on MacConkey agar and both can cause urinary-tract infections. Nurses see “pink slime” in sinks and assume it’s harmless E. coli; meanwhile, clinicians get blood cultures showing gram-negative rods and must decide whether it’s the typical gut bug or the sneaky hospital invader that laughs at many first-line antibiotics.
Key Differences
E. coli ferments lactose (pink colonies), shows indole-positive, and is usually sensitive to ampicillin. Serratia marcescens is lactose-negative (colorless or red pigment), indole-negative, and resistant to penicillins and first-gen cephalosporins. In the lab, a quick oxidase-negative, DNase-positive test plus the red pigment at 25 °C points straight to Serratia.
Which One Should You Choose?
If you’re a microbiologist choosing which isolate to report, flag Serratia marcescens for infection-control review: it signals possible catheter or equipment contamination. E. coli, on the other hand, prompts a standard enteric work-up and simpler antibiotic choices unless ESBL is detected. In short, red pigment and hospital source equals Serratia alert.
Can red color always confirm Serratia marcescens?
No. The pigment forms only at 20–28 °C; at 37 °C colonies are cream to pink and mimic other Enterobacterales. Always run biochemical tests.
Is E. coli ever antibiotic-resistant?
Absolutely. ESBL and carbapenemase-producing strains are rising; always perform susceptibility testing before prescribing.
How do you label cultures in the lab?
Write full genus-species on the report: “Escherichia coli” or “Serratia marcescens.” Never abbreviate to “E. coli” or “S. marcescens” on patient charts to avoid transcription errors.