Serratia marcescens
Gram-negative bacilli
Identification & Growth
Identification
- Oxidase-negative, DNase-positive, gelatinase-positive, lipase-positive
- Lactose-negative or late lactose fermenter; ONPG-positive
Growth Conditions
- May produce a red pigment (prodigiosin) at 25 °C
- MacConkey agar: colourless to pale-pink colonies; 35 °C, 24 h
Clinical Significance
- Outbreaks in neonatal ICUs, pneumonia, catheter-associated UTI, bacteremia
Intrinsic Resistance
- Ampicillin, amoxicillin-clavulanate, cefalotin, cefoxitin (chromosomal AmpC)
- Colistin/polymyxin B and nitrofurantoin (EUCAST Expected Resistant Phenotypes v1.2)
Bench Alerts
- Never report a polymyxin as a therapeutic option
- Isolates clustered in time suggest a common source — notify infection control
Clinical Notes
Clinical presentations
- Causes hospital infections such as ventilator-associated pneumonia, catheter-related bacteremia, and urinary tract infection.
- Associated with bloodstream infection outbreaks related to contamination of intravenous solutions and infusion equipment.
- Can cause keratitis and endophthalmitis associated with contaminated contact lenses or ophthalmologic procedures.
Specimens and collection
- Blood cultures and catheter tip culture should be obtained in suspected intravascular device-related bacteremia.
- Ocular specimens (corneal scraping, aqueous/vitreous humor) require collection by an ophthalmologist with sterile technique and immediate transport.
Epidemiology and at-risk populations
- A predominantly nosocomial pathogen, capable of surviving in aqueous solutions and hospital surfaces for prolonged periods.
- Neonatal and adult intensive care patients are particularly vulnerable to outbreaks related to common environmental sources.
- Included in carbapenem-resistant Enterobacterales surveillance due to its capacity to acquire carbapenemases.
Resistance and therapeutic implications
- Carries an inducible chromosomal AmpC beta-lactamase, with expected resistance to ampicillin, amoxicillin-clavulanate, and first-generation cephalosporins.
- It is intrinsically resistant to polymyxins (colistin), which should be reflected in the report regardless of the in vitro result.
- Derepressed AmpC mutants can emerge during prolonged third-generation cephalosporin therapy, leading to treatment failure.
Bench and reporting notes
- Gram-negative rod, late lactose-fermenter, oxidase-negative; some strains produce a red pigment (prodigiosin) at room temperature.
- The report should state intrinsic resistance to colistin and to nitrofurantoin/nitrofurazone, avoiding inappropriate use of these classes.
- Isolation from multiple patients with the same susceptibility profile within a short period should prompt investigation for a common-source outbreak.
Sources
- PMC — A Primer on AmpC β-Lactamases
- EUCAST — Expected Phenotypes
- CDC — About Carbapenem-resistant Enterobacterales (CRE)
Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.
FAQ: Frequently Asked Questions
How to identify Serratia marcescens in the lab?
Serratia marcescens is identified through oxidase-negative, dnase-positive, gelatinase-positive, lipase-positive, lactose-negative or late lactose fermenter; onpg-positive.
What are the intrinsic resistances of Serratia marcescens?
This organism is naturally resistant to ampicillin, amoxicillin-clavulanate, cefalotin, cefoxitin (chromosomal ampc), colistin/polymyxin b and nitrofurantoin (eucast expected resistant phenotypes v1.2). These drugs should not be reported as susceptible.
Where is Serratia marcescens commonly found?
It is typically associated with outbreaks in neonatal icus, pneumonia, catheter-associated uti, bacteremia.
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