Is Serratia Part of MDR SPICE Resistant Gram-Negative Bacteria?
Yes, Serratia marcescens is definitively part of the multidrug-resistant (MDR) SPICE group of Gram-negative bacteria, which includes Serratia, Pseudomonas, indole-positive Proteus, Citrobacter, and Enterobacter species. 1
Understanding SPICE Organisms and Serratia's Role
Serratia marcescens possesses an inducible chromosomal AmpC β-lactamase that confers resistance to multiple β-lactam antibiotics, making it inherently prone to multidrug resistance. 1 This mechanism is identical to that found in other SPICE organisms like Citrobacter and Enterobacter species, where the enzyme can be easily expressed at high levels through mutation, resulting in resistance to oxyimino-β-lactams and β-methoxy-β-lactams while maintaining susceptibility to carbapenems 1.
Key Resistance Mechanisms
- Serratia has intrinsic resistance to multiple antibiotics including penicillin, ampicillin, first-generation cephalosporins, and notably colistin 2, 3
- The organism can acquire plasmid-mediated metallo-β-lactamases (such as IMP-type enzymes) that confer carbapenem resistance, though this was initially more common in Asia and has spread to other regions including the United States 1
- Multidrug-resistant efflux pumps (particularly SdeAB-HasF complex) actively expel fluoroquinolones and other antimicrobials, contributing to treatment failures 4
- Extended-spectrum β-lactamase (ESBL) production has been documented in clinical isolates, further limiting treatment options 2
Clinical Significance in Healthcare Settings
Serratia marcescens ranks among the most common MDR Gram-negative pathogens causing hospital-acquired pneumonia, following Pseudomonas aeruginosa, Klebsiella species, and Enterobacter species. 1 The organism is particularly problematic in:
- Ventilator-associated pneumonia and intensive care unit infections 1, 5
- Bloodstream infections with mortality rates reaching 31-40% in outbreak settings 2, 3
- Urinary tract infections and surgical site infections 3
Critical Pitfall to Avoid
Never use colistin for empiric or targeted therapy against Serratia—it has intrinsic resistance to polymyxins. 1, 2 A documented outbreak occurred after colistin use for Acinetobacter infections, which created selective pressure favoring Serratia marcescens emergence, demonstrating how inappropriate antibiotic selection can drive MDR organism proliferation 2.
Recommended Antimicrobial Therapy
For serious Serratia marcescens infections, carbapenems (meropenem, imipenem, or doripenem) represent first-line therapy, often combined with aminoglycosides for critically ill patients. 1, 5, 3
Treatment Algorithm:
- For susceptible isolates: Extended-spectrum cephalosporins (ceftriaxone, ceftazidime) or carbapenems 5
- For ESBL-producing strains: Meropenem is preferred as it maintains activity 1
- For carbapenem-resistant isolates: Consider newer agents like ceftolozane/tazobactam or ceftazidime/avibactam, though clinical data for Serratia specifically are limited 1
- Aminoglycosides (amikacin preferred over gentamicin) should be added for severe infections, as resistance rates are lower 3
- Cotrimoxazole may be considered for uncomplicated urinary tract infections only 3
Agents with Documented Activity:
According to FDA labeling, tigecycline demonstrates in vitro activity against Serratia marcescens 6, and imipenem has established efficacy with high stability against Serratia-produced β-lactamases 7. However, tigecycline should be used cautiously given concerns about bacteremia outcomes 1.
Surveillance and Infection Control
Recent genomic studies demonstrate that multidrug-resistant Serratia clones have emerged independently across multiple healthcare facilities and can transmit between hospitals. 8 These clones have acquired resistance determinants for β-lactams, fluoroquinolones, and tetracyclines on multiple occasions, suggesting ongoing horizontal gene transfer 8, 9.
Standard infection control interventions can successfully interrupt transmission even when selective antibiotic pressure (like colistin use) continues, emphasizing the critical importance of contact precautions and environmental decontamination 2.