Cephalexin is NOT Effective Against Serratia marcescens
Cephalexin should not be used to treat Serratia marcescens infections, as this organism is intrinsically resistant to first-generation cephalosporins like cephalexin. 1, 2
Why Cephalexin Fails Against Serratia
Serratia marcescens exhibits inherent resistance to multiple antibiotic classes, including:
- Penicillins (including ampicillin) 2, 3
- First-generation cephalosporins (cephalexin, cephalothin) 2, 3
- Colistin (intrinsic resistance) 1
The resistance pattern is well-documented, with studies showing 100% resistance to cephalothin (the parenteral equivalent of cephalexin) among clinical isolates 3.
Appropriate Treatment Options for Serratia marcescens
First-Line Therapy
For serious Serratia marcescens infections, combination therapy is strongly recommended 1, 2:
- Extended-spectrum cephalosporins (ceftazidime, cefotaxime, ceftriaxone, or cefepime) PLUS an aminoglycoside (gentamicin or amikacin) 1, 2
- Carbapenems (meropenem, imipenem, or doripenem) are highly effective alternatives, particularly for severe infections or ESBL-producing strains 1, 3
- Piperacillin-tazobactam is appropriate for susceptible isolates 1
Site-Specific Considerations
For endocarditis: Combination therapy with a third-generation cephalosporin plus aminoglycoside for minimum 6 weeks, often requiring surgical intervention 1
For CNS infections: Meropenem is preferred over imipenem due to better CSF penetration and lower seizure risk 1
For uncomplicated urinary infections: Cotrimoxazole may be considered if susceptibility is confirmed 2
Resistance Patterns to Guide Selection
Recent surveillance data shows 4:
- Lowest resistance rates: Cefotaxime (0.6%), gentamicin (0.6%), amikacin (0% in one study) 2, 4
- Higher resistance rates: Ceftriaxone (22.7%), ceftazidime (19.6%) 4
- Imipenem: Universally susceptible in most studies 3
Critical Clinical Pitfall
Never use cephalexin or other first-generation cephalosporins for suspected or confirmed Serratia infections, as this will result in treatment failure and potentially increased morbidity and mortality (overall mortality from Serratia bacteremia reaches 31-50%) 2, 3. The organism's intrinsic resistance mechanisms render these agents completely ineffective regardless of dosing 2, 3.