Is cephalexin (Cephalexin) effective for treating a urinary tract infection caused by Serratia marcescens?

Cephalexin is Not Effective for Urinary Tract Infections Caused by Serratia marcescens

Cephalexin should not be used to treat urinary tract infections caused by Serratia marcescens as this organism is commonly resistant to first-generation cephalosporins. 1, 2

Microbiology and Resistance Patterns of Serratia marcescens

• Serratia marcescens is commonly found in complicated UTIs, particularly in healthcare settings, and is associated with urological instrumentation and catheterization 1, 3
• S. marcescens belongs to a group of organisms (along with Pseudomonas, Klebsiella, and other Enterobacteriaceae) that are more likely to demonstrate antimicrobial resistance than typical uropathogens 1
• Studies have shown that S. marcescens isolates have significant resistance to multiple antibiotics, making treatment challenging 2, 4
• S. marcescens has been associated with nosocomial outbreaks in urologic patients, often with multidrug-resistant strains 5, 6

Recommended Treatment Approaches for S. marcescens UTI

First-line Options:

• For uncomplicated S. marcescens UTI:
  • Fluoroquinolones (ciprofloxacin or levofloxacin) if local resistance rates are <10% 1
  • Trimethoprim-sulfamethoxazole if the organism is susceptible 1

For Complicated S. marcescens UTI:

• Obtain urine culture and susceptibility testing before initiating therapy 1
• Initial empiric therapy options while awaiting culture results:
  • Combination therapy with amoxicillin plus an aminoglycoside 1
  • Third-generation cephalosporin (e.g., ceftriaxone) 1
  • Aminoglycosides (particularly gentamicin) have shown good efficacy against S. marcescens 2

For Multidrug-Resistant S. marcescens:

• For carbapenem-resistant strains, consider:
  • Ceftazidime-avibactam 1
  • Meropenem-vaborbactam or imipenem-cilastatin-relebactam 1
  • Plazomicin or other aminoglycosides 1, 2

Why Cephalexin is Not Appropriate

• First-generation cephalosporins like cephalexin have "inferior efficacy and more adverse effects compared with other UTI antimicrobials" even for typical uropathogens 1
• Guidelines specifically note that β-lactams other than pivmecillinam "should be used with caution for uncomplicated cystitis" 1
• S. marcescens commonly demonstrates resistance to older cephalosporins 2
• Even for third-generation cephalosporins like ceftriaxone and ceftazidime, resistance rates of 22.7% and 19.6% respectively have been reported for S. marcescens 2

Important Clinical Considerations

• Always obtain urine culture and susceptibility testing before initiating therapy for suspected S. marcescens UTI 1
• Treatment duration should be 7-14 days for complicated UTIs (14 days for men when prostatitis cannot be excluded) 1
• If the patient has an indwelling catheter, it should be changed prior to collecting urine for culture 1
• Address any underlying urological abnormalities or complicating factors as part of management 1
• Monitor for development of increasing resistance during therapy, which has been reported with S. marcescens 4

Common Pitfalls to Avoid

• Do not treat asymptomatic bacteriuria, even in catheterized patients, as this promotes antimicrobial resistance without clinical benefit 1
• Avoid empiric use of antibiotics with known high resistance rates for S. marcescens (including cephalexin) 2
• Do not rely solely on dipstick testing for diagnosis, as pyuria is common in catheterized patients and has no predictive value in differentiating symptomatic UTI from asymptomatic bacteriuria 1
• Be aware that S. marcescens can develop resistance during therapy, particularly with aminoglycosides 4