Cephalexin for Staphylococcus aureus Urinary Tract Infections
Cephalexin is effective for treating Staphylococcus aureus urinary tract infections due to its excellent urinary concentration and activity against methicillin-susceptible S. aureus (MSSA). Cephalexin achieves high urinary concentrations (approximately 1000-5000 mcg/mL) following standard oral dosing, making it an appropriate choice for S. aureus UTIs when the organism is susceptible 1.
Antimicrobial Activity Against S. aureus
- Cephalexin demonstrates good activity against methicillin-susceptible S. aureus (MSSA) strains 1
- Cephalexin is excreted primarily by the kidney (glomerular filtration and tubular secretion) with over 90% of the drug excreted unchanged in urine within 8 hours 1, 2
- Peak urine concentrations following 250 mg, 500 mg, and 1 g doses are approximately 1000,2200, and 5000 mcg/mL respectively, providing concentrations well above the MIC for susceptible S. aureus 1
- Methicillin-resistant S. aureus (MRSA) strains are resistant to cephalexin and require alternative therapy 1
Treatment Recommendations for S. aureus UTIs
- For uncomplicated lower UTIs caused by susceptible S. aureus, cephalexin is an appropriate oral treatment option 3
- For complicated UTIs with systemic symptoms, initial therapy may require parenteral agents before transitioning to oral cephalexin once clinical improvement occurs 4
- When MRSA is suspected or confirmed, alternative agents such as trimethoprim-sulfamethoxazole, doxycycline, or clindamycin should be used instead of cephalexin 4
- Urine culture and susceptibility testing should be performed to guide definitive therapy 4
Dosing Recommendations
- For adults with uncomplicated UTIs: 500 mg orally twice or three times daily for 7-14 days 3
- For complicated UTIs: Treatment duration should be 7-14 days (14 days for men when prostatitis cannot be excluded) 4
- For children with S. aureus infections: 22-45 mg/kg twice daily or 15-25 mg/kg three times daily depending on the MIC of the organism 5
Clinical Efficacy
- Cephalexin has demonstrated clinical efficacy in urinary tract infections in numerous studies since the 1970s 6
- It is essentially non-toxic at recommended doses with a favorable safety profile 6
- For uncomplicated lower UTIs, cephalexin achieves good early bacteriological and clinical cure rates comparable to many first-line agents 3
- In the era of increasing antibiotic resistance, cephalexin represents a valuable fluoroquinolone-sparing alternative for susceptible organisms 3
Important Considerations
- Always obtain urine culture before initiating antimicrobial therapy in patients with suspected UTI to guide definitive therapy 4
- Cephalexin is not active against most strains of Enterobacter species, Morganella morganii, Proteus vulgaris, Pseudomonas species, or Acinetobacter calcoaceticus 1
- For catheter-associated UTIs, treatment should follow recommendations for complicated UTIs 4
- Asymptomatic bacteriuria should not be treated, even in catheterized patients, except in specific circumstances such as pregnancy or before urologic procedures with anticipated mucosal bleeding 4
Pitfalls to Avoid
- Do not use cephalexin for MRSA UTIs as it will be ineffective 4, 1
- Avoid treating asymptomatic bacteriuria with cephalexin or any antibiotic as this practice leads to antimicrobial resistance without clinical benefit 4
- Do not rely solely on urine dipstick testing for diagnosis, as pyuria is common in catheterized patients and has no predictive value in differentiating symptomatic UTI from asymptomatic bacteriuria 4
- Remember that cephalexin is not appropriate for febrile infants with UTIs if parenchymal involvement (pyelonephritis) is suspected, as it may not achieve adequate serum concentrations 4