Keflex (Cephalexin) Will Not Work for Lactobacillus and Should Be Discontinued
Lactobacillus species at 50,000 CFU/mL does not represent a true urinary tract infection and should not be treated with antibiotics, including Keflex. 1
Why This Is Not a True UTI
Lactobacillus is explicitly identified as a contaminant, not a pathogen, in urine cultures. The American Academy of Pediatrics guidelines specifically state that "organisms such as Lactobacillus spp, coagulase-negative staphylococci, and Corynebacterium spp are not considered clinically relevant urine isolates for otherwise healthy" patients. 1
This represents either:
- Vaginal/periurethral contamination during specimen collection 1
- Normal urogenital flora that colonizes the distal urethra 1
- Asymptomatic bacteriuria that should not be treated 1
Why Cephalexin Won't Work (And Shouldn't Be Used)
Even if treatment were indicated (which it is not), cephalexin has no meaningful activity against Lactobacillus species:
- Lactobacillus are Gram-positive rods that are intrinsically resistant to most cephalosporins 2
- Cephalexin's spectrum covers typical uropathogens like E. coli and Klebsiella, not Lactobacillus 2, 3
- The high urinary concentrations achieved by cephalexin (500-1000 mcg/mL) are irrelevant when the organism is not susceptible 2
The Harm of Treating This
Treating asymptomatic bacteriuria or contaminants promotes antimicrobial resistance without clinical benefit. 4
Key risks include:
- Unnecessary antibiotic exposure leading to resistance development 4
- Disruption of protective vaginal flora, potentially increasing future UTI risk 1
- Adverse effects including gastrointestinal symptoms and potential C. difficile infection 5
- Misdiagnosis of the actual clinical problem if symptoms are present 1
What Should Be Done Instead
If the patient is symptomatic:
- Reassess for true UTI with proper specimen collection technique (catheterized or midstream clean-catch) 1
- Look for pyuria (≥5 WBC/hpf or positive leukocyte esterase) to distinguish true infection from contamination 1
- Culture should grow ≥50,000 CFU/mL of a true uropathogen (E. coli, Klebsiella, Proteus, Enterococcus) 1
If the patient is asymptomatic:
- Stop the antibiotic immediately 4
- No further treatment or testing is needed 1, 4
- Reassure the patient that Lactobacillus in urine is normal flora, not infection 1
Common Pitfall to Avoid
Do not treat based solely on a positive culture without considering the organism identity and clinical context. The presence of bacteria in urine does not automatically equal infection requiring treatment. 1, 4 The key distinction is whether there is a true uropathogen causing inflammation (pyuria) with symptoms, versus colonization or contamination with non-pathogenic organisms like Lactobacillus. 1