What organisms is cephalexin (Cephalexin) effective against in treating Urinary Tract Infections (UTIs)?

Cephalexin Susceptibility Coverage for UTI

Cephalexin is effective against common gram-positive and gram-negative uropathogens, specifically Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, penicillinase-producing Staphylococcus aureus, and Streptococcus species, but lacks activity against Pseudomonas, Enterococcus, methicillin-resistant staphylococci, and most Enterobacter species. 1

Spectrum of Activity

Gram-Negative Coverage

• Primary uropathogens covered: E. coli, Klebsiella pneumoniae, Proteus mirabilis, Haemophilus influenzae, and Moraxella catarrhalis 1
• These organisms account for the majority of uncomplicated UTIs, with E. coli representing approximately 85% of cases 2
• Achieves urinary concentrations exceeding 1000 mcg/mL after standard dosing, far above the MIC for susceptible organisms 1, 3

Gram-Positive Coverage

• Effective against: Staphylococcus aureus (including penicillinase-producing strains), Streptococcus pneumoniae (penicillin-susceptible strains), and Streptococcus pyogenes 1
• Staphylococcus saprophyticus, a common cause of UTI in young women, is typically susceptible 4

Organisms NOT Covered

Critical Gaps in Coverage

• Methicillin-resistant staphylococci (MRSA): Uniformly resistant to all cephalosporins including cephalexin 1
• Enterococci (Enterococcus faecalis): Most strains are resistant to cephalosporins 1
• Pseudomonas aeruginosa: No activity whatsoever 1
• Acinetobacter calcoaceticus: No activity 1
• Most strains of Enterobacter spp., Morganella morganii, and Proteus vulgaris: Not reliably covered 1
• Penicillin-resistant Streptococcus pneumoniae: Usually cross-resistant to beta-lactam antibiotics 1

Clinical Application

When to Use Cephalexin

• Uncomplicated UTIs with susceptible organisms: Recent studies demonstrate 81-88% clinical success rates when used empirically for uncomplicated UTIs 5, 2
• Local antibiogram support: Should only be used empirically when local resistance patterns show >80% susceptibility of E. coli to first-generation cephalosporins 5
• Culture-directed therapy: Ideal when urine culture confirms susceptibility (MIC ≤8 mcg/mL for cephalothin as surrogate marker) 1

When NOT to Use Cephalexin

• Complicated UTIs requiring empiric therapy: Guidelines do not recommend cephalexin as first-line for complicated UTIs without culture data 6
• Suspected Pseudomonas or Enterococcus infection: Requires alternative agents 1
• High local resistance rates: Avoid if local E. coli resistance to first-generation cephalosporins exceeds 20% 7

Susceptibility Testing Interpretation

MIC Breakpoints (using cephalothin as surrogate)

• Susceptible: MIC ≤8 mcg/mL 1
• Intermediate: MIC 16 mcg/mL 1
• Resistant: MIC ≥32 mcg/mL 1

Disk Diffusion (30 mcg cephalothin disk)

• Susceptible: Zone diameter ≥18 mm 1
• Intermediate: Zone diameter 15-17 mm 1
• Resistant: Zone diameter ≤14 mm 1

Important Clinical Pearls

Pharmacokinetic Advantages

• Over 90% excreted unchanged in urine within 8 hours, achieving concentrations of 1000-5000 mcg/mL depending on dose 1, 3
• This exceptional urinary penetration allows effective treatment even when serum levels would be subtherapeutic for systemic infections 4

Common Pitfalls

• Do not assume coverage of Enterococcus: If gram-positive cocci in chains are seen on urinalysis, consider alternative agents as enterococci are inherently resistant 1
• Avoid in catheter-associated UTIs without culture: These infections have higher rates of resistant organisms and polymicrobial infections 7
• Not appropriate for pyelonephritis requiring empiric therapy: Use broader-spectrum agents until culture results available 6

Resistance Considerations

• Treatment of asymptomatic bacteriuria with cephalexin promotes resistance: 47% of reinfecting organisms became highly resistant versus 26% in untreated controls 7
• Always obtain urine culture before initiating therapy to guide definitive treatment and detect resistance 6