To which antibiotics are Proteus mirabilis and Escherichia coli susceptible?

Antibiotic Susceptibility of Proteus mirabilis and E. coli

Both Proteus mirabilis and E. coli demonstrate excellent susceptibility to carbapenems (imipenem/meropenem at 100%), with E. coli also showing high susceptibility to nitrofurantoin and fosfomycin, while P. mirabilis requires different first-line agents due to intrinsic nitrofurantoin resistance. 1

First-Line Antibiotic Options by Pathogen

For E. coli (Uncomplicated UTI):

The 2025 AUA/CUA/SUFU guidelines establish clear first-line options 2:

• Nitrofurantoin: 100 mg twice daily for 5 days (88-93% cure rate, resistance <5%)
• Trimethoprim-sulfamethoxazole (TMP-SMX): One double-strength tablet twice daily for 3 days (90-100% cure rate) - only if local resistance <20%
• Fosfomycin: 3g single dose (83-91% cure rate, resistance <2%)

Critical caveat: Fluoroquinolone resistance in E. coli has increased significantly (14.5% in 2009 to 19.3% in 2016), making quinolones less reliable for empiric therapy 3. TMP-SMX resistance exceeds 20% in many regions, limiting its utility 3.

For P. mirabilis:

P. mirabilis has intrinsic resistance to nitrofurantoin and fosfomycin 3, requiring alternative agents:

• TMP-SMX: Effective when susceptibility confirmed (resistance <35% reported) 3
• Fluoroquinolones (ciprofloxacin/levofloxacin): Resistance remains <18% but increasing 3
• 3rd-generation cephalosporins: Resistance <2%, highly effective 3

Complicated UTI/Pyelonephritis Treatment

For both pathogens in complicated infections 1:

Oral Options (if clinically stable):

• Ciprofloxacin: 500-750 mg twice daily for 7 days
• Levofloxacin: 750 mg daily for 5 days
• TMP-SMX: 160/800 mg twice daily for 14 days
• Cefpodoxime: 200 mg twice daily for 10 days

Parenteral Options (severe illness/sepsis):

• Carbapenems (meropenem 1g three times daily): 100% susceptibility for both organisms 1, 4
• Ceftriaxone: Initial dose recommended before oral step-down
• Piperacillin-tazobactam: >87% susceptibility even for ESBL-producing E. coli 5
• Amikacin: >87% susceptibility for ESBL producers 5

ESBL Considerations

ESBL production significantly compromises susceptibility to fluoroquinolones and most β-lactams (except carbapenems) 5, 6. ESBL rates:

• E. coli: 32.9% in Lebanon 4, 6% in UK 7
• Geographic variation substantial (highest in China, Vietnam, India) 5

For ESBL-producing strains, carbapenems remain the gold standard with 100% susceptibility, while piperacillin-tazobactam and amikacin serve as carbapenem-sparing alternatives 1, 5.

Resistance Patterns to Avoid

Do not use empirically:

• Ampicillin: 53% E. coli resistance, 28% P. mirabilis resistance 7
• Fluoroquinolones in high-resistance areas (>10% resistance threshold exceeded in many regions) 1, 3
• Nitrofurantoin or fosfomycin for P. mirabilis (intrinsic resistance) 3

Key Clinical Pitfalls

1. Geographic variation matters: Resistance patterns differ dramatically by region - always consider local antibiograms 5
2. Hospital vs. community acquisition: Hospital-acquired infections show significantly higher ESBL rates and broader resistance 5
3. P. mirabilis and stones: If persistent P. mirabilis growth detected, exclude urinary stone formation (urease-producing organism) 1
4. Culture before treatment: For complicated UTIs, obtain culture and tailor therapy based on susceptibilities 1

Newer FDA-Approved Options

Recent approvals provide alternatives for multidrug-resistant infections 2:

• Sulopenem etzadroxil: Specifically approved for E. coli, K. pneumoniae, and P. mirabilis when limited alternatives exist
• Pivmecillinam: Minimal community resistance, effective for MDR UTI
• Gepotidacin: Novel mechanism, 51% therapeutic success comparable to nitrofurantoin