Is Proteus mirabilis sensitive to flucloxacillin?

Proteus mirabilis is NOT Sensitive to Flucloxacillin

Proteus mirabilis is naturally resistant to flucloxacillin and should not be treated with this antibiotic. 1

Rationale for Resistance

Proteus mirabilis, a gram-negative bacterium, demonstrates natural resistance to several classes of antibiotics including:

• Penicillin G and oxacillin (and by extension, flucloxacillin)
• Macrolides
• Lincosamides
• Streptogramins
• Glycopeptides
• Rifampicin
• Fusidic acid

This natural resistance pattern is well-established in microbiology literature, with studies confirming that P. mirabilis possesses intrinsic resistance to these agents 1.

Antimicrobial Susceptibility Profile of P. mirabilis

P. mirabilis is typically susceptible to:

• Aminoglycosides
• Acylureidopenicillins
• Most cephalosporins
• Carbapenems (100% sensitivity to imipenem and meropenem) 2
• Aztreonam
• Quinolones
• Sulfamethoxazole and co-trimoxazole

Clinical Implications

When treating infections caused by P. mirabilis (commonly urinary tract infections, wounds, and intra-abdominal infections), appropriate antibiotic selection is crucial:

1. For mild to moderate infections:

   • Cephalosporins (ceftriaxone, cefotaxime)
   • Fluoroquinolones (ciprofloxacin, levofloxacin) with consideration of local resistance patterns
   • Trimethoprim-sulfamethoxazole (if susceptible)
   • Amoxicillin-clavulanic acid

2. For severe infections:

   • Carbapenems (imipenem, meropenem)
   • Extended-spectrum cephalosporins with or without aminoglycosides

Resistance Patterns to Monitor

P. mirabilis has shown increasing resistance to certain antibiotics:

• Ampicillin (38.5%)
• Sulfamethoxazole/trimethoprim (39.0%)
• Ciprofloxacin (35.2%)
• Gentamicin (25.4%)
• Chloramphenicol (33.8%) 2

Common Pitfalls in Treatment

1. Misidentification of appropriate therapy: Assuming that anti-staphylococcal penicillins like flucloxacillin would be effective against P. mirabilis due to its activity against other common pathogens.

2. Overlooking natural resistance patterns: P. mirabilis has well-documented natural resistance to several antibiotic classes, including isoxazolyl penicillins like flucloxacillin.

3. Failure to obtain cultures: Treatment should ideally be guided by antimicrobial susceptibility testing, especially in complicated infections.

In summary, flucloxacillin should not be used for P. mirabilis infections as this organism demonstrates natural resistance to this antibiotic. Alternative agents with documented activity against P. mirabilis should be selected based on the site and severity of infection, local resistance patterns, and patient-specific factors.