Can fosfomycin be used to treat Staphylococcus (Staph) infections of the urinary tract and what other bacteria is this antibiotic effective against in the bladder?

Fosfomycin for Staphylococcus Urinary Tract Infections

Fosfomycin can be used to treat Staphylococcus saprophyticus urinary tract infections in women with uncomplicated cystitis, but it is restricted to this specific indication only—not for complicated UTIs, pyelonephritis, or infections in men. 1, 2

Spectrum of Activity Against Urinary Pathogens

Fosfomycin demonstrates in vitro activity against the following bacteria commonly causing bladder infections:

Primary Uropathogens (Well-Established Efficacy)

• Escherichia coli (including ESBL-producing strains): 99.6% susceptibility rate, representing the most common cause of uncomplicated cystitis 3, 4, 5
• Staphylococcus saprophyticus: Specifically mentioned as a susceptible pathogen in uncomplicated UTIs 4
• Enterococcus species (including vancomycin-resistant Enterococcus/VRE): 94.4% susceptibility rate with the same single 3-gram oral dose recommended 1, 5

Additional Susceptible Organisms

• Proteus mirabilis: Demonstrates good in vitro activity 3, 4
• Klebsiella pneumoniae: Shows susceptibility, though with a trend toward higher MICs 4, 6
• Citrobacter, Enterobacter, and Serratia species: In vitro activity documented 3

Critical Limitations for Staphylococcus Infections

The European Association of Urology restricts fosfomycin to uncomplicated cystitis only in women—it should NOT be used for complicated UTIs, pyelonephritis, or in men regardless of the pathogen. 1, 2 This means:

• Fosfomycin is appropriate for Staphylococcus saprophyticus causing simple bladder infections in non-pregnant women 4
• Do NOT use for Staphylococcus aureus (including MRSA) causing UTIs, as these typically represent complicated infections requiring different management 1
• Do NOT use in men with any staphylococcal UTI, as clinical efficacy data is limited in this population 1

Dosing and Administration

• Single 3-gram oral dose provides therapeutic urinary concentrations for 24-48 hours 1, 3
• Peak urinary concentrations occur within 4 hours and remain high (>128 mg/L) for 24-48 hours 3
• Clinical recovery rates of 96-99% and bacteriological eradication rates of 75-90% at 5-11 days post-therapy 3, 4

Common Pitfalls to Avoid

• Do not use for any complicated UTI: This includes patients with anatomic abnormalities, functional abnormalities, immunosuppression, or catheter-associated infections 2
• Do not use for pyelonephritis: Fluoroquinolones and cephalosporins are preferred for upper tract infections 1
• Do not use routinely in men: Limited efficacy data exists for male UTIs 1
• Avoid in Enterobacter cloacae infections: Low essential agreement and high very major error rates documented 6

Advantages Over Other Agents

• Minimal collateral damage to intestinal flora compared to fluoroquinolones and broad-spectrum beta-lactams 1
• Single-dose convenience improves adherence compared to 3-7 day regimens 7, 3
• Low resistance rates: Little cross-resistance with other antibacterial agents due to unique mechanism of action (MurA enzyme inhibition) 3, 4
• Safe in pregnancy: Recommended for asymptomatic bacteriuria in pregnant women with standard short-course treatment or single-dose administration 7, 1

Tolerability Profile

• Most common adverse events are gastrointestinal (diarrhea, nausea, vomiting), which are transient, mild, and self-limiting 1, 3
• Skin rash reported but generally mild 1
• Overall well-tolerated with low incidence of adverse events 3, 4