Moxifloxacin Should NOT Be Used for Enterococcus Bacteremia
Moxifloxacin has poor activity against Enterococcus species and is not an appropriate treatment choice for enterococcal bacteremia. While the FDA label lists Enterococcus faecalis as a susceptible organism 1, the clinical reality is that moxifloxacin demonstrates only moderate activity with MIC values that are clinically inadequate for serious bloodstream infections.
Susceptibility Data: The Evidence Shows Limited Activity
In Vitro Susceptibility Findings
The available research consistently demonstrates suboptimal activity of moxifloxacin against enterococci:
For ciprofloxacin-susceptible E. faecalis: MIC90 values range from 0.25-0.5 mg/L 2, with other studies showing MICs of 0.5-16 mg/L for E. faecalis and 2-4 mg/L for E. faecium 3
For fluoroquinolone-resistant strains: The situation is far worse. High-level ciprofloxacin-resistant E. faecalis (MIC 32 mg/L) shows moxifloxacin MICs of 8-16 mg/L 4, which are well into the resistant range
FDA breakpoints: The susceptibility breakpoint for E. faecalis is ≤1 mg/L (susceptible), 2 mg/L (intermediate), and ≥4 mg/L (resistant) 1
Cross-Resistance is a Major Problem
Moxifloxacin exhibits complete cross-resistance with other fluoroquinolones against enterococci 4. If an isolate is resistant to ciprofloxacin, it will be resistant to moxifloxacin. This is critical because fluoroquinolone resistance among enterococci is increasingly common in healthcare settings.
Clinical Context: Why This Matters for Bacteremia
Bacteremia Requires Bactericidal Therapy
Time-kill studies reveal the fundamental problem: even at 3 mcg/mL (well above typical MICs), moxifloxacin shows variable bactericidal activity:
- Against E. faecalis with MIC 0.25 mg/L: 4.0-4.2 log10 reduction at 24 hours
- Against E. faecalis with MIC 2 mg/L: only 1.8-2.0 log10 reduction 5
For serious bloodstream infections, this level of activity is insufficient, particularly when the MIC approaches or exceeds 1 mg/L.
Guidelines Explicitly Do Not Recommend Enterococcal Coverage in Most Settings
The 2010 IDSA/SIS guidelines for intra-abdominal infections specifically state: "Empiric coverage of Enterococcus is not necessary in patients with community-acquired intra-abdominal infection" 6. When enterococcal coverage IS needed (such as in bacteremia), moxifloxacin is not among the recommended agents.
Geographic Considerations
In regions with high fluoroquinolone resistance (>20% among E. coli, which correlates with enterococcal resistance), fluoroquinolones including moxifloxacin are explicitly not recommended 7. Even in Taiwan, where resistance rates are lower, moxifloxacin showed intermediate resistance patterns against E. faecalis 8.
What Should Be Used Instead?
For enterococcal bacteremia, appropriate agents include:
- Ampicillin (for ampicillin-susceptible E. faecalis)
- Vancomycin (for ampicillin-resistant strains)
- Daptomycin (for vancomycin-resistant enterococci)
- Linezolid (for VRE, though bacteriostatic)
The choice depends on species identification (E. faecalis vs. E. faecium) and susceptibility testing 9.
Critical Pitfalls to Avoid
Do not assume FDA-labeled activity equals clinical efficacy: The FDA label's inclusion of E. faecalis is based on in vitro data, not clinical outcomes in bacteremia 1
Do not use moxifloxacin for polymicrobial bacteremia involving enterococci: Even if covering other organisms, the enterococcal component will be inadequately treated
Resistance testing is mandatory: Over 50% of E. faecalis and E. faecium may require >2 mg/L moxifloxacin for inhibition 10, making them intermediate or resistant
In summary: Moxifloxacin susceptibility for Enterococcus bacteremia is poor to intermediate at best, with MIC90 values of 0.5-2 mg/L for susceptible strains and 8-16 mg/L for resistant strains. This agent should not be used for enterococcal bacteremia treatment.