Is Haemophilus influenzae (H flu) sensitive to ciprofloxacin?

Haemophilus influenzae Sensitivity to Ciprofloxacin

Yes, Haemophilus influenzae is highly sensitive to ciprofloxacin and is included in the FDA-approved spectrum of activity for this antibiotic. 1

Evidence of H. influenzae Sensitivity to Ciprofloxacin

The FDA drug label explicitly lists Haemophilus influenzae as one of the aerobic gram-negative microorganisms susceptible to ciprofloxacin 1. This is based on extensive clinical testing and standardized susceptibility testing methods.

Susceptibility Testing Parameters

According to standardized testing protocols:

• For H. influenzae, a minimum inhibitory concentration (MIC) of ≤1 μg/mL indicates susceptibility to ciprofloxacin 1
• Quality control testing with H. influenzae ATCC 49247 should yield MIC values of 0.004-0.03 μg/mL 1
• Zone diameter testing shows that zones ≥21 mm indicate susceptibility when using Haemophilus Test Medium (HTM) 1

Clinical Relevance and Resistance Concerns

While H. influenzae is generally sensitive to ciprofloxacin, several important clinical considerations should be noted:

• Research has documented rare cases of ciprofloxacin-resistant H. influenzae, particularly in patients with:

  • Chronic lung disease 2
  • Cystic fibrosis 3
  • History of multiple courses of ciprofloxacin therapy 2, 3

• Cross-resistance among quinolones has been observed, meaning that resistance to ciprofloxacin typically indicates resistance to other quinolones as well 4

Practical Clinical Application

When treating H. influenzae infections with ciprofloxacin:

1. Consider ciprofloxacin as an effective option for most H. influenzae infections

2. Be vigilant for potential resistance in patients with:

   • Chronic respiratory conditions
   • History of repeated fluoroquinolone exposure
   • Prior documented resistant isolates

3. In experimental models, ciprofloxacin has demonstrated superior intrapulmonary killing of H. influenzae compared to ampicillin and chloramphenicol, making it particularly effective for respiratory infections 5

Monitoring and Resistance Prevention

To preserve ciprofloxacin's effectiveness against H. influenzae:

• Use judiciously and avoid repeated courses when possible
• Consider susceptibility testing in high-risk patients or treatment failures
• Monitor for clinical response within 48-72 hours of initiating therapy

The emergence of ciprofloxacin-resistant H. influenzae strains, while still uncommon, highlights the importance of antimicrobial stewardship to maintain the long-term utility of this important antibiotic.