The in vitro impact of moxifloxacin and gatifloxacin concentration (0.5% vs 0.3%) and the addition of benzalkonium chloride on antibacterial efficacy.

Purpose: Varied concentrations of moxifloxacin (MOX) and gatifloxacin (GAT) and the addition of 0.005% benzalkonium chloride (BAK) were evaluated for eliminating Staphylococcus aureus (SA), Pseudomonas aeruginosa (PA), and coagulase-negative Staphylococcus (CNS).

Design: In vitro laboratory investigation.

Methods: The time-kill survival of SA, PA, and CNS were tested at one, two, three, six, eight, and 24 hours to: (1) Mueller-Hinton broth, (2) BAK, (3) 0.5% MOX, (4) 0.5% GAT, (5) 0.3% MOX, (6) 0.3% GAT, (7) 0.3% GAT plus BAK, (8) 0.5% MOX plus BAK, (9) 8 microg/ml GAT, and (10) 8 mug/ml MOX. Antibiotic interactions (GAT and BAK) were determined by checkerboard testing. The outcome measures were (1) time-to-kill, (2) killing-rates, and (3) fractional inhibitory concentration (FIC) indices.

Results: MOX and GAT at either 0.5% or 0.3% had equivalent antibacterial effects. BAK alone or the addition of BAK to either antibiotic eliminated SA and CNS within one hour, whereas 0.3% GAT plus BAK eliminated bacteria faster than 0.5% MOX (P = .016). For PA, BAK alone had no antibacterial effect. The kill rates of MOX and GAT were equivalent. FIC indices indicated that GAT and BAK were indifferent against SA and CNS, but antagonistic to PA.

Conclusion: As a preservative, MOX and GAT have equivalent antibacterial activity with similar killing rates. BAK appears to independently complement GAT for eliminating SA and CNS, but has no effect on PA. The in vitro predictive clinical effect due to varied antibiotic concentration and the addition of BAK requires confirmatory clinical studies for validation.