In vitro synergy studies using aztreonam and fluoroquinolone combinations against six species of gram-negative bacilli

Background: Combination antimicrobial therapy is often necessary to eradicate infections caused by gram-negative bacteria. Methods: To evaluate the potential benefit of aztreonam and fluoroquinolone combination therapy, the activity of aztreonam in combination with ciprofloxacin, gatifloxacin, levofloxacin and moxifloxacin was assessed using checkerboard testing for four clinical isolates of each of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Enterobacter cloacae, Burkholderia cepacia and Pseudomonas aeruginosa. Results: All aztreonam and fluoroquinolone combinations demonstrated additive activity [fractional inhibitory concentration (FIC) index >0.5-4] or synergy (FIC index less than or equal to 50.5) for all 24 isolates tested. Aztreonam plus ciprofloxacin was the most effective combination, demonstrating synergy against 16.7% of isolates (2 P. mirabilis, 1 E. cloacae, 1 B. cepacia). Aztreonam in combination with moxifloxacin was synergistic against 2 isolates (P. mirabilis, E cloacae), and in combination with gatifloxacin against 1 isolate (E cloacae). Aztreonam and levofloxacin demonstrated only additive activity. Conclusion: Additive activity was observed for the majority of the aztreonam and fluoroquinolone combinations tested against six species of gram-negative bacilli. Synergy involving aztreonam and fluoroquinolone combinations was less common than additive activity and was isolate dependent. Although in vivo clinical successes have been documented using aztreonam in combination with other agents, confirmation by laboratory techniques requires further investigation. Copyright (C) 2003 S. Karger AG, Basel.