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 Pseudomanas aeruginosa.


RESULTS
All aztreonam and fluoroquinolone combinations demonstrated additive activity [fractional inhibitory concentration (FIC) index >0.5-4] or synergy (FIC index <or=0.5) 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.


CONCLUSIONS
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.
Critchley
Ian A
IA
Focus Technologies, Herndon, VA 20171, USA. icritchley@focusanswers.com
Sahm
Daniel F
DF
Kelly
Laurie J
LJ
Karlowsky
James A
JA
D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000494:pharmacology*; D001398:Aztreonam / Q000008:administration & dosage / Q000494:pharmacology*; D016956:Burkholderia cepacia / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004359:Drug Therapy, Combination / Q000008:administration & dosage / Q000494:pharmacology*; D016972:Enterobacter cloacae / Q000187:drug effects; D004926:Escherichia coli / Q000187:drug effects; D024841:Fluoroquinolones / Q000008:administration & dosage / Q000494:pharmacology*; D006092:Gram-Negative Facultatively Anaerobic Rods / Q000187:drug effects*; D007711:Klebsiella pneumoniae / Q000187:drug effects; D008826:Microbial Sensitivity Tests; D011513:Proteus mirabilis / Q000187:drug effects; D011550:Pseudomonas aeruginosa / Q000187:drug effects*; D013045:Species Specificity
D000900:Anti-Bacterial Agents; D024841:Fluoroquinolones; D001398:Aztreonam