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Antimicrobial Agents and Chemotherapy, August 1998, p. 2002-2005, Vol. 42, No. 8
Department of Pathology (Clinical
Microbiology), Hershey Medical Center, Hershey, Pennsylvania
17033,1 and
Department of Pathology,
Case Western Reserve University, Cleveland, Ohio
441062
Received 24 March 1998/Returned for modification 6 May
1998/Accepted 28 May 1998
The present study examined the activities of trovafloxacin,
levofloxacin, and ciprofloxacin, alone and in combination with cefoperazone, ceftazidime, cefpirome, and gentamicin, against 100 strains of Stenotrophomonas maltophilia by the MIC
determination method and by synergy testing of the combinations by the
time-kill and checkerboard titration methods for 20 strains. The
respective MICs at which 50% and 90% of isolates were inhibited for
the drugs used alone were as follows: trovafloxacin, 0.5 and 2.0 µg/ml; levofloxacin, 2.0 and 4.0 µg/ml; ciprofloxacin, 4.0 and
16.0 µg/ml; cefoperazone, >128.0 and >128.0 µg/ml; ceftazidime,
32.0 and >128.0 µg/ml; cefpirome, >128.0 and >128.0 µg/ml; and
gentamicin, 128.0 and >128.0 µg/ml. Synergistic fractional
inhibitory concentration indices (
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Activities of Three Quinolones, Alone and in
Combination with Extended-Spectrum Cephalosporins or Gentamicin,
against Stenotrophomonas maltophilia
0.5) were found for
50% of
strains for trovafloxacin-cefoperazone, trovafloxacin-ceftazidime,
levofloxacin-cefoperazone, levofloxacin-ceftazidime, ciprofloxacin-cefoperazone, and ciprofloxacin-ceftazidime, with other
combinations affecting fewer strains. For 20 strains tested by the
checkerboard titration and time-kill methods, synergy (
100-fold drop
in count compared to the count achieved with the more active compound)
was more pronounced after 12 h due to regrowth after 24 h. At
12 h, trovafloxacin at 0.004 to 0.5 µg/ml showed synergy with
cefoperazone for 90% of strains, with ceftazidime for 95% of strains
with cefpirome for 95% of strains, and with gentamicin for 65% of
strains. Levofloxacin at 0.03 to 0.5 µg/ml and ciprofloxacin at 0.5 to 2.0 µg/ml showed synergy with cefoperazone for 80% of strains,
with ceftazidime for 90 and 85% of strains, respectively, with
cefpirome for 85 and 75% of strains, respectively, and with gentamicin
for 65 and 75% of strains, respectively. Time-kill assays were more
discriminatory than checkerboard titration assays in demonstrating
synergy for all combinations.
*
Corresponding author. Mailing address: Department of
Pathology, Hershey Medical Center, P.O. Box 850, Hershey, PA 17033. Phone: (717) 531-5113. Fax: (717) 531-7953. E-mail:
pappelbaum{at}psghs.edu.
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