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Sung, Choi, Yoo, and Kim: In vitro Antimicrobial Synergy against Imipenem-Resistant Acinetobacter baumannii
Original Article

Korean J Leg Med 2007;27(2):111-117.

Published online: 10 February 2007

DOI: https://doi.org/10.3343/kjlm.2007.27.2.111

In vitro Antimicrobial Synergy against Imipenem-Resistant Acinetobacter baumannii

Heungsup Sung, M.D., Soo Jin Choi, M.D., Soojin Yoo, M.D., Mi-Na Kim, M.D.

Department of Laboratory Medicine, University of Ulsan College of Medicine and Asan Medical Center, Seoul, Korea

Received 30 October 2006       Revised 5 December 2006       Accepted 5 December 2006

Copyright © 2007 The Korean Society for Legal Medicine

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background

Most imipenem-resistant Acinetobacter baumannii (IRAB) isolates are multiresistant, leaving few options for an effective antimicrobial therapy. We purposed to select possible candidates for the combinations of antimicrobials that are synergistic in vitro for inhibitory or bactericidal activities against IRAB and evaluate the usefulness of double disk synergy test (DDS) in predicting synergistic bactericidal activity.

Methods

Fifty-five IRAB isolates recovered from patients during the period from August 1999 to November 2000 were tested for susceptibilities to amikacin, gentamicin, tobramycin, piperacillin, piperacillin/tazobactam, cefotaxime, cefepime, cefoperazone/sulbactam (C/S), imipenem, meropenem, ciprofloxacin, levofloxacin, trimethoprim/sulfamethoxazole, chloramphenicol, minocycline, and colistin by the Clinical and Laboratory Standard Institute agar dilution method. Three isolates showing different susceptibility profiles were tested for antimicrobial synergy by DDS and then by time-kill study (TKS) using DDS-positive combinations.

Results

Colistin, C/S, and minocycline were active in 50 (90.9%), 50, and 44 (80.0%) isolates, respectively, and all the other drugs were active in less than 20% of isolates. Minocycline-imipenem, minocycline-C/S, minocycline-amikacin, imipenem-tobramycin, C/S-amikacin, and C/S-tobramycin combinations showed synergistic inhibitory or bactericidal activity by TKS when the same combinations were synergistic in DDS; however, C/S-imipenem was found synergistic on DDS, but not by TKS.

Conclusions

Colistin, C/S, and minocycline were relatively active against IRAB. DDS might help predict the synergistic antimicrobial effect of TKS if one of the combinations was susceptible.

Keywords: Imipenem-resistant, Acinetobacter baumannii, Atimicrobial synergy, Double disk synergy, Time kill study

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Fig. 1.
Double disk synergy tests of three A. baumannii isolates. (A) The isolate 1 showed synergy at the combination of MC+IP, IP+C/S, and C/S+AK. (B) The isolate 2 showed synergy at the combination of MC+IP, MC+C/S, MC+AK, IP+AK, IP+TM, IP+C/S, C/S +TM, and C/S+AK. (C) The isolate3 showed synergy at the combination of MC+AK, IP+AK, and IP+C/S. The black dots denote synergy between the two drugs.
kjlm-27-111f1.tif
Fig. 2.
Time-kill tests for three A. baumannii isolates. (A) IRAB1 showed synergistic killing at the combination of MC+IP, MC+AK, and C/S+AK. (B) IRAB2 showed synergistic killing at the combination of MC+IP, MC+TM, MC+C/S, MC+AK, IP+TM, C/S+TM, and C/S+AK. (C) IRAB3 showed no synergistic killing at any combination of antimicrobials. The number in parentheses denotes the concentration (μg/mL) of antimicrobials tested.
kjlm-27-111f2.tif
Table 1.
MICs of 55 A. baumannii isolates against common antimicrobials
Antimicrobial (s) %S %I MIC50 (μg/mL) MIC90 (μg/mL)
Azithromycin NA NA 64 128
Piperacillin 10.9 9.1 256 256
Piperacillin/tazobactam 16.3 3.6 128 256
Cefotaxime 0 0 >128 >128
Cefoperazone/sulbactam 90.9 0 16 16
Aztreonam 0 1.8 >128 >128
Cefepime 9.1 3.6 128 128
Meropenem 0 3.6 16 32
Imipenem 0 18.1 64 32
Ciprofloxacin 18.1 0 >32 >32
Levofloxacin 18.1 1.8 16 16
Gentamicin 0 1.8 >64 >64
Tobramycin 7.2 0 64 >64
Amikacin 5.4 23.6 64 128
Arbekacin NA NA 8 16
Minocycline 80 0 2 2
Colistin 90.9 0 0.5 1
Rifampin NA NA 4 8
Sulbactam NA NA 16 32
Trimethoprim/sulfamethoxazole 12.7 0 32 64
Chloramphenicol 0 0 >128 >128

Abbreviations: NA, not available due to the lack of interpretative criteria; %S, percentage of susceptible isolates; %I, percentage of intermediate resistant isolates; MIC50 and MIC90, MICs at which 50% and 90% of isolates are inhibited, respectively.

Table 2.
Antimicrobial susceptibility and synergy results of the isolates tested by double disk synergy tests and time-kill studies
Strain No. MICs (μg/mL)
 Antimicrobial combination tested by
Double disk synergy tests
 Time-kill studies
IP C/S AK TM MC CI Synergistic Non-synergistic Synergistic Non-synergistic
IRAB1 32 16 16 16 2 4 MC+IP MC+AK MC+IP MC+C/S
              C/S+AK MC+C/S MC+AK IP+C/S
              IP+C/S   C/S+AK  
IRAB2 32 8 64 1 0.12 4 MC+IP MC+TM MC+IP IP+AK
              MC+C/S   MC+TM IP+C/S
              MC+AK   MC+C/S  
              IP+AK   MC+AK  
              IP+TM   IP+TM  
              IP+C/S   C/S+TM  
              C/S+TM   C/S+AK  
              C/S+AK      
IRAB3 64 16 128 16 0.12 0.25 MC+AK MC+IP None MC+IP
              IP+AK MC+CI   MC+C/S
              IP+C/S MC+C/S   MC+CI
                IP+CI   IP+AK
                C/S+AK   IP+C/S
                C/S+CI   IP+CI
                    CS+AK
                    CS+CI

Abbreviations: IP, imipenem; C/S, cefoperazone/sulbactam; AK, amikacin; TM, tobramycin; MC, minocycline; CI, ciprofloxacin.

Abbreviations: See Table 2.

Abbreviations: See Table 2.

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