In vitro activity of rifampicin alone and in combination with imipenem against multidrug-resistant Acinetobacter baumannii harboring the blaOXA-72 resistance gene.
BACKGROUND
The growing incidence of multidrug resistance (MDR) in bacteria is an emerging challenge in the treatment of infections. Acinetobacter baumannii is an opportunistic pathogen prone to exhibit MDR that contributes significantly to nosocomial infections, particularly in severely ill patients. Thus, we performed research on rifampicin activity against selected MDR OXA-72 carbapenemase-producing A. baumannii strains. Since it is widely accepted that rifampicin should not be used as monotherapy in order to avoid the rapid development of rifampicin resistance, we evaluated the efficacy of combination therapy with imipenem.


METHODS
Minimal inhibitory concentrations (MICs) of both rifampicin and imipenem were determined by use of the broth microdilution method. Evaluations of the interactions between rifampicin and imipenem were performed by analysis of the fractional inhibitory concentration index (∑FIC), determined using the checkerboard titration method.


RESULTS
All tested isolates showed full susceptibility to rifampicin. The checkerboard method revealed synergism in 5 isolates (29%) and an additive effect in another 5 isolates (29%); no difference was reported in the remaining 7 isolates (41%). Strains moderately resistant to imipenem (MIC ≤ 64 mg/l) tended to show synergy or additive interaction.


CONCLUSIONS
We conclude that in vitro synergism or an additive interaction between rifampicin and imipenem most likely occurs in A. baumannii strains showing moderate resistance to imipenem (MIC ≤ 64 mg/l). Moreover, utilizing this combination in the therapy of infections caused by strains exhibiting higher levels of resistance (MIC > 64 mg/l) is not recommended since in this setting imipenem could not prevent the development of rifampicin resistance.
From the Department of Microbiological Diagnostics and Infectious Immunology, Medical University of Bialystok , Bialystok , Poland.
Piotr
P
Majewski
Piotr
P
Wieczorek
Dominika
D
Ojdana
Paweł Tomasz
PT
Sacha
Anna
A
Wieczorek
Elżbieta Anna
EA
Tryniszewska
D000151:Acinetobacter Infections / Q000188:drug therapy / Q000382:microbiology*; D040981:Acinetobacter baumannii / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics; D006801:Humans; D015378:Imipenem / Q000494:pharmacology*; D008826:Microbial Sensitivity Tests; D012293:Rifampin / Q000494:pharmacology*; D001618:beta-Lactamases / Q000235:genetics*
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D015378:Imipenem; D001618:beta-Lactamases; D012293:Rifampin