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Pseudo-outbreak of imipenem-resistant Acinetobacter baumannii resulting from false susceptibility testing by a rapid
A Tsakris1, A Pantazi, S Pournaras
1Department of Microbiology, Medical School, Aristotle University of Thessaloniki, Thessaloniki, Greece. atsakris@med.auth.gr
Abstract:
Introduction of the Vitek GNS-506 susceptibility testing cards in the Hippokration General Hospital, Thessaloniki, Greece, resulted in an apparently high prevalence of imipenem-resistant Acinetobacter baumannii. When 35 of these isolates were further tested by disk diffusion, broth microdilution, and agar dilution assays, 32 were imipenem sensitive by all tests and three were sensitive or intermediate, depending on the method. The pseudoresistant acinetobacters did not form a genetically homogeneous group. It is suggested that the detection of imipenem-resistant A. baumannii isolates by this system should be confirmed by an additional susceptibility test.
Insights
The Vitek GNS-506 system may overestimate imipenem resistance in Acinetobacter baumannii. Further susceptibility testing is recommended to confirm resistance, as many isolates initially flagged as resistant were actually sensitive.
Area of Science:
- Clinical Microbiology
- Infectious Diseases
- Antimicrobial Resistance
Background:
- The introduction of the Vitek GNS-506 susceptibility testing cards at Hippokration General Hospital revealed a high prevalence of imipenem-resistant Acinetobacter baumannii.
- Acinetobacter baumannii is a significant opportunistic pathogen, and resistance to carbapenems like imipenem poses a major treatment challenge.
Purpose of the Study:
- To investigate the accuracy of the Vitek GNS-506 system in detecting imipenem resistance in Acinetobacter baumannii.
- To determine the true prevalence of imipenem resistance among isolates initially identified as resistant by the Vitek system.
Main Methods:
- Thirty-five Acinetobacter baumannii isolates, initially identified as imipenem-resistant by the Vitek GNS-506 system, were re-tested.
- Susceptibility testing was performed using disk diffusion, broth microdilution, and agar dilution assays.
Main Results:
- Out of 35 isolates, 32 were found to be imipenem-sensitive across all three confirmatory methods.
- The remaining three isolates showed sensitivity or intermediate results, varying by the specific testing method employed.
- The isolates exhibiting apparent 'pseudoresistance' did not represent a genetically uniform group.
Conclusions:
- The Vitek GNS-506 system may lead to an overestimation of imipenem resistance in Acinetobacter baumannii.
- Confirmatory testing using alternative methods like disk diffusion, broth microdilution, or agar dilution is crucial for accurate susceptibility assessment.
- Clinical laboratories should validate Vitek GNS-506 findings for imipenem resistance in A. baumannii with additional methods.