Tigecycline resistance has emerged recently and has shown diverse mechanisms. The aim of this study was to assess the role of efflux activity in tigecycline resistance in 120 clinical isolates of A. baumannii using two methods: the H33342 accumulation assay and adeB real-time reverse transcriptase polymerase chain reaction. In addition, we analyzed the correlation between the expression level of adeB and H33342 accumulation level. Methods: A. baumannii clinical isolates was divided into

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... ine-resistant (49 strains), intermediate (40 strains), and susceptible (31 strains) groups. The H33342 accumulation was measured in the absence or presence of the efflux pump inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP). Real-time RT-PCR was performed to determine the relative expression of the adeB gene in A. baumannii clinical isolates. Results: The level of H33342 accumulation in the resistant group was relatively lower than those in the other groups. The addition of CCCP caused a significantly increased fold change in H33342 accumu-lation in the tigecycline-resistant group. Significant difference in the fold change level in H33342 accumulation was found between tigecycline-susceptible and resistant isolates. Those findings support the role of efflux pumps of which substrates are H33342 in the resistance of tigecycline. Significant differences in the relative expression levels of adeB were shown between tigecycline-susceptible and resistant groups also. Conclusion: The results showed that several efflux pumps of which substrates were H33342 can contribute to tigecycline resistance. The adeB overexpression can also contribute to tigecycline resistance. It is possible that efflux pumps other than adeB efflux pumps contribute to tigecycline resistance because there was no correlation between fold change level in H33342 accumulation and adeB expression level.