Abstract: Objective To investigate the mechanism of drug resistance of sulbactam mediated by TEM-1 betalactamases and porin CarO in clinical strains of acinetobacter baumannii. Methods Twenty-four unrepeated clinical acinetobacter baumannii strains were divided into sensitive strain group (n=6) and insensitive strain group (n=18) by susceptibility testing to sulbactam. Antibiotics susceptibility test was carried out using the Kirby-Bauer method. BlaTEM-1 and carO genes were amplified by PCR. Four blaTEM-1 positive strains (A3, A5, 1327 and C1) were selected, and their amplified products were sequenced. The quantitative real-time RT-PCR was used to analyze mRNA transcriptional levels of blaTEM-1 and carO genes. Results Resistant rates of the sensitive strain group for meropenem, imipenem, cefoperazone,ciprofloxacin, gentamicin and ampicillin were 1/6, 2/6, 3/6, 1/6, 5/6 and 5/6, and resistant rates of the insensitive strain group were 6/18, 10/18, 18/18, 18/18 and 18/18. BlaTEM-1 genes were amplified in 16 insensitive strains, and blaTEM-1 was negative in sensitive strains. The carO genes were amplified in all 24 strains. There was no significative mutation in the 4 strains of blaTEM-1 genes. The promoters of the strains A3, A5 and 1327 were P4, and C1 was P3. There was a positive correlation between the mRNA expression of blaTEM-1 and the MIC value of sulbactam (rs=0.551, P=0.027). There was no difference in the mRNA expression of carO between the two groups. Conclusion The clinical strains are seriously resistant to antibiotics. The main resistance mechanism of clinical strains to sulbactam is the high mRNA expression of blaTEM-1, and the promoter may be one of the reasons of high expression of TEM-1

Key words: Acinetobacter baumannii, beta-Lactamases, microbial sensitivity tests, carO gene, blaTEM-1 gene, TEM-1 β-lactamase