Correlation between antibiotic resistance and serum resistance in Acinetobacter baumannii

Objective: This study aimed to investigate the relationship between antibiotic resistance and serum resistance in clinical isolates of Acinetobacter baumannii (A. baumannii). Methods: The tested 67 clinical isolates were collected from several hospitals in China. Antibiotic resistance to 21 antibiotics from 9 antimicrobial categories was assessed by Kirby-Bauer disc diffusion or broth microdilution methods. Multilocus sequence typing (MLST) was used to group the A. baumannii isolates. At last, the in vivo mice model was used to detect the relationship between drug resistance and mortality from inoculation with A. baumannii. Results: Among all 67 isolates, 16 were defined as multidrug-resistant (MDR), and 46 were extensively drug-resistant (XDR). MLST grouped the A. baumannii isolates into 9 existing sequence types (STs). ST208 accounted for 44.8% (30/67) of the isolates, which belonged to clonal complex (CC) 92. The serum resistance testing showed that 53 out of 68 strains of A. baumannii (67 clinical isolates and reference strain ATCC19606) were highly resistant to killing by complement system in normal human serum (NHS). The comparison of the antibiotic resistance and serum resistant strains showed that antibiotic resistant isolates had stronger serum resistance than susceptible strains. Furthermore, mice infected by XDR isolates had higher mortality rate. Conclusion: Drug-resistant A. baumannii strains have stronger serum resistance. These results sounds alarming and should be considered in the clinical treatment of drug-resistant A. baumannii.