Objective  To explore the significance of the resistance to polymyxin resistance of the extensively drug resistant Acinetobacter baumannii (XDRAB) lipopolysaccharide (LPS) lpx A, lpx C, lpx D and to screen appropriate combination therapy.

  Methods  In the past two years, 72 XDRAB in the secretions of our patients were selected as the research object. According to the minimum inhibitory concentration (MIC) of the XDRAB strain on polymyxin, they were included in the drug resistance group and the sensitive group. The gene sequences of strains lpx A, lpx C, lpx D were compared with the standard strains to analyze gene mutations and compared the mutation rates in the drug resistant group and the sensitive group. The efficacy of the combination drugs was evaluated by microcheckerboard dilution method, including polymyxin+imipenem group, polymyxin+meropenem group, polymyxin+cefoperazone/sulbactam group, polymyxin+levofloxacin group, and polymyxin+fosfomycin group. Calculated the fractional inhibitory concentration (FIC) index of the combined medication regimen and compared the percentage of strains that exhibited synergistic, additive, irrelevant, and antagonistic effects.

  Results  Tentyone were in the drug resistant group, accounting for 21 (29.17%,) and 51 were in the sensitive group, accounting for 70.83%. Some strains had mutations in lpx A, lpx C, lpx D genes. The mutation rate in the drug resistant group was 90.48%, which was significantly higher than 11.76% in the sensitive group, the difference was statistically significant (P<0.05). The combined drug sensitivity test showed, compared with the polymyxin+fosfomycin group, the mycotin+fosfomycin group had a higher percentage of strains with synergistic FIC index in the polymyxin+imipenem group, the difference was statistically significant (P<0.01).

  Conclusion  XDRAB is resistant to polymyxin, which is related to mutations in LPS lipid A biosynthesis genes lpx A, lpx C, lpx D. Clinical treatment should adopt a combination of polymyxin+imipenem/meropenem and other drug combination to reduce the secondary infection of drug resistant bacteria.