Analysis of the mechanism of drug resistance of VIM-2-type metallo-β-lactamase-producing Acinetobacter baumannii isolated from burn patients and its homology

Objective To study the drug resistance of Acinetobacter baumannii (AB) producing VIM–2–type metallo–β–lactamase (MBL) isolated from burn patients of our ward against carbapenem antibiotics and its homology. Methods A total of 400 strains of AB (identified) were isolated from sputum, urine, blood, pus, and wound drainage of burn patients hospitalized in our ward from September 2011 to March 2014. Drug resistance of the 400 strains of AB to 15 antibiotics, including compound sulfamothoxazole, aztreonam, etc., was tested using the automatic microorganism identifying and drug sensitivity analyzer. Among the carbapenems–resistant AB isolates, modified Hodge test was applied to screen carbapenemase–producing strains. The carbapenemase genes of the carbapenemase–producing strains, and the mobile genetic elements class 1 integron (Intl1) gene and conserved sequence (CS) of carbapenemase–producing strains carrying bla_VIM–2 gene were determined with PCR and DNA sequencing. For carbapenemase–producing strains carrying bla_VIM–2 gene, synergism test with imipenem–ethylene diamine tetraacetic acid (EDTA) and enhancement test with imipenem–EDTA and ceftazidime–EDTA were used to verify the MBL–producing status. Drug resistance of the VIM–2–type MBL–producing AB strains was analyzed. For VIM–2–type MBL–producing AB strains, plasmid conjugation experiment was used to explore the transfer of plasmid; outer membrane protein (OMP) CarO gene was detected by PCR. For VIM–2–type MBL–producing AB strains carrying CarO gene, the protein content of CarO was analyzed with sodium dodecyl sulfate polyacrylamide gel electrophoresis. The repetitive consensus sequence of Enterobacteriaceae genome PCR (ERIC–PCR) was carried out for gene typing of VIM–2–type MBL–producing AB strains to analyze their homology. Results (1) The resistant rates of the 400 strains of AB against levofloxacin and compound sulfamethoxazole were low. A total of 381 carbapenems–resistant AB strains were screened, including 240 carbepenemase–producing strains. (2) Out of the 240 carbepenemase–producing strains, 18 strains were found to harbor the bla_VIM–2 gene, accounting for 7.5%; 133 strains carried the bla_TEM–1 gene, accounting for 55.42%; 195 strains carried the bla_OXA–23 gene, accounting for 81.25%; 188 strains carried the bla_armA gene, accounting for 78.33%. (3) Eighteen carbepenemase–producing strains which carried the bla_VIM–2 gene were found to carry the Intl1 gene, showing the Intl1–VIM linkage. Simultaneously, Intl1 variable area CS showed diversity. (4) Eighteen carbepenemase–producing strains which carried the bla_VIM–2 gene were verified to produce MBL. The resistant rates of the 18 strains of AB against compound sulfamethoxazole were the lowest, followed by levofloxacin and cefoperazone/sulbactam, and those against the other antibiotics were above 60.00%. (5) Through multiple joint tests, plasmid conjugation experiment positive transfer strain was not found in 18 VIM–2–type MBL–producing AB strains. (6) Nine out of the 18 VIM–2–type MBL–producing AB strains were found to carry CarO gene. The OMP CarO of VIM–2–type MBL–producing AB strains carrying CarO gene was lost or lowered in the protein content. (7) The 18 VIM–2–type MBL–producing AB strains were classified into 6 genotypes by the ERIC–PCR. There were respectively 6, 4, 3, and 1 stain (s) in genotypes A, B, C, and F, and there were 2 strains in genotypes D and E respectively. Conclusions The resistance mechanism of AB against carbapenems is mainly mediated by bla_TEM–1, bla_OXA–23, and bla_arma; meanwhile, VIM–2–type MBL–producing and lack or change in OMP CarO are attributable to carbapenems resistance of clinically isolated AB from burn wards, and the Intl1 gene may take a part in bla_VIM–2 gene transmission.