Comparative analysis of carbapenemases, RND family efflux pumps and biofilm formation potential among Acinetobacter baumannii strains with different carbapenem susceptibility.

AIM: This study has conducted a comparative analysis of common carbapenemases harboring, the expression of resistance-nodulation-cell division (RND) family efflux pumps, and biofilm formation potential associated with carbapenem resistance among Acinetobacter baumannii (A. baumannii) strains with different carbapenem susceptibility. METHODS: A total of 90 isolates of A. baumannii from two tertiary hospitals of China were identified and grouped as carbapenem susceptible A. baumannii (CSAB) strains and carbapenem non-susceptible A. baumannii (CnSAB) strains based on the susceptibility to imipenem. Harboring of carbapenemase genes, relative expression of RND family efflux pumps and biofilm formation potential were compared between the two groups. RESULT: Among these strains, 12 (13.3 %) strains were divided into the CSAB group, and 78 (86.7 %) strains into the CnSAB group. Compared with CSAB strains, CnSAB strains increased distribution of blaOXA-23 (p < 0.001) and ISAba1/blaOXA-51-like (p = 0.034) carbapenemase genes, and a 6.1-fold relative expression of adeB (p = 0.002), while CSAB strains led to biofilm formation by 1.3-fold than CnSAB strains (p = 0.021). CONCLUSIONS: Clinically, harboring more blaOXA-23-like and ISAba1/blaOXA-51-like complex genes and overproduction of adeABC are relevant with carbapenem resistance, while carbapenem susceptible strains might survive the stress of antibiotic through their ability of higher biofilm formation.

Distribution pattern of carbapenemases and solitary contribution to resistance in clinical strains of Acinetobacter baumannii.

BACKGROUND: This study aimed to investigate the distribution pattern of carbapenemases and evaluate their solitary contribution to carbapenem resistance. METHODS: One hundred and twelve isolates of Acinetobacter baumannii (A. baumannii) isolated from the intensive care unit (ICU) of a southern China tertiary hospital were identified, and antimicrobial susceptibility tests (ASTs) of these strains were determined. Common carbapenemases were detected and the distribution pattern of carbapenemases was analyzed. Logistic regression and general linear model analyzed were performed to identify the correlation between antimicrobial susceptibility and carbapenemase genes. RESULTS: These 112 strains were classified into a carbapenem-resistant A. baumannii (CRAB) group (71.7%) and a carbapenem-susceptible A. baumannii (CSAB) group (28.3%). Carbapenemase genes, including blaOXA-51-like (100.0%), blaOXA-23 (93.4%), ISAba1/blaOXA-51-like (27.5%), blaNDM-1 (8.8%), blaOXA-24 (2.2%) and blaOXA-58 (2.2%) were detected in CRAB strains, and no blaSIM, blaVIM and blaIMP gene in these 112 isolates. There was a statistically significant difference between CSAB and CRAB group in carrying blaOXA-23 (P<0.001) and ISAba1/blaOXA-51-like (P=0.024). CONCLUSIONS: A pattern of blaOXA-51-like (100.0%), blaOXA-23 (93.4%), blaNDM-1 (8.8%), blaOXA-24 (2.2%) and blaOXA-58 (2.2%) was detected in CRAB strains. BlaOXA-23-like and ISAba1/blaOXA-51-like complex might be more relevant to carbapenem resistance in A. baumannii. Harboring blaOXA-23-like and ISAba1/blaOXA-51-like complex might increase the possibility of resistance 2.16 times [risk ratio (RR): 2.16; 95% confidence interval (CI): 1.04-4.51] and 1.29 times (RR: 1.29; 95% CI: 1.07-1.56), respectively.

Overproduction of efflux pumps caused reduced susceptibility to carbapenem under consecutive imipenem-selected stress in Acinetobacter baumannii.

PURPOSE: Acinetobacter baumannii is an important pathogen in the nosocomial infections worldwide. Combining with carbapenemases, efflux pumps and outer membrane proteins (OMPs) have been thought to affect the development of carbapenem resistance in A. baumannii. This study aimed to investigate the contributions of different efflux pumps and OMPs in developing carbapenem resistance in a clinical isolate of A. baumannii and reveal the possible mechanism of overproduction of main efflux pumps. PATIENTS AND METHODS: In this study, an imipenem-susceptible clinical isolate was identified as A. baumannii and named SZE. Several common carbapenemases were detected by polymerase chain reaction (PCR). Imipenem-selected mutants were selected from SZE by serial subcultivations on Mueller-Hinton agar, and the minimum inhibitory concentration (MIC) was detected. Gene expressions of four families of efflux pumps, five OMPs, and blaOXA-51 were determined by reverse transcription quantitative PCR, and comparisons were made between SZE strain and the imipenem-selected mutants. The adeRS system in SZE and its mutant was sequenced and aligned. RESULTS: Under consecutive imipenem-selected stress, the MIC to imipenem increased gradually from 0.125 µg/mL to 8 µg/mL. The effect of resistance inducement was almost neutralized when treated with an efflux pump inhibitor. The expression of efflux pumps, adeB, adeG, and adeJ, was increased by 6.9-, 4.0-, and 2.1-fold in mutants, respectively, compared to SZE. A single mutation (G to A) at position 58 was detected in the regulatory adeRS system and possibly upregulated the adeB expression, and then affected the carbapenem resistance in A. baumannii strains. CONCLUSION: In conclusion, under consecutive imipenem-selected stress in vitro, A. baumannii strain evolved the ability to reduce susceptibility to a variety of antimicrobials by overproduction of efflux pumps. Especially, the resistance-nodulation-cell division super family and a nucleotide mutant in adeRS regulating system caused the overexpression of adeABC.