Acinetobacter baumannii transfers the blaNDM-1 gene via outer membrane vesicles

To investigate the transmission of the gene encoding New Delhi metallo-β-lactamase-1 ( bla NDM-1 ) through outer membrane vesicles (OMVs) released from an Acinetobacter baumannii strain (A_115).

Isolation and purification of OMVs by density gradient from a carbapenem-resistant clinical strain of A. baumannii harbouring plasmid-mediated bla NDM-1 and aac(6')-Ib-cr genes was performed. DNA was purified from the OMVs and used for PCR and dot-blot analysis. Vesicles treated with DNase I and proteinase K were used to transform A. baumannii ATCC 19606 and Escherichia coli JM109 strains. MIC values for the transformants were determined, followed by PCR and restriction digestion of plasmids. PFGE was done for A_115 and transformants of ATCC 19606 and JM109.

The A. baumannii strain (ST 1462) released vesicles (25-100 nm) during in vitro growth at late log phase. PCR and dot-blot analysis confirmed the presence of bla NDM-1 and aac(6')-Ib-cr genes in intravesicular DNA. bla NDM-1 and aac(6')-Ib-cr genes were transferred to both the A. baumannii ATCC 19606 and E. coli JM109 recipient cells. The transformation frequency of the purified OMVs was in the range of 10 -5 -10 -6 and gradually reduced with storage of OMVs. The sizes of the plasmids in the transformants and their restriction digestion patterns were identical to the plasmid in A_115. The transformants showed elevated MIC values of the β-lactam group of antibiotics, which confirmed the presence of a bla NDM-1 -harbouring plasmid.

This is the first experimental evidence of intra- and inter-species transfer of a plasmid harbouring a bla NDM-1 gene in A. baumannii via OMVs with high transformation frequency.