Gram-negative bacteria produce nanoparticles known as outer membrane vesicles (OMVs) that contain bacterial products including DNA, protein and lipids. Due to the ability of OMVs to package DNA within them, it is suggested that they are able to facilitate the transfer of genes between bacterial species. This study examines the ability of OMVs isolated from Gram-negative Pseudomonas aeruginosa to package and transfer DNA to other bacteria.
In this study, OMVs were isolated from P. aeruginosa (PAO9505) harbouring a plasmid encoding for antibiotic resistance. Examination of OMVs using DNA staining revealed that DNA was associated on the outside, in addition to being contained within OMVs. Plasmid-containing OMVs were then used to transform the recipient P. aeruginosa strain (PAO9503), resulting in antibiotic resistant transformants. In order to confirm that the plasmid was contained within OMVs, vesicles were treated with DNase and PCR was used to confirm the presence of intact plasmid DNA post-treatment. We are currently determining the ability of DNase-treated OMVs to transfer plasmids encoding for antibiotic resistance between strains. In addition, DNA packaging is suggested to be increased in response to conditions of stress, such as antibiotic treatment. To test this hypothesis, we purified OMVs from cultures treated with sub-lethal doses of antibiotics. We found that OMVs produced in response to antibiotic stress contained more DNA than OMVs generated during normal conditions, and we are currently testing the transformation efficiency of these OMVs.
Collectively, these findings identify that OMVs are a mechanism used by bacteria to transfer DNA encoding for antibiotic resistance to other bacteria, and that stress may alter the selective packaging of DNA within OMVs. The outcomes of this study suggest that DNA transfer mediated via OMVs may contribute to the spread of antibiotic resistance between bacterial strains.