The Gram-negative oral anaerobe Porphyromonas gingivalis is a major pathogen of chronic periodontitis, the major cause of tooth loss in adults. P. gingivalis requires iron and protoporphyrin IX, which the bacterium obtains as heme and stores on its cell surface.
Outer membrane vesicles (OMVs) are secreted by most Gram-negative bacteria as a stress response. OMVs are formed as the bacterial outer membrane blebs outwards and pinches off the cells, consequently containing periplasm, outer membrane proteins, lipopolysaccharides and phospholipids. OMVs reportedly transport toxins and other virulence factors between the bacterial and target cells to initiate a host immune response. It has been proposed that OMVs are involved in horizontal gene transfer by facilitating the transfer of DNA between bacterial cells and species. Other studies have observed an increase in OMV secretion by Gram-negative bacteria in response to iron limitation, a common condition confronted in the host.
Given the aetiological implications of OMVs, this study aimed to characterise OMV production by P. gingivalis during heme excess and heme limited conditions.
An anaerobic chemostat system was used to continuously culture P. gingivalis W50 in heme excess (5 μg/mL) or heme limitation (0.05 μg/mL). Scanning and transmission electron microscopy analyses revealed two populations of OMVs in heme excess with mean diameters of 50 and 90 nm in a 1:1.5 ratio. In low heme conditions the production of the larger diameter vesicles largely ceased. Dynamic light scattering data was consistent with the electron microscopy results. Confocal laser scanning microscopy was used to visualise DNA within bacterial vesicles.
The data collected here indicates an association between bacterial stress and differential OMV production and warrants further research into the impact iron availability has on the production, composition and function of OMVs. The data may also suggest an additional role for OMVs in iron acquisition from the host.