The Gram-negative oral anaerobe Porphyromonas gingivalis is a major pathogen of chronic periodontitis, the major cause of tooth loss in adults. P. gingivalis is a protoporphyrin IX auxotroph, which it obtains mainly as host derived haem through the action of surface located protease/adhesins and specific haem-binding lipoproteins. Outer membrane vesicles (OMVs) are secreted by most Gram-negative bacteria under a variety of growth conditions. This study aimed to characterise OMV production by P. gingivalis during haem excess and haem limited conditions.
An anaerobic chemostat system was used to continuously culture P. gingivalis W50 in haem excess (5.0 μg/mL) or haem limitation (0.1 μg/mL). The bacterium was grown at 37oC with a mean generation time of 6.9 h (0.5 relmax). Scanning and transmission electron microscopy analyses revealed haem limited grown bacteria produced an altered morphology whereby cells became elongated with a relatively larger number of OMVs associated with the bacterial surface. Cryo-electron microscopy was used to visualise an electron density surface layer associated with all OMV. Differential proteomics studies were carried out using Dimethyl labelling experiment on OMV preparations from haem limited and haem excess cultures that were subsequently analysed on a LTQ Orbitrap Elite Mass Spectrometer. Thirty five differentially regulated proteins were identified, 19 of which were found in greater abundance in haem limited derived OMVs. These included the haem-binding lipoprotein IhtB, which is encoded as part of an iron transport operon, and the outer membrane proteins PG0191 and ompH-1.
The data indicates an upregulation of OMV biogenesis and a difference in the protein composition when P. gingivalis is grown under conditions of micronutrient limitation. The data may also suggest a role for OMVs in iron acquisition from the host.